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Purpose This study aims to reduce the delivery time of CyberKnife m6 treatments by allowing for noncoplanar continuous arc delivery. To achieve this, a novel noncoplanar continuous arc delivery optimization algorithm was developed for the CyberKnife m6 treatment system (CyberArc-m6). Methods and materials CyberArc-m6 uses a five-step overarching strategy, in which an initial set of beam geometries is determined, the robotic delivery path is calculated, direct aperture optimization is conducted, intermediate MLC configurations are extracted, and the final beam weights are computed for the continuous arc radiation source model. This algorithm was implemented on five prostate and three brain patients, previously planned using a conventional step-and-shoot CyberKnife m6 delivery technique. The dosimetric quality of the CyberArc-m6 plans was assessed using locally confined mutual information (LCMI), conformity index (CI), heterogeneity index (HI), and a variety of common clinical dosimetric objectives. Results Using conservative optimization tuning parameters, CyberArc-m6 plans were able to achieve an average CI difference of 0.036 ± 0.025, an average HI difference of 0.046 ± 0.038, and an average LCMI of 0.920 ± 0.030 compared with the original CyberKnife m6 plans. Including a 5 s per minute image alignment time and a 5-min setup time, conservative CyberArc-m6 plans achieved an average treatment delivery speed up of 1.545x ± 0.305x compared with step-and-shoot plans. Conclusions The CyberArc-m6 algorithm was able to achieve dosimetrically similar plans compared to their step-and-shoot CyberKnife m6 counterparts, while simultaneously reducing treatment delivery times.

Purpose Although collimator rotation is an optimization variable that can be exploited for dosimetric advantages, existing Volumetric Modulated Arc Therapy (VMAT) optimization uses a fixed collimator angle in each arc and only rotates the collimator between arcs. In this study, we develop a novel integrated optimization method for VMAT, accounting for dynamic collimator angles during the arc motion. Methods Direct Aperture Optimization (DAO) for Dynamic Collimator in VMAT (DC-VMAT) was achieved by adding to the existing dose fidelity objective an anisotropic total variation term for regulating the fluence smoothness, a binary variable for forming simple apertures, and a group sparsity term for controlling collimator rotation. The optimal collimator angle for each beam angle was selected using the Dijkstra's algorithm, where the node costs depend on the estimated fluence map at the current iteration and the edge costs account for the mechanical constraints of multi-leaf collimator (MLC). An alternating optimization strategy was implemented to solve the DAO and collimator angle selection (CAS). Feasibility of DC-VMAT using one full-arc with dynamic collimator rotation was tested on a phantom with two small spherical targets, a brain, a lung and a prostate cancer patient. The plan was compared against a static collimator VMAT (SC-VMAT) plan using three full arcs with 60 degrees of collimator angle separation in patient studies. Results With the same target coverage, DC-VMAT achieved 20.3% reduction of R50 in the phantom study, and reduced the average max and mean OAR dose by 4.49% and 2.53% of the prescription dose in patient studies, as compared with SC-VMAT. The collimator rotation co-ordinated with the gantry rotation in DC-VMAT plans for deliverability. There were 13 beam angles in the single-arc DC-VMAT plan in patient studies that requires slower gantry rotation to accommodate multiple collimator angles. Conclusions The novel DC-VMAT approach utilizes the dynamic collimator rotation during arc delivery. In doing so, DC-VMAT affords more sophisticated intensity modulation, alleviating the limitation previously imposed by the square beamlet from the MLC leaf thickness and achieves higher effective modulation resolution. Consequently, DC-VMAT with a single arc manages to achieve superior dosimetry than SC-VMAT with three full arcs.

Purpose: Due to the potentially periodic collimator dynamic in volumetric modulated arc therapy (VMAT) dose deliveries with the sweeping-window arc therapy (SWAT) technique, additional manifestations of dosimetric deviations in the presence of intrafractional motion may occur. With a fast multileaf collimator (MLC), and a flattening filter free dose delivery, treatment times close to 60 s per fraction are clinical reality. For these treatment sequences, the human breathing period can be close to the collimator sweeping period. Compared to a random arrangement of the segments, this will cause a further degradation of the dose homogeneity. Methods: Fifty VMAT sequences of potentially moving target volumes were delivered on a two dimensional ionization chamber array. In order to detect interplay effects along all three coordinate axes, time resolved measurements were performed twice—with the detector aligned in vertical (V) or horizontal (H) orientation. All dose matrices were then moved within a simulation software by a time-dependent motion vector. The minimum relative equivalent uniform dose EUD r,m for all breathing starting phases was determined for each amplitude and period. Furthermore, an estimation of periods with minimum EUD was performed. Additionally, LINAC logfiles were recorded during plan delivery. The MLC, jaw, gantry angle, and monitor unit settings were continuously saved and used to calculate the correlation coefficient between the target motion and the dose weighed collimator motion component for each direction (CC, LR, AP) separately. Results: The resulting EUD r,m were EUD r,m (CC V ) = (98.3 ± 0.6)%, EUD r,m (CC H ) = (98.6 ± 0.5)%, EUD r,m (AP V ) = (97.7 ± 0.9)%, and EUD r,m (LR H ) = (97.8 ± 0.9)%. The overall minimum relative EUD observed for 360∘ arc midventilation treatments was 94.6%. The treatment plan with the shortest period and a minimum relative EUD of less than 97% was found at T = 6.1 s. For a partial 120∘ arc, an EUD r,m = 92.0% was found. In all cases, a correlation coefficient above 0.5 corresponded to a minimum in EUD. Conclusions: With the advent of fast VMAT delivery techniques, nonrobust treatment sequences for human breathing patterns can be generated. These sequences are characterized by a large correlation coefficient between a target motion component and the corresponding collimator dynamic. By iteratively decreasing the maximum allowed dose rate, a low correlation coefficient and consequentially a robust treatment sequence are ensured.

Purpose: To systematically analyze penumbra characteristics in co‐planar arc‐beams and demonstrate how optimal aperture margin (OAM) can be systematically estimated. Methods: Full arc‐beam delivery was simulated with a Pinnacle RTP for a total of 32 cases. Considered factors were energy (6X, 6F, 10X, and 10F, where ‘F’ indicates flattening‐filter‐free), field‐size (2×2, 4×4, 6×6, and 8×8), and two materials (50×50×50 cm×cm×cm water and 50×50×50 cmxcmxcm water with a 20 cm diameter sphere of 0.25 g/cc lung density at the center). The highest‐dose‐gradient (HDG) and its range in both radial‐dose‐distribution (RDD) and longitudinal‐dose‐distribution (LDD) were evaluated. For OAM estimation, start with simulated dose distributions in interest. Measure coverage size (CS) at the chosen PIL in the RDD. The difference between the target size (TS) and the CS is the margin needed. In radial‐direction, choose a PIL where the CS is larger than the TS [resulting in ‘negative margin (NM)’] to enhance conformation capability.Next, measure the CS at the chosen PIL in the LDD. Longitudinally, positive margin is expected in general. This process was applied to 6X and 10F cases in lung to illustrate its applicability. Results: Overall, larger HDG was observed in 6 MV (than 10 MV), smaller fields, LDD (than RDD), and in water (than lung). 6F showed slightly larger HDG than 6X while 10X did than 10F. Dose fall‐off started significantly farther from the CAX in RDD than LDD, supporting the NM concept. With the systematic margin estimation applied, expected PILs matched closely with isodose points having tight target coverage, indicating the applicability of the method. Conclusions: Among tested, the 6F beam provides the most optimal dose distribution for dose conformation. The range of optimal PIL decreases as field‐size increases. It has been demonstrated optimal AMs can be systematically estimated as proposed in this study.

Purpose: To evaluate the dosimetric implications of using VMAT (Volume Modulated Arc Therapy) treatment planning techniques compared to traditional Arc therapy methods for patients undergoing SBRT (Stereotactic Body Radiation Therapy) for early‐stage Non‐Small Cell LungCancer(NSCLC).Methods: Ten NSCLCcancer patients are planned with both VMAT and Arc techniques. The SBRTtreatment plans comparison was quantified by several Dose‐Volume Histogram (DVH) indicators including mean, maximum and minimum doses for GTV, ITV, PTV, OAR (Organs At Risk), and V95 (volume receiving at least 95% of the prescribed dose) for PTV. Results: On average VMAT plans require for treatment delivery 16.6 ± 20.2 % more monitor units (MU) than the traditional Arc plans. The average PTV minimum, maximum and mean doses as a percentage of prescribed dose are 94.5 ± 3.9 %, 114.1 ± 3.3 % and 106.6 ± 1.6 % for VMAT vs 91.6 ± 4.4 %, 119.5 ± 5.3 % and 109.5 ± 2.5 % for the Arc technique. The V95 PTV coverage for VMAT plans range from 99.4 % to 100 % with a mean of 99.7 %, compared with a range of 96.8 % to 100 % with a mean of 99 % for the Arc plans. The maximum dose received by the lungs, spinal cord and chest wall show on average significant increases for Arc plans as opposed to VMAT plans (5.7 ± 6 % increase for lungs, 4.4 ± 9.2 % for cord and 2.4 ± 6.3 % for chest wall ). The average mean doses and minimum doses for the OAR are similar for both techniques. Conclusions: The comparison of VMAT vs Arc plans for SBRT of NSCLC patients is subject to many variables, including GTV and PTV volume sizes, shape and their proximity relative to the OAR.

Purpose: A treatment planning study was performed to compare fixed field IMRT, coplanar and noncoplanar VMAT. Methods:Treatment planningCT scans of three lungcancer patients with small (2244cc), medium (2845cc) and large (3531cc) lungs were used to create simulated patient cases. For each simulated patient, three spherical tumors (GTV) of varying diameters: 1, 3, 5 cm were contoured onto lungCT scans. The ITV was set as GTV+2 cm and the PTV was set as ITV+1 cm. The location of the tumor was set in the right middle lobe of lung. For each patient scenario, the IMRT (nine static fields), single coplanar arc VMAT and 5 noncoplanar arcs VMAT treatment plans were generated using Pinnacle treatment planning system. The arcs of noncoplanar VMAT were set at −30° (clockwise and counterclockwise), 0° and 30° (clockwise and counterclockwise). Thetreatment plans were evaluated by calculation of V100%, V80%, V50% and V20% , DVH of dose constrain rings (DCR) at PTV+1cm, +2cm, +3cm, +4cm and Paddick Conformity Index (CI: 0–1 best) and Gradient Index (GI:the smaller the better). Results: The V100% and V80% were similar for all the plans but the V50% and V20% were the largest for IMRT, and the smallest for noncoplanar VMAT. The noncoplanar VMAT had larger V 20 for all the DCRs comparing to the other two plans. The CI of noncoplanar VMAT was slightly larger than the other two plans. The GI was the smallest for noncoplanar VMAT and the largest for IMRT for all the simulation scenarios. Conclusions: The noncoplanar VMAT consistently provides high dose conformity to the target and low dose to healthy tissue. The noncoplanar VMAT has larger low dose volume while smaller high dose volume comparing to IMRT and coplanar VMAT.

Volumetric modulated arc therapy (VMAT) is a time efficient treatment delivery platform capable of producing highly conformal dose distributions with a single 360° arc. However, additional arcs can be used to further improve the conformal dose distribution. For these reasons, VMAT is often used for stereotactic body radiation therapy (SBRT) in which the treatment deliveries are hypofractionated. The dosimetric impact of tumour motion, especially in lung SBRT where tumour motion is most significant and treatments are hypofractionated, has always been a clinical concern. Through the use of 4-dimensional computed tomography (4D-CT), 4D dose distributions can be calculated that account for dosimetric errors due to motion and temporal variation in lung density that are not accounted for in clinical treatment plans. The purpose of this study was to quantify the dosimetric differences that arise due to tumour motion and variations in lung density between single and dual VMAT SBRT treatment plans. Six patients previously treated for stage I/II non-small-cell lung cancer with SBRT were included in this retrospective study. 3D and 4D dose distributions were calculated for both single and dual arc plans for each of the six patients. Dose-volume histogram metrics are reported for the target and critical structures. The results show significant differences (p ≤ 0.05) between the 3D and 4D dose distributions for the ratio of the prescription isodose volume to the primary target volume (PTV). This result was consistent for both single and dual arc VMAT plans.

Purpose:Dosimetric comparison of beam arrangements of coplanar, non‐ coplanar and conformal arc for Stereotactic body radiotherapy of lung lesion. Methods: Four spherical targets with diameters of 2, 4, 6 and 7cm are contoured in the geometric center of right lung of a selected SBRTCT scan. For each target, treatment plans of optimized 7, 9 and 11 coplanar, non‐ coplanar and conformal arc beam arrangements are generated in Pinnacle (Pinnacle Version 9.0, Philips RadiationOncology Systems, WI). Prescription isodoses are chosen to cover at least 95% of target volumes. The volumes enclosed by the prescription and 50% of prescription isodose surfaces are computed. Conformality index (CI) and effective dose fall off gradient are computed for dosimetric comparison. Results: All plans with optimized coplanar and conformal arc beam arrangements have similar CIs while optimized non‐coplanar beam arrangements have the smallest CIs. For each target size, optimized 7 and 9 coplanar beam arrangements have the largest half prescription isodose volumes while optimized 11 coplanar beam arrangements have slightly less half prescription isodose volumes than conformal arc plans. All 3 optimized non‐coplanar beam arrangements Result in faster dose gradient than conformal arcs except for the smallest target cases, for a 2cm diameter target, 9 or 11 non‐coplanar beams are needed to achieve better dose gradient than their rival conformal arc plan. Conclusions:Treatment plans using optimized 9 and 11 non‐coplanar beam arrangements are superior to conformal arc plans for SBRT of lung lesions based on dosimetric comparison. To achieve a superior plan for small lesions, 11 or more optimized coplanar beams may be required.

Purpose: Traditional SBRT employs approximately 10 static beams with up to 20 Gy per fraction, requiring lengthy treatments which can be difficult for patients to tolerate, increasing the risk of movement, and causing discrepancies in the reproducibility of the breathing cycle. Commercial VMAT systems offer shorter treatment times with modulated beams; however, modulation is often not necessary or desired for small fields. Conformai arc therapy offers efficient beam delivery, but with only one aperture shape and constant beam weighting over all gantry angles. This study evaluates the efficiency of a new SBRTdeliveryMethod: a conformai arc with multiple aperture shapes and variable dose rate. Methods: Three clinical SBRT cases were chosen for this study. Each static field was converted into an arc segment to create a StereoArc plan. Gantry angle ranges were determined from the clinical monitor units, with the MU/degree chosen to maximize the dose rate. All segments were merged into a single arc with variable dose rate. Dose distributions from the StereoArc plans were compared to the clinical static field plans using Pinnacle. Delivery times were compared between the static SBRT plans, both with and without Beam Automation, and equivalent StereoArc plans. All plans were delivered on a Varian TrueBeam using a dose rate of 1000 MU/min. Results:Dose differences between StereoArc and static plans were minimal. Delivery times for the static plans were 5–8 minutes, while delivery time with StereoArc was less than 3 minutes for all cases, which was equivalent to delivering the static plans with Beam Automation. Conclusions:Delivery efficiency was improved up to 60%: from 8 minutes for static fields, to less than 3 minutes for StereoArc. StereoArc appears to be both an effective and efficient way of deliveringSBRT for centers not wishing to modulate SBRT and without access to Beam Automation. This study is partially supported by NIH grant 1R01CA133539‐01A2.

Purpose: To compare low dose volume and integral dose of various rotational arc radiation therapy modalities, including helical tomotherapy (HT) and other volumetric modulated arc therapies (VMAT). Methods: A digital cylindrical phantom and an anthropomorphic pelvic phantom were used in the study. HT treatment was planned on TomoTherapy Planning Station (Accuray) and VMAT plans were generated on Pinnacle (Philips) and Eclipse (Varian). Same objectives and criteria were used for all the planning tasks. Tomotherapy plans were made with both 1cm jaw and 2.5cm jaw. Pinnacle SmartArc and Eclipse RapidArc plans used 1 arc and 2 arcs. Plan quality was evaluated by dose‐volume histograms, conformity index (CI), and heterogeneity index (HI). V10% and V20% of body were used in quantifying low dose volume. Integral doses were computed for each plan and compared. Dose profiles across the target center were retrieved to show the rate of dose falls off. Results: All arc TPS generated clinically acceptable plan with comparable target CI and HI. The V20% of body of the cylindrical phantom case were 3551cc, 4054cc, 3375cc, 3438cc, 3714cc and 3679cc for tomotherapy 1 cm and 2 cm jaw, SmartArc 1 arc and 2 arc, and RapidArc 1 arc and 2 arc plans, respectively. The integral doses were 19.67, 22.78, 18.68, 18.50, 19.78 and 19.73 Joule, respectively. For the anthropomorphic phantom, the integral doses were 81.69, 102.77, 84.5 and 82.5 Joule for tomotherapy 1cm, 2cm, SmartArc 1arc and RapidArc 1arc, respectively. Dose profiles showed tomotherapy 1cm plan had similar longitudinal penumbra as SmartArc and RapidArc, while tomotherapy 2.5cm plan produced significant broader penumbra. Conclusions: Similar low dose volume and integral dose are found in SmartArc, Rapid Arc, and as well as helical tomotherapy plan of 1cm jaw. Tomotherapy plans of 2.5cm jaw, however, shows higher low dose volume and integral dose.

Purpose: Gated Volumetric Modulated Arc Therapy (VMAT) is an emerging treatment modality for Stereotactic Body Radiotherapy(SBRT). However, gating significantly prolongs treatment time. In order to enhance treatment efficiency, a novel dual gated VMAT, in which dynamic arc deliveries are executed sequentially in alternating exhale and inhale phases, is proposed and evaluated experimentally. Methods: The essence of dual gated VMAT is to take advantage of the natural pauses that occur at inspiration and exhalation by alternatively delivering the dose at the two phases, instead of the exhale window only. The arc deliveries at the two phases are realized by rotating gantry forward at the exhale window and backward at the inhale in an alternative fashion. Custom XML scripts were developed in Varian's TrueBeam STx Developer Mode to enable dual gated VMAT delivery. RapidArc plans for a lung case were generated for both inhale and exhale phases. The two plans were then combined into a dual gated arc by interleaving the arc treatment nodes of the two RapidArc plans. The dual gated plan was delivered in the development mode of TrueBeam LINAC onto a motion phantom and the delivery was measured by using pinpoint chamber/film/diode array (delta 4). The measured dose distribution was compared with that computed using Eclipse AAA algorithm. The treatment delivery time was recorded and compared with the corresponding single gated plans. Results: Relative to the corresponding single gated delivery, it was found that treatment time efficiency was improved by 95.5% for the case studied here. Pinpoint chamber absolute dose measurement agreed the calculation to within 0.7%. Diode chamber array measurements revealed that 97.5% of measurement points of dual gated RapidArc delivery passed the 3% and 3mm gamma‐test criterion. Conclusions: A dual gated VMAT treatment has been developed and implemented successfully with nearly doubled treatment delivery efficiency.

This study compared the dosimetry between prostate plans using the single-arc and double-arc technique in volumetric modulated arc therapy (VMAT). Single-arc and double-arc prostate VMAT plans were created in five patients with prostate volumes ranging from 39.0-68.1 cm3 . 6 MV photon beams generated by a Varian 21EX linac with 120-leaf multileaf collimator, and prescription dose of 78 Gy with 39 fractions were used. Dose-volume histograms of the target and critical organs, dose criteria such as the planning target volume (PTV) D99%, rectum D30% and rectum D50%, and dose profiles along the anterior-posterior and lateral directions crossing the isocenter were determined using the Eclipse treatment planning system with RapidArc optimization. It is found that the double-arc technique provided a better PTV coverage and rectal sparing compared to the single-arc for all patients. For dose profiles crossing the isocenter, it is seen that the dose in the anterior-posterior direction was shifted to the lateral, when the single-arc technique was replaced by the double-arc. This dosimetric change made the dose in the rectal position shift laterally to the femoral head, resulting in a lower rectal dose. The calculated monitor units (MUs) were 471-600 for the single-arc plans and 587-730 for the double-arc. Although the double-arc plans on average required more MUs of about 20%, and almost twice the treatment time compared to the single-arc, it is worthwhile to be considered particularly in some cases, where the dose criteria are difficult to achieve using the single-arc technique.

Intensity Modulated Arc Therapy (IMAT) is a rotational variant of Intensity Modulated Radiation Therapy (IMRT) that can be implemented with or without angular dose rate optimization. The purpose of this study is to determine the inter-relationship among arc range, Multi-Leaf Collimator (MLC) leaf positions and angular dose rate optimizations for IMAT delivery of a concave target. A concave planning target volume (PTV) with central cylindrical organ at risk (OAR) was used in dissecting the inter-relationships. Plans with full and limited arc range were generated using leaf position optimization (LPO) alone, angular dose rate optimization (ADRO) alone or LPO followed by ADRO. Two initial IMAT arcs were conformal avoidance arcs created with 5° angular increments where MLC leaf positions were determined from the beams eye view to irradiate the PTV but avoid the OAR. The objective function value (evaluating dose to PTV and OAR), a conformity index, dose homogeneity index, mean dose to OAR and normal tissues were computed and used to evaluate the treatment plans. Dose rate variations and MLC leaf movements as a function of gantry angle were examined in this approach. The results demonstrate that LPO followed by ADRO provided the lowest objective function values, best conformity and dose homogeneity indices, and third in mean dose to OAR and normal tissues for the complete arc range. Future work will address different strategies for simultaneously optimizing both the MLC leaf positions and the variable angular dose rate for IMAT delivery and compare single and multiple arc plans.

Purpose: For fast adaptation of step and shoot intensity modulated radiotherapy(IMRT) plans, monitor units (MU)-preserving methods which modify only the segment shapes have been proposed in the literature. In this work, two such adaptation methods are applied to intensity modulated arc therapy (IMAT) and their results are compared to that of a newly optimized IMAT plan. Methods: In a simplified cylindrically symmetric model, the organ at risk (OAR) is surrounded by the planning target volume (PTV). For the initial plan, a steep dose gradient is produced by variants of double arc (IMAT) plans. To simulate situations which require adaptation, the OAR radius and the inner PTV radius have been varied. One adaptation method (Warp) is based on a mesh spanned over structures identified within the beam’s eye view (BEV). Changes to the structure projections warp the mesh. For the adaptation, the segment shapes are fixed to the mesh. The other method (2-Step) uses geometrical 3D information from the computed tomography(CT). For comparison, the objective function representing the dose to the PTV as well as the mean and the maximum dose to the OAR is used. Results: For the narrow segments that compensate the underdosage in the PTV areas proximate to the OAR, the Warp method suggests contrary adaptation rules compared to the 2-Step method. In contrast to Warp, the 2-Step method approximates the behavior of a newly optimized plan and leads to better dose homogeneity in the clinical target volume (CTV) and the PTV, whilst simultaneously sparing the OAR. Conclusions: For minor changes associated with less steep dose gradients, both Warp and 2-Step methods are suitable. However, the 2-Step method should be preferred for more challenging cases, where steep dose gradients between the OAR and the concave PTV are needed. For considerable interfractional reductions of the gap between the OAR and the PTV, where especially steep dose gradients have to be generated, MU-preserving adaptation techniques are not adequate. In this case, narrower segments in the initial plan can be used to facilitate the adaptation. Otherwise, non-MU-preserving adaptation methods have to be applied. Further work is needed to include clinical cases with more complex geometries and expand the methods to IMRT techniques.

Purpose: To evaluate a commercial volumetric modulated arc therapy (VMAT), using multiple noncoplanar arcs, for linac-based cranial radiosurgery, as well as evaluate the combined accuracy of the VMAT dose calculations and delivery. Methods: Twelve patients with cranial lesions of variable size (0.1-29 cc) and two multiple metastases patients were planned (Eclipse RapidArc AAA algorithm, v8.6.15) using VMAT (1-6 noncoplanar arcs), dynamic conformal arc (DCA, {approx}4 arcs), and IMRT (nine static fields). All plans were evaluated according to a conformity index (CI), healthy brain tissue doses and volumes, and the dose to organs at risk. A 2D dose distribution was measured (Varian Novalis Tx, HD120 MLC, 1000 MU/min, 6 MV beam) for the {approx}4 arc VMAT treatment plans using calibrated film dosimetry. Results: The CI (0-1 best) average for all plans was best for {approx}4 noncoplanar arc VMAT at 0.86 compared with {approx}0.78 for IMRT and a single arc VMAT and 0.68 for DCA. The volumes of healthy brain receiving 50% of the prescribed target coverage dose or more (V{sub 50%}) were lowest for the four arc VMAT [RA(4)] and DCA plans. The average ratio of the V{sub 50%} for the other plans to the RA(4) V{sub 50%} were 1.9 formore » a single noncoplanar arc VMAT [RA(1nc)], 1.4 for single full coplanar arc VMAT [RA(1f)] and 1.3 for IMRT. The V{sub 50%} improved significantly for single isocenter multiple metastases plan when two noncoplanar VMAT arcs were added to a full single coplanar one. The maximum dose to 5 cc of the outer 1 cm rim of healthy brain which one may want to keep below nonconsequential doses of 300-400 cGy, was 2-3 times greater for IMRT, RA(1nc) and RA(1f) plans compared with the multiple noncoplanar arc DCA and RA(4) techniques. Organs at risk near (0-4 mm) to targets were best spared by (i) single noncoplanar arcs when the targets are lateral to the organ at risk and (ii) by skewed nonvertical planes of IMRT fields when the targets are not lateral to the organ at risk. The highest dose gradient observed between an organ at risk and a target at the edge of a VMAT arc plane or plane of IMRT fields was 17%/mm. The average absolute percent difference between the measured and calculated central axis dose for all the VMAT plans was 3.6 {+-} 2.2%. The measured perpendicular profile widths and shifts were on average within 0.5 mm of planned values. The average total MUs for VMAT plans was double the DCA average and similar to the IMRT average. Conclusions: For the aforementioned planning and delivery system and cranial lesions greater than 7 mm in diameter, multiple noncoplanar arc VMAT consistently provides accurate and high quality cranial radiosurgery dose distributions with low doses to healthy brain tissue and high dose conformity to the target. These qualities may make multiple noncoplanar arc VMAT suitable for a greater range of prescription doses or larger and more irregular lesions. For smaller and/or rounder lesions there are other clinically acceptable treatment techniques that may involve fewer couch angles or arcs and reduce treatment times.« less

Purpose: The RapidArc volumetric modulated arc therapy (VMAT) planning process is based on a core engine, the so-called progressive resolution optimizer (PRO). This is the optimization algorithm used to determine the combination of field shapes, segment weights (with dose rate and gantry speed variations), which best approximate the desired dose distribution in the inverse planning problem. A study was performed to assess the behavior of two versions of PRO. These two versions mostly differ in the way continuous variables describing the modulated arc are sampled into discrete control points, in the planning efficiency and in the presence of some new features. The analysis aimed to assess (i) plan quality, (ii) technical delivery aspects, (iii) agreement between delivery and calculations, and (iv) planning efficiency of the two versions. Methods: RapidArc plans were generated for four groups of patients (five patients each): anal canal, advanced lung, head and neck, and multiple brain metastases and were designed to test different levels of planning complexity and anatomical features. Plans from optimization with PRO2 (first generation of RapidArc optimizer) were compared against PRO3 (second generation of the algorithm). Additional plans were optimized with PRO3 using new features: the jaw tracking, the intermediate dose and the air cavity correction options. Results: Results showed that (i) plan quality was generally improved with PRO3 and, although not for all parameters, some of the scored indices showed a macroscopic improvement with PRO3. (ii) PRO3 optimization leads to simpler patterns of the dynamic parameters particularly for dose rate. (iii) No differences were observed between the two algorithms in terms of pretreatment quality assurance measurements and (iv) PRO3 optimization was generally faster, with a time reduction of a factor approximately 3.5 with respect to PRO2. Conclusions: These results indicate that PRO3 is either clinically beneficial or neutral in terms of dosimetric quality while it showed significant advantages in speed and technical aspects.

Purpose: The cylindrical symmetry of vertebrae favors the use of volumetric modulated arc therapy in generating a dose ''hole'' on the center of the vertebrae limiting the dose to the spinal cord. The authors have evaluated if collimator angle is a significant parameter for dose distribution optimization in vertebral metastases. Methods: Three patients with one-three vertebrae involved were considered. Twenty-one differently optimized plans (nine single-arc and 12 double-arc plans) were performed, testing various collimator angle positions. Clinical target volume was defined as the whole vertebrae, excluding the spinal cord canal. The planning target volume (PTV) was defined as CTV+5 mm. Dose prescription was 5x4 Gy{sup 2} with normalization to PTV mean dose. The dose at 1 cm{sup 3} of spinal cord was limited to 11.5Gy. Results: The best plans in terms of target coverage and spinal cord sparing were achieved by two arcs and Arc1-80 deg. and Arc2-280 deg. collimator angles for all the cases considered (i.e., leaf travel parallel to the spinal cord primary orientation). If one arc is used, only 80 deg. reached the objectives. Conclusions: This study demonstrated the role of collimation rotation for the vertebrae metastasis irradiation, with the leaf travel parallel to the spinal cordmore » primary orientation to be better than other solutions. Thus, optimal choice of collimator angle increases the optimization freedom to shape a desired dose distribution.« less

An efficient method for volumetric intensity modulated arc therapy (VMAT) planning was developed, where a single arc (360 degrees or less) is delivered under continuous variation of multileaf collimator (MLC) segments, dose rate, and gantry speed. Plans can be generated for any current linear accelerator that supports these degrees of freedom. MLC segments are derived from fluence maps at relatively coarsely sampled angular positions. The beam segments, dose rate, and gantry speed are then optimized using direct machine parameter optimization based on dose volume objectives and leaf motion constraints to minimize arc delivery time. The method can vary both dose rate and gantry speed or alternatively determine the optimal plan at constant dose rate and gantry speed. The method was used to retrospectively generate variable dose rate VMAT plans to ten patients (head and neck, prostate, brain, lung, and tonsil). In comparison to step-and-shoot intensity modulated radiation therapy, dosimetric plan quality was comparable or improved, estimated delivery times ranged from 70 to 160 s, and monitor units were consistently reduced in nine out of the ten cases by an average of approximately 6%. Optimization and final dose calculation took between 5 and 35 min depending on plan complexity.

Intensity modulated arc therapy (IMAT) is an intensity modulated radiation therapy delivery technique originally proposed as an alternative to tomotherapy. IMAT uses a series of overlapping arcs to deliver optimized intensity patterns from each beam direction. The full potential of IMAT has gone largely unrealized due in part to a lack of robust and commercially available inverse planning tools. To address this, we have implemented an IMAT arc-sequencing algorithm that translates optimized intensity maps into deliverable IMAT plans. The sequencing algorithm uses simulated annealing to simultaneously optimize the aperture shapes and weights throughout each arc. The sequencer enforces the delivery constraints while minimizing the discrepancies between the optimized and sequenced intensity maps. The performance of the algorithm has been tested for ten patient cases (3 prostate, 3 brain, 2 head-and-neck, 1 lung, and 1 pancreas). Seven coplanar IMAT plans were created using an average of 4.6 arcs and 685 monitor units. Additionally, three noncoplanar plans were created using an average of 16 arcs and 498 monitor units. The results demonstrate that the arc sequencer can provide efficient and highly conformal IMAT plans. An average sequencing time of approximately 20 min was observed.

This project investigated the potential of summing fixed-beam dose distributions calculated using the pencil-beam redefinition algorithm (PBRA) at small angular steps ( 1 ° ) to model an electron arc therapy beam. The PRBA, previously modified to model skincollimation, was modified further by incorporating two correction factors. One correction factor that is energy, SSD (source-to-surface distance), and field-width dependent constrained the calculated dose output to be the same as the measureddose output for fixed-beam geometries within the range of field widths and SSDs encountered in arc therapy. Another correction factor (single field-width correction factor for each energy) compensated for large-angle scattering not being modeled, allowing a more accurate calculation of dose output at mid arc. The PBRA was commissioned to accurately calculate dose in a water phantom for fixed-beam geometries typical of electron arc therapy. Calculated central-axis depth doses agreed with measureddoses to within 2% in the low-dose gradient regions and within 1- mm in the high-dose gradient regions. Off-axis doses agreed to within 2 mm in the high-dose gradient regions and within 3% in the low-dose gradient regions. Arced-beam calculations of dose output and depth dose at mid arc were evaluated by comparing to data measured using two cylindrical water phantoms with radii of 12 and 15 cm at 10 and 15 MeV . Dose output was measured for all combinations of phantom radii of curvature, collimator widths (4, 5, and 6 cm ), and arc angles ( 0 ° , 20 ° , 40 ° , 60 ° , 80 ° , and 90 ° ) for both beam energies. Results showed the calculated mid-arc dose output to agree within 2% of measurement for all combinations. For a 90 ° arc angle and 5 × 20 cm 2 field size, the calculated mid-arc depth dose in the low-dose gradient region agreed to within 2% of measurement for all depths at 10 MeV and for depths greater than depth of dose maximum R 100 at 15 MeV . For depths in the buildup region at 15 MeV the calculations overestimated the measureddose by as much as 3.4%. Mid-arc depth dose in the high-dose gradient region agreed to within 2.2 mm of measureddose. Calculated two-dimensional relative dose distributions in the plane of rotation were compared to dosemeasurements using film in a cylindrical polystyrene phantom for a 90 ° arc angle and field widths of 4, 5, and 6 cm at 10 and 15 MeV . Results showed that off-axis dose at the ends of arc (without skincollimation) agreed to within 2% in the low-dose gradient region and to within 1.2 mm in the high-dose gradient region. This work showed that the accuracy of the PBRA arced-beam dose model met the criteria specified by Van Dyk et al. [Int. J. Radiat. Oncol. Biol. Phys.26, 261–273 (1993)] with the exception of the buildup region of the 15 MeV beam. Based on the present results, results of a previous study showing acceptable accuracy in the presence of skincollimation, and results of a previous study showing acceptable accuracy in the presence of internal heterogeneities, it is concluded that the PBRA arced-beam dose model should be adequate for planning electron arc therapy.

The intensity modulated arc has been proposed as an alternative to tomotherapy. Treatment planing systems more typically model the conventional step and shoot or sliding window dynamic multileaf collimator (DMLC) deliveries, and may not support intensity modulated arc therapy (IMAT). As well, another potential drawback to this technique is that increasing the number of intensity levels required to achieve certain dose distributions necessitates increasing the number of gantry passes, as may occur if the desired dose distribution is complex (e.g., concave or bifurcated), potentially increasing the overall treatment time. A technique is presented here for the delivery of tomotherapy like dose distributions in a single gantry pass by the use of a large number of fields modulated by a sliding window DMLC technique from fixed equally spaced gantry positions. This serves as a good approximation to either IMAT or tomotherapy deliveries. The planning of these fields is achieved using iterative filtered back projection. Measured results of deliveries of varying degrees of complexity on a homogeneous phantom are compared to desired distributions.

Based on the study of treatment arc positioning versus target length, a method that allowed periodic shift of arc abutment regions through the course of intensity modulated radiotherapy (IMRT) was developed. In this method, two treatment plans were developed for the same tumor. The first plan contained the original target (Planning Target Volume as defined by radiation oncologist) and the second one contained a modified target. The modification of the original target consisted of simply increasing its length, adding a small extension to it, or creating a distant pseudo target. These modifications cause arc abutment regions in the second plan to be shifted relative to their positions in the first plan. Different methods of target modification were investigated because in some cases (for instance, when a critical structure might overlap with the target extension) a simple extension of the target would cause an unacceptable irradiation of the sensitive structures. The dose prescribed to the modified portion of the target varied from 10% to 100% of the original target dose. It was found that a clinically significant shift (> or =5 mm) in abutment region locations occurred when the dose prescribed to the extended portion of the target was > or =95% of the original target dose. On the other hand, the pseudo target required only approximately 10% to 20% of the original target dose to produce the same shift in arc positions. Results of the film dosimetry showed that when a single plan was used for the treatment delivery, the dose nonuniformity was 17% and 25% of the prescribed dose with 0.5 and 1 mm errors in couch indexing, respectively. The dose nonuniformity was reduced by at least half when two plans were used for IMRT delivery.

Purpose: Integration of coordinated robotic table motion with inversely-planned arc delivery has the potential to resolve table-top delivery limitations of large-field treatments such as Total Body Irradiation (TBI), Total Lymphoid Irradiation (TLI), and Cranial-Spinal Irradiation (CSI). We formulate the foundation for Trajectory Modulated Arc Therapy (TMAT), and using Varian Developer Mode capabilities, experimentally investigate its practical implementation for such techniques. Methods: A MATLAB algorithm was developed for inverse planning optimization of the table motion, MLC positions, and gantry motion under extended-SSD geometry. To maximize the effective field size, delivery trajectories for TMAT TBI were formed with the table rotated at 270° IEC and dropped vertically to 152.5cm SSD. Preliminary testing of algorithm parameters was done through retrospective planning analysis. Robotic delivery was programmed using custom XML scripting on the TrueBeam Developer Mode platform. Final dose was calculated using the Eclipse AAA algorithm. Initial verification of delivery accuracy was measured using OSLDs on a solid water phantom of varying thickness. Results: A comparison of DVH curves demonstrated that dynamic couch motion irradiation was sufficiently approximated by static control points spaced in intervals of less than 2cm. Optimized MLC motion decreased the average lung dose to 68.5% of the prescription dose. The programmed irradiation integrating coordinated table motion was deliverable on a TrueBeam STx linac in 6.7 min. With the couch translating under an open 10cmx20cm field angled at 10°, OSLD measurements along the midline of a solid water phantom at depths of 3, 5, and 9cm were within 3% of the TPS AAA algorithm with an average deviation of 1.2%. Conclusion: A treatment planning and delivery system for Trajectory Modulated Arc Therapy of extended volumes has been established and experimentally demonstrated for TBI. Extension to other treatment techniques such as TLI and CSI is readily achievable through the developed platform. Grant Funding by Varian Medical Systems.

Purpose: Overall low dose to lung for patients with locally advanced lung cancer with large PTVs is a concern, prompting investigation into the preferred arc geometry for lung VMAT. Methods: 8 subjects (6 males, 64±7 years, stage III/IV=5/3) were enrolled with a mean PTV size of 595±303cc. A standard dose scheme of 60Gy in 30 fractions was used for all subjects. For each subject, four VMAT plans with different combinations of the number of arcs (1 arc vs. two arcs) and the arc range (full vs. partial arc) were generated in Eclipse. Dosimetric characteristics for each plan were evaluated by PTV coverage and OAR sparing. PTV coverage was quantified by mean, max dose, homogeneity index HI5/95 defined as (D5%–D95%)/Mean_dose and conformity index (CI). OAR sparing was measured by mean, V10 and V20 for lung minus GTV, mean, max, V30 and V40 for heart and max dose for cord. For each metric abovementioned, a two-way repeated ANOVA analysis was performed to evaluate whether the number of arcs and the arc range will significantly affect dose distribution of VMAT plans. Results: In general partial arc geometry provided better PTV coverage with significantly smaller HI5/95 (p

The characteristic angle-beta concept provides a simple semiempirical method for determination of dose distributions in electron arc therapy. Initially, the method required a set of measured radial depth dose distributions for each electron beam energy used for arc therapy. In this paper, we report an extension of the angle-beta concept that enables the determination of arc therapy depth doses for an arbitrary electron energy from the known set of depth dose data at a reference energy. Depth dose distributions of stationary and arc electron beams have been studied in the energy range from 9 to 18 MeV. The stationary electron beams used for electron arc therapy were collimated by photon collimators only, no secondary collimation was used in our experiments. For stationary electron beams and for arc electron beams with a given characteristic angle beta, the depths of dose maximum as well as the depths of a given percentage depth dose beyond the depth of dose maximum are linearly proportional to the mean incident electron energy. This simple geometrical and dosimetric relationship increases the potential usefulness of the angle-beta concept in clinical electron arc therapy.

A comparison was made of different treatment plans to determine the effect on the three-dimensional dose distributions of varying the allowed parameters in linac-based stereotactic radiosurgery with circular collimators; these parameters are arc position, length, and weighting, and collimator diameter. For the class of eccentrically shaped target volumes that are not so irregular as to require several separate isocenters, it was found that superior dose distributions could be achieved by varying arc length, arc position, arc weighting, and collimator diameter. An analysis of the results achieved with an automated planning program indicates that, in general, the variables of arc position and arc length are of greater importance than collimator size or beam weighting. However, there are cases where varying these latter two parameters does result in markedly better dose distributions. A deeper investigation into the effects of multiple collimators on the dose distribution in the area of steepest gradient demonstrated that multiple collimator sizes do not significantly degrade the dose falloff, which is in fact mostly determined by the effects of intersecting arcs.

Purpose: Radiosurgery of multiple (n>4) brain metastasis lesions requires 3–4 noncoplanar VMAT arcs with excessively high monitor units and long delivery time. We investigated whether an improved optimization technique would decrease the needed arc numbers and increase the delivery efficiency, while improving or maintaining the plan quality. Methods: The proposed 4pi arc space optimization algorithm consists of two steps: automatic couch angle selection followed by aperture generation for each arc with optimized control points distribution. We use a greedy algorithm to select the couch angles. Starting from a single coplanar arc plan we search through the candidate noncoplanar arcs to pick a single noncoplanar arc that will bring the best plan quality when added into the existing treatment plan. Each time, only one additional noncoplanar arc is considered making the calculation time tractable. This process repeats itself until desired number of arc is reached. The technique is first evaluated in coplanar arc delivery scheme with testing cases and then applied to noncoplanar treatments of a case with 12 brain metastasis lesions. Results: Clinically acceptable plans are created within minutes. For the coplanar testing cases the algorithm yields singlearc plans with better dose distributions than that of two-arc VMAT, simultaneously with a 12–17% reduction in the delivery time and a 14–21% reduction in MUs. For the treatment of 12 brain mets while Paddick conformity indexes of the two plans were comparable the SCG-optimization with 2 arcs (1 noncoplanar and 1 coplanar) significantly improved the conventional VMAT with 3 arcs (2 noncoplanar and 1 coplanar). Specifically V16 V10 and V5 of the brain were reduced by 11%, 11% and 12% respectively. The beam delivery time was shortened by approximately 30%. Conclusion: The proposed 4pi arc space optimization technique promises to significantly reduce the brain toxicity while greatly improving the treatment efficiency.

Purpose: To identify the weakness of dose calculation algorithm in a treatment planning system for volumetric modulated arc therapy (VMAT) and sliding window (SW) techniques using a two-dimensional diode array. Methods: The VMAT quality assurance(QA) was implemented with a diode array using multiple partial arcs that divided from a VMAT plan; each partial arc has the same segments and the original monitor units. Arc angles were less than ± 30°. Multiple arcs delivered through consecutive and repetitive gantry operating clockwise and counterclockwise. The source-toaxis distance setup with the effective depths of 10 and 20 cm were used for a diode array. To figure out dose errors caused in delivery of VMAT fields, the numerous fields having the same segments with the VMAT field irradiated using different delivery techniques of static and step-and-shoot. The dose distributions of the SW technique were evaluated by creating split fields having fine moving steps of multi-leaf collimator leaves. Calculated doses using the adaptive convolution algorithm were analyzed with measured ones with distance-to-agreement and dose difference of 3 mm and 3%.. Results: While the beam delivery through static and step-and-shoot techniques showed the passing rate of 97 ± 2%, partial arc delivery of the VMAT fields brought out passing rate of 85%. However, when leaf motion was restricted less than 4.6 mm/°, passing rate was improved up to 95 ± 2%. Similar passing rate were obtained for both 10 and 20 cm effective depth setup. The calculated doses using the SW technique showed the dose difference over 7% at the final arrival point of moving leaves. Conclusion: Error components in dynamic delivery of modulated beams were distinguished by using the suggested QA method. This partial arc method can be used for routine VMAT QA. Improved SW calculation algorithm is required to provide accurate estimated doses.

Purpose: To compare the dosimetry of coplanar and non-coplanar RapidArc whole brain plans for hippocampal sparing Methods: We studied the RapidArc plans of patient with brain metastases, with the prescription of 3750 cGy in 15 fractions. The coplanar approach used a full clockwise (CW) arc and a full counterclockwise (CCW) arc, with the couch angle to be 0°. The non-coplanar approach used a full arc with a couch angle of 0°, and a partial arc with a couch angle of 90°. Treatment planning system is Eclipse Ver. 11. Constraints for eyes, lens, brainstem, optical nerves and chiasm are employed in the optimization so that these OARs’ dose are below tolerance. Constraints for hippocampus are employed so that they receive dose as low as possible while maintain good coverage to whole brain. The beam delivery machine is Varian 21 IX. T1-weighted MRI images were used for hippocampus contouring. Results: The target coverage index for coplanar and non-coplanar RapidArc plans are 94.9% and 95.4%, respectively, with homogeneity index of 0.223 vs 0.226, which is defined as (D2% – D98%)/Dmean of target volume. V95 and V100 are 99.0% and 94.8% for coplanar plan, vs 99.1% and 95.4% for non-coplanar plan, while the mean dose of hippocampus are 1244.5 cGy for coplanar plan vs 1212.3 cGy for non-coplanar plan. Dose for eyes, lens, optical nerves, optical chiasm and brainstem are all below tolerance. Conclusion: Coplanar RapidArc plan provides good target coverage while achieves good hippocampal sparing, and there is no benefit to use non-coplanar approach.

Purpose: Electron arc therapy provides excellent dose distributions for treating superficial tumors along curved surfaces. However this modality has not received widespread application due to the lack of needed advancement in electron beam delivery, accurate electron dose calculation and treatment plan optimization. The aim of the current work is to investigate possible parameters that can be optimized for electron arc (eARC) therapy. Methods: The MCBEAM code was used to generate phase space files for 6 and 12MeV electron beam energies from a Varian trilogy machine. An Electron Multi-leaf collimator eMLC of 2cm thickness positioned at 82 cm source collimated distance was used in the study. Dose distributions for electron arcs were calculated inside a cylindrical phantom using the MCSIM code. The Cylindrical phantom was constructed with 0.2cm voxels and a 15cm diameter. Electron arcs were delivered with two different approaches. The first approach was to deliver the arc as segments of very small field widths. In this approach we also tested the impact of the segment size and the arc increment angle. The second approach is to deliver the arc as a sum of large fields each covering the whole target as seen from the beam eye view. Results: In considering 90 % as the prescription isodose line, the first approach showed a region of buildup proceeding before the prescription zone. This build up is minimizing with the second approach neglecting need for bolus. The second approach also showed less x-ray contamination. In both approaches the variation of the segment size changed the size and location of the prescription isodose line. The optimization process for eARC could involve interplay between small and large segments to achieve desired coverage. Conclusion: An advanced modulation of eARCs will allow for tailored dose distribution for superficial curved target as with challenging scalp cases

Purpose: To choose appropriate gantry starting angle for partial left breast irradiation using volumetric modulated arc therapy (VMAT). Methods: A random patient of left breast carcinoma was selected for this study. The slice which was selected for this mathematical formulation was having maximum breast thickness and maximum medial and lateral tangential distance. After this appropriate isocenter was chosen on that CT slice. The distances between various points were measured by the measuring tool in Monaco 5.00.04. Using the various trigonometric equations, a final equation was derived which shows the relationship between Gantry start angle, isocenter Location and tissue thickness. Results: The final equation for gantry start for right medial tangential arc is given asStarting angle = 270°+tan^(−1)(sin(θ)/(x_1/x_2 +cosθ))The above equation was tested for 10 cases and it was found to be appropriate for all the cases. Conclusion: Gantry starting angle for partial arc irradiation depends upon Breast thickness, Distance between Medial and lateral tangent and isocenter location.

Purpose: To develop a clinical workflow for delivering a modulated-arc total body irradiation (TBI) with RayStation scripting. This technique uses arc fields with the patient lying at floor level on a padded table and is validated through measurements taken on a custom-made TBI phantom. Methods: Treatment planning was performed for a retrospective cohort of eight patients with a diverse range of heights and body types. Each was replanned using an open-field dual arc method, with the patient in supine and prone positions on the floor of the vault. All plans were optimized using Raystation Planning 4.7.2.5 (RaySearch Laboratories, Stockholm, Sweden), with 200 cGy prescribed to the 95% of the body contour − 5mm. This results in an open-field beam that sweeps craniocaudally across the length of the patient. The technique is validated with measurements at 10 cm intervals in a custom-milled, 5 cm thick acrylic phantom. A centrally located CC13 ion chamber and a Mobile MOSFET (Best Medical Canada, Ottawa, ON) detector array were used to measure dose. Supine and prone arcs for each patient were consecutively delivered, and the aggregate dose at each point was compared to the planned dose calculated in the phantom. Results: The ion chamber measurements differed from the planned dose by an average of .5%, with a standard deviation of 2.1%. All measured data for the MOSFETS were within 10% of the corresponding planned dose except for two outlying points. The standard deviation of dose differences across the entire cohort was 4.0%. Most significant discrepancies occurred either in inhomogeneous regions with large gradients, or at inferior points where beam angle was steepest. Conclusion: We have confirmed that the planned dose is well matched to our measurements within 10% for this method of planning and delivery. We are currently incorporating this technique into our clinical workflow. This work is supported by RaySearch.

Purpose: Volumetric modulated arc therapy (VMAT) has revolutionized radiation treatment by decreasing treatment time and monitor units, thus reducing scattered and whole body radiation dose. As the collimator angle changes the apparent leaf gap becomes larger which can impact plan quality, organ at risk (OAR) sparing as well as IMRT QA passing rate which is investigated. Methods: Two sites (prostate and head and neck) that have maximum utilization of VMAT were investigated. Two previously treated VMAT patients were chosen. For each patient 10 plans were created by maintaining constant optimization constraints while varying collimator angles from 0-90 deg at an interval of 10 degrees for the first arc and the appropriate complimentary angle for the second arc. Plans were created with AAA algorithm using 6 MV beam on a Varian IX machine with Millennium 120 MLC. The dose-volume histogram (DVH) for each plan was exported and dosimetric parameters (D98, D95, D50, D2) as well homogeneity index (HI) and conformity index (CI) were computed. Each plan was validated for QA using ArcCheck with gamma index passing criteria of 2%/2 mm and 3%/3 mm. Additionally, normal tissue complication probability (NTCP) for each OAR was computed using Uzan-Nahum software. Results: The CI values for both sites had no impact as target volume coverage in every collimator angle were the same since it was optimized for adequate coverage. The HI which is representative of DVH gradient or dose uniformity in PTV showed a clear trend in both sites. The NTCP for OAR (brain and cochlea) in H&N plan and (bladder and rectum) in prostate plan showed a distinct superiority for collimator angles between 15-30 deg. The gamma passing rates were not correlated with angle. Conclusion: Based on CI, HI, NTCP and gamma passing index, it can be concluded that collimator angles should be maintained within 15–30 deg.

Background: Commercial treatment planning system Pinnacle3 (Philips, Fitchburg, WI, USA) employs a convolution-superposition algorithm for volumetric-modulated arc radiotherapy (VMAT) optimization and dose calculation. Study of Monte Carlo (MC) dose recalculation of VMAT plans for advanced-stage nasopharyngeal cancers (NPC) is currently limited. Methods: Twenty-nine VMAT prescribed 70Gy, 60Gy, and 54Gy to the planning target volumes (PTVs) were included. These clinical plans achieved with a CS dose engine on Pinnacle3 v9.0 were recalculated by the Monaco TPS v5.0 (Elekta, Maryland Heights, MO, USA) with a XVMC-based MC dose engine. The MC virtual source model was built using the same measurement beam dataset as for the Pinnacle beam model. All MC recalculation were based on absorbed dose to medium in medium (Dm,m). Differences in dose constraint parameters per our institution protocol (Supplementary Table 1) were analyzed. Results: Only differences in maximum dose to left brachial plexus, left temporal lobe and PTV54Gy were found to be statistically insignificant (p> 0.05). Dosimetric differences of other tumor targets and normal organs are found in supplementary Table 1. Generally, doses outside the PTV in the normal organs are lower with MC than with CS. This is also true in the PTV54-70Gy doses but higher dose in the nasal cavity near the bone interfaces is consistently predicted by MC, possibly due to the increased backscattering of short-range scattered photons and the secondary electrons that is not properly modeled by the CS. The straight shoulders of the PTV dose volume histograms (DVH) initially resulted from the CS optimization are merely preserved after MC recalculation. Conclusion: Significant dosimetric differences in VMAT NPC plans were observed between CS and MC calculations. Adjustments of the planning dose constraints to incorporate the physics differences from conventional CS algorithm should be made when VMAT optimization is carried out directly with MC dose engine.

Purpose: To test a novel total body irradiation (TBI) system using conformal partial arc with patient lying on the stationary couch which is biologically equivalent to a moving couch TBI. This improves the scanning field TBI, which is previously presented. Methods: The Uniform MU Modulated arc Segments TBI or UMMS-TBI scans the treatment plane with a constant machine dose rate and a constant gantry rotation speed. A dynamic MLC pattern which moves while gantry rotates has been designed so that the treatment field moves same distance at the treatment plane per each gantry angle, while maintaining same treatment field size (34cm) at the plane. Dose across the plane varies due to the geometric differences including the distance from the source to a point of interest and the different attenuation from the slanted depth which changes the effective depth. Beam intensity is modulated to correct the dose variation across the plane by assigning the number of gantry angles inversely proportional to the uncorrected dose. Results: Measured dose and calculated dose matched within 1 % for central axis and 3% for off axis for various patient scenarios. Dose from different distance does not follow the inverse square relation as it is predicted from calculation. Dose uniformity better than 5% across 180 cm at 10cm depth is achieved by moving the gantry from −55 to +55 deg. Total treatment time for 2 Gy AP/PA fields is 40–50 minutes excluding patient set up time, at the machine dose rate of 200 MU/min. Conclusion: This novel technique, yet accurate but easy to implement enables TBI treatment in a small treatment room with less program development preparation than other techniques. The VMAT function of treatment delivery is not required to modulate beams. One delivery pattern can be used for different patients by changing the monitor units.

Purpose: To compare the plan quality between Eclipse (Varian) and Monaco (Elekta) TPS. To ascertain, if SBRT lung treatment could be delivered in a single coplanar arc (360 degrees) with both Elekta and Varian platforms. To assess if the smaller leaf width in Varian Millennium and Elekta Agility MLC heads have a dosimetric advantage over Elekta MLCi2 head Methods: Ten SBRT lung patients (PTV volumes ranging from 11 cc to 103cc) who were previously treated on Varian Linac with non-coplanar arcs and received 50Gy in 5 fractions were chosen for this study. The patients were replanned in Eclipse TPS (AAA algorithm) using a 360 degree coplanar single arc (SA) delivery technique and 2 partial complimentary 180 degree arcs (PA). Treatment planning using single coplanar arc (360 degree arc) was also done on Monaco TPS (Montecarlo) for both Agility (160 leaf) and MLCi2 (80 leaf) Elekta MLC heads Results: The average monitor units to deliver 10 Gy across all delivery methods were 3000 ± 474 MU and did not vary with PTV size. Coplanar single arc and partial arc techniques did not compromise either the RTOG 0813 or 0915 low dose spillage criteria for R50% or the maximum dose to any point 2cm away from the PTV. OAR doses to spinal cord, heart, great vessels, esophagus, rib and lung were comparable on both Eclipse (Varian) and Monaco (Elekta) platforms regardless of the delivery method. Conclusion: SBRT lung tumors can be treated with a single coplanar 360 degree arc in both Varian and Elekta platforms. Non coplanar arcs and increasing arc degrees more than 360 degrees had no benefit in this study regardless of the volume of PTV. 0.5 cm leaf width used in Millennium and Agility MLC heads had no significant dosimetric improvement over 1 cm leaves in the MLCi2 head.

Purpose: Spine metastases have traditionally been treated using a parallel opposed 3-D technique: an anterior field and a posterior field (AP/PA). This has proven to give the lesions adequate coverage. The downside to this type of treatment is that the full prescription dose covers a large area of the patient and can go through critical organs such as the heart, spine, lungs, bowel, and others depending on the vertebrae being treated. Conformal arc therapy can also be used for this type of treatment. These two techniques will be compared for target coverage and doses to organs at risk (OARs). Methods: 20 multi-vertebra metastases patients were chosen for this study, with a range of vertebrae being treated amongst each case. The original AP/PA treatments were re-planned with 1–3 conformal arcs placed depending on the location of nearby OARs. The GTV coverage was normalized so that both techniques had equal coverage. The OAR doses and the volume of the 50% prescription dose value were then compared between the two planning techniques. Results: Initial results on average show lower doses for the heart maximum, heart mean, lung V20, spinal cord maximum, and bowel maximum for the conformal arc plans. The AP/PA plans however on average showed lower kidney mean dose, liver mean dose, and parotid mean dose. The 50% prescription dose volume was smaller in all the conformal arc plans except for one. Conclusion: From the preliminary results the conformal arc plans spare critical OARs better than the two field technique, except when treating the vertebrae near the kidneys, liver, and parotids. The conformal arc plans also have a lower volume of the 50% prescription dose value. The data suggests that conformal arcs can be used for certain palliative spine treatments and that this technique should be considered when beginning planning.

Purpose: The current clinical Volumetric Modulated Arc Therapy (VMAT) optimization is formulated as a non-convex problem and various greedy heuristics have been employed for an empirical solution, jeopardizing plan consistency and quality. We introduce a novel global direct aperture optimization method for VMAT to overcome these limitations. Methods: The global VMAT (gVMAT) planning was formulated as an optimization problem with an L2-norm fidelity term and an anisotropic total variation term. A level set function was used to describe the aperture shapes and adjacent aperture shapes were penalized to control MLC motion range. An alternating optimization strategy was implemented to solve the fluence intensity and aperture shapes simultaneously. Single arc gVMAT plans, utilizing 180 beams with 2° angular resolution, were generated for a glioblastoma multiforme (GBM), lung (LNG), and 2 head and neck cases—one with 3 PTVs (H&N3PTV) and one with 4 PTVs (H&N4PTV). The plans were compared against the clinical VMAT (cVMAT) plans utilizing two overlapping coplanar arcs. Results: The optimization of the gVMAT plans had converged within 600 iterations. gVMAT reduced the average max and mean OAR dose by 6.59% and 7.45% of the prescription dose. Reductions in max dose and mean dose were as high as 14.5 Gy in the LNG case and 15.3 Gy in the H&N3PTV case. PTV coverages (D95, D98, D99) were within 0.25% of the prescription dose. By globally considering all beams, the gVMAT optimizer allowed some beams to deliver higher intensities, yielding a dose distribution that resembles a static beam IMRT plan with beam orientation optimization. Conclusions: The novel VMAT approach allows for the search of an optimal plan in the global solution space and generates deliverable apertures directly. The single arc VMAT approach fully utilizes the digital linacs’ capability in dose rate and gantry rotation speed modulation. Varian Medical Systems, NIH grant R01CA188300, NIH grant R43CA183390.

Purpose: The purpose of this study is to determine the optimal treatment planning strategy among the different arc arrangements for prostate stereotactic body radiotherapy (SBRT) plans with volumetric modulated arc therapy (VMAT). Methods: Ten patients with prostate cancer were selected. The SBRT-VMAT plans for each patient were generated with single-full (181° to 179°; 1FA), single-partial (240° to 120°; 1PA), double-full (181° to 179° and 179° to 181°; 2FA), and double-partial (240° to 120° and 120° to 240°; 2PA) arc arrangements. The prescription dose was 42.7 Gy in 7 fractions. Dose distribution was calculated using a 10-MV flattening-filter-free beam and the Acuros XB algorithm. Dosimetric parameters of target volume and organs at risk (OARs) were evaluated from cumulative dose-volume histograms on prostate SBRT-VMAT plans between single-arc (1FA and 1PA) and double-arc (2FA and 2PA) arrangements. Results: All plans using four arc arrangements were highly conformal with conformity index (CI)

Purpose: Some modern linear accelerators are equipped with one low energy flat beam and two flattening filter free (FFF) beams at high and low energies. The purpose of this study is to investigate whether the high energy FFF beam can produce the same plan quality as the conventional low energy flat beam, using a volumetric modulated arc (VMAT) technique for prostate patients. Methods: Ten prostate cancer patients were selected with a prescription of 78Gy. For each patient, three plans were created: (a) double arc flat 6MV plan used clinically; (b) double arc 10MV FFF plan; (c) single arc 10MV FFF plan. Each plan was prescribed so that at least 95% of the PTV received the prescription dose. The following dosimetric endpoints were evaluated: volume receiving 78Gy (V78) of the CTV and PTV, PTV conformality index (CI, ratio of prescription isodose volume to the PTV volume), bladder volume receiving 70Gy (V70) and 60Gy (V60), rectum volume receiving 70Gy (V70) and 50Gy (V50), dose to 10cc of the rectum, and volume of both femoral heads receiving 50Gy (V50). Total monitor units for each plan were recorded. Results: No significant difference was found for all dosimetric endpoints between all plans (p>0.05). Compared to the 6MV plans, monitor units were higher with the double arc 10MV FFF plans and lower with the single arc 10MV FFF plans, 29% and 4% respectively. Conclusion: Both single arc and double arc 10MV FFF VMAT can achieve equivalent plan quality as 6MV flat beam double arc treatment plans. With the gantry speed restriction, a high dose rate of 2400MU/min may allow the optimizer to use more MUs than actually needed. Single arc 10MV FFF VMAT plans are a reasonable alternative to double arc 6MV flat beam VMAT plans.

Purpose: The purpose of this study was to evaluate the dosimetric and practical effects of the Monaco treatment planning system “max arcs-per-beam” optimization parameter in pelvic radiotherapy treatments. Methods: A total of 17 previously treated patients were selected for this study with a range of pelvic disease site including prostate(9), bladder(1), uterus(3), rectum(3), and cervix(1). For each patient, two plans were generated, one using a arc-per-beam setting of ‘1’ and another with setting of ‘2’. The setting allows the optimizer to add a gantry direction change, creating multiple arc passes per beam sequence. Volumes and constraints established from the initial clinical treatments were used for planning. All constraints and dose coverage objects were kept the same between plans, and all plans were normalized to 99.7% to ensure 100% of the PTV received 95% of the prescription dose. We evaluated the PTV conformity index, homogeneity index, total monitor units, number of control points, and various dose volume histogram (DVH) points for statistical comparison (alpha=0.05). Results: We found for the 10 complex shaped target volumes (small central volumes with extending bilateral ‘arms’ to cover nodal regions) that the use of 2 arcs-per-beam achieved significantly lower average DVH values for the bladder V20 (p=0.036) and rectum V30 (p=0.001) while still meeting the high dose target constraints. DVH values for the simpler, more spherical PTVs were not found significantly different. Additionally, we found a beam delivery time reduction of approximately 25%. Conclusion: In summary, the dosimetric benefit, while moderate, was improved over a 1 arc-per-beam setting for complex PTVs, and equivalent in other cases. The overall reduced delivery time suggests that the use of multiple arcs-per-beam could lead to reduced patient on table time, increased clinical throughput, and reduced medical physics quality assurance effort.

Purpose: To develop a delivery-efficient proton spot-scanning arc therapy technique with robust plan quality. Methods: We developed a Scanning Proton Arc(SPArc) optimization algorithm integrated with (1)Control point re-sampling by splitting control point into adjacent sub-control points; (2)Energy layer re-distribution by assigning the original energy layers to the new sub-control points; (3)Energy layer filtration by deleting low MU weighting energy layers; (4)Energy layer re-sampling by sampling additional layers to ensure the optimal solution. A bilateral head and neck oropharynx case and a non-mobile lung target case were tested. Plan quality and total estimated delivery time were compared to original robust optimized multi-field step-and-shoot arc plan without SPArc optimization (Arcmulti-field) and standard robust optimized Intensity Modulated Proton Therapy(IMPT) plans. Dose-Volume-Histograms (DVH) of target and Organ-at-Risks (OARs) were analyzed along with all worst case scenarios. Total delivery time was calculated based on the assumption of a 360 degree gantry room with 1 RPM rotation speed, 2ms spot switching time, beam current 1nA, minimum spot weighting 0.01 MU, energy-layer-switching-time (ELST) from 0.5 to 4s. Results: Compared to IMPT, SPArc delivered less integral dose(−14% lung and −8% oropharynx). For lung case, SPArc reduced 60% of skin max dose, 35% of rib max dose and 15% of lung mean dose. Conformity Index is improved from 7.6(IMPT) to 4.0(SPArc). Compared to Arcmulti-field, SPArc reduced number of energy layers by 61%(276 layers in lung) and 80%(1008 layers in oropharynx) while kept the same robust plan quality. With ELST from 0.5s to 4s, it reduced 55%–60% of Arcmulti-field delivery time for the lung case and 56%–67% for the oropharynx case. Conclusion: SPArc is the first robust and delivery-efficient proton spot-scanning arc therapy technique which could be implemented in routine clinic. For modern proton machine with ELST close to 0.5s, SPArc would be a popular treatment option for both single and multi-room center.

Purpose: To develop a fast optimization method for station parameter optimized radiation therapy (SPORT) and show that SPORT is capable of improving VMAT in both plan quality and delivery efficiency. Methods: The angular space from 0° to 360° was divided into 180 station points (SPs). A candidate aperture was assigned to each of the SPs based on the calculation results using a column generation algorithm. The weights of the apertures were then obtained by optimizing the objective function using a state-of-the-art GPU based Proximal Operator Graph Solver (POGS) within seconds. Apertures with zero or low weight were thrown out. To avoid being trapped in a local minimum, a stochastic gradient descent method was employed which also greatly increased the convergence rate of the objective function. The above procedure repeated until the plan could not be improved any further. A weighting factor associated with the total plan MU also indirectly controlled the complexities of aperture shapes. The number of apertures for VMAT and SPORT was confined to 180. The SPORT allowed the coexistence of multiple apertures in a single SP. The optimization technique was assessed by using three clinical cases (prostate, H&N and brain). Results: Marked dosimetric quality improvement was demonstrated in the SPORT plans for all three studied cases. Prostate case: the volume of the 50% prescription dose was decreased by 22% for the rectum. H&N case: SPORT improved the mean dose for the left and right parotids by 15% each. Brain case: the doses to the eyes, chiasm and inner ears were all improved. SPORT shortened the treatment time by ∼1 min for the prostate case, ∼0.5 min for brain case, and ∼0.2 min for the H&N case. Conclusion: The superior dosimetric quality and delivery efficiency presented here indicates that SPORT is an intriguing alternative treatment modality.

Purpose: Stereotactic radiosurgery for spine metastases involves irradiation using a single high dose fraction. The purpose of this study was to dosimetrically compare stereotactic spine radiosurgery(SRS) plans using a recently new volumetric modulated arc therapy(VMAT) technique against fix-field intensity-modulated radiotherapy(IMRT). Plans were evaluated for target conformity and spinal cord sparing. Methods: Fifteen previously treated patients were replanned using the Eclipse 10.1 TPS AAA calculation algorithm. IMRT plans with 7 fields were generated. The arc plans used 2 full arc configurations. Arc and IMRT plans were normalized and prescribed to deliver 16.0 Gy in a single fraction to 90% of the planning target volume(PTV). PTVs consisted of the vertebral body expanded by 3mm, excluding the PRV-cord, where the cord was expanded by 2mm.RTOG 0631 recommendations were applied for treatment planning. Partial spinal cord volume was defined as 5mm above and below the radiosurgery target volume. Plans were compared for conformity and gradient index as well as spinal cord sparing. Results: The conformity index values of fifteen patients for two different treatment planning techniques were shown in table 1. Conformity index values for 2 full arc planning (average CI=0.84) were higher than that of IMRT planning (average CI=0.79). The gradient index valuesmore » of fifteen patients for two different treatment planning techniques were shown in table 2. Gradient index values for 2 full arc planning (average GI=3.58) were higher than that of IMRT planning (average GI=2.82).The spinal cord doses of fifteen patients for two different treatment planning techniques were shown in table 3. D0.35cc, D0.03cc and partial spinal cord D10% values in 2 full arc plannings (average D0.35cc=819.3cGy, D0.03cc=965.4cGy, 10%partial spinal=718.1cGy) were lower than IMRT plannings (average D0.35cc=877.4cGy, D0.03c=1071.4cGy, 10%partial spinal=805.1cGy). Conclusions: The two arc VMAT technique is superior to 7 field IMRT technique in terms of both spinal cord sparing and better conformity and gradient indexes.« less

Purpose: Electron arc therapy has long been proposed as the most suitable technique for the treatment of superficial tumors that follow circularly curved surfaces. However it was challenged by unsuitability of the conventional applicators and the lack of adequate 3-D dose calculation tools for arc electron beams in the treatment planning systems (TPS). Now with the availability of an electron specific multi-leaf collimator (eMLC) and an in-house Monte Carlo (MC) based TPS, we were motivated to investigate more advanced modulated electron arc (MeARC) therapy and its beneficial outcome. Methods: We initiated the study by a film measurement conducted in a head and neck phantom, where we delivered electron arcs in a step and shoot manner using the light field as a guide to avoid fields abutments. This step was done to insure enough clearance for the arcs with eMLC. MCBEAM and MCPLAN MC codes were used for the treatment head simulation and phantom dose calculation, respectively. Treatment plans were generated for targets drawn in real patient CTs and head and neck phantom. We utilized beams eye view available from a commercial planning system to create beamlets having same isocenter and adjoined at the scalp surface. Then dose-deposition coefficients from those beamlets were calculated for all electron energies using MCPLAN. An in-house optimization code was then used to find the optimum weights needed from individual beamlets. Results: MeARC showed a nicely tailored dose distribution around the circular curved target on the scalp. Some hot spots were noticed and could be attributed to fields abutment problem owing to the bulging nature of electron profiles. Brain dose was shown to be at lower levels compared to photon treatment. Conclusion: MeARC was shown to be a promising modality for treating scalp cases and could be beneficial to all superficial tumors with a circular curvature.

Purpose: To clinically evaluate the differences in volumetric modulated arc therapy (VMAT) treatment plan and delivery between two commercial treatment planning systems. Methods: Two commercial VMAT treatment planning systems with different VMAT optimization algorithms and delivery approaches were evaluated. This study included 16 clinical VMAT plans performed with the first system: 2 spine, 4 head and neck (HN), 2 brain, 4 pancreas, and 4 pelvis plans. These 16 plans were then re-optimized with the same number of arcs using the second treatment planning system. Planning goals were invariant between the two systems. Gantry speed, dose rate modulation, MLC modulation, plan quality, number of monitor units (MUs), VMAT quality assurance (QA) results, and treatment delivery time were compared between the 2 systems. VMAT QA results were performed using Mapcheck2 and analyzed with gamma analysis (3mm/3% and 2mm/2%). Results: Similar plan quality was achieved with each VMAT optimization algorithm, and the difference in delivery time was minimal. Algorithm 1 achieved planning goals by highly modulating the MLC (total distance traveled by leaves (TL) = 193 cm average over control points per plan), while maintaining a relatively constant dose rate (dose-rate change

Purpose: The purpose of this study was to propose a method to implement arc therapy that is compatible with existing particle therapy systems having gantries and pencil-beam scanning capacities. Furthermore, we sought to demonstrate expected benefits of this method for selected clival chordoma patients. Methods: We propose that a desired particle arc treatment plan can be discretized into a finite number of fixed beams and that only one (or a subset) of these beams be delivered in any single treatment fraction; the target should receive uniform dose during each fraction. For 3 clival-chordoma patients, robust-optimized, scanned proton beams were simulated to deliver 78 Gy (RBE) to clinical target volumes (CTVs), using either a single-field plan with a posterior-anterior (PA) beam or a discrete-arc plan with 16 beams that were equally spaced throughout a 360-degree axial arc. Dose-volume metrics were compared with emphasis on the brainstem, since risk of radiation necrosis there can often restrict application of tumoricidal doses for chordomas. Results: The mean volume of brainstem receiving a dose of 60 Gy (RBE) or higher (V60Gy) was 10.3±0.9 cm 3 for the single-field plan and 4.7±1.8 cm 3 for the discrete-arc plan, a reduction of 55% in favor of arcs. The mean dose to the brainstem was also reduced using arcs, by 18%, while the maximum dose was nearly identical for both methods. For the whole brain, V60Gy was reduced by 23%, in favor of arcs. Mean dose to the CTVs were nearly identical for both strategies, within 0.3%. Conclusion: Discrete arc treatments can be implemented using existing scanned particle-beam facilities. Aside from the physical advantages, the biological uncertainties of particle therapy, particularly high in the distal edge, can be reduced by arc therapy via rotational smearing, which may be of benefit for tumors near the brainstem.

Purpose: To produce an insight on the influence of selected parameters on the pass/fail rates of in-vivo dosimetry (IVD) controls for dynamic arc treatments. Methods: A group of 25 patient treatment plans optimized with the treatment planning system Eclipse™ (Varian) for dual-arc RapidArc treatment was analyzed. All plans were tracked with the IVD solution EPIgray (DOSIsoft S.A) over all sessions of the treatment course. Based on the acquired a-Si EPID images, the dose differences to the TPS at given reference points and the corresponding gamma indices were computed. In addition, the dose difference variations over the course were determined post-treatment. Five parameters — considered as possible fail or pass causes of IVD for dynamic arc plans — were chosen. Based on the treatment plan, two types of modulation indices qualifying the movement of the MLC were calculated. An “in-field” coefficient and a “weighted-in-field” coefficient were established to quantify the position of the dose reference points in the planned volume. Finally, the dose rate variations for each arc were also determined. The described parameters were confronted to the results of the IVD controls. Results: The analyzed plans have a mean relative dose difference of 0.2% (±7.2%). No correlation could be established between the modulation indices and the dose differences, the gamma indices or the dose difference variation. Slightly better passing rates for individual sessions and total treatment plans were observed for “in-field” coefficients superior to 50%. Similar passing rates were observed for arcs with below and above average dose rate deviations. Conclusion: Though all the analyzed parameters are cited as possible pass/fail criteria, only the position of the dose reference point can be linked to dose differences in IVD controls. Automatically generated dose measurement points could avoid false interpretations and an overflow of false negative alerts. The author’s PhD research is funded by DOSIsoft S.A.

Purpose: Enable a scanning field total body irradiation (TBI) technique, using dynamic arcs, which is biologically equivalent to a moving couch TBI. Methods: Patient is treated slightly above the floor and the treatment field scans across the patient by a moving gantry. MLC positions change during gantry motion to keep same field opening at the level of the treatment plane (170 cm). This is done to mimic the same geometry as the moving couch TBI technique which has been used in our institution for over 10 years. The dose rate and the gantry speed are determined considering a constant speed of the moving field, variations in SSD and slanted depths resulting from oblique gantry angles. An Eclipse (Varian) planning system is commissioned to accommodate the extended SSD. The dosimetric foundations of the technique have been thoroughly investigated using phantom measurements. Results: Dose uniformity better than 2% across 180 cm length at 10cm depth is achieved by moving the gantry from −55 to +55 deg. Treatment range can be extended by increasing gantry range. No device such as a gravity-oriented compensator is needed to achieve a uniform dose. It is feasible to modify the dose distribution by adjusting the dose rate at each gantry angle to compensate for body thickness differences. Total treatment time for 2 Gy AP/PA fields is 40–50 minutes excluding patient set up time, at the machine dose rate of 100 MU/min. Conclusion: This novel yet transportable moving field technique enables TBI treatment in a small treatment room with less program development preparation than other techniques. Treatment length can be extended per need, and. MLC-based thickness compensation and partial lung blocking are also possible.

Purpose: Different parameters Result in different plans. As the developing of volumetric modulated arc therapy for rectal cancer, the purpose of this study was to work out appropriate parameters for the optimization of rectal cancer. Methods: For ten cases which were optimized using Oncentra4.3 version, varying systematically the number of arcs, collimator angle and the delivery time to create different plans. Homogeneity in planning target volume, median dose in the organs at risk and number of monitor units were evaluated for statistical analysis. Measurements were performed with Delt4, taking the delivery time, and compared to the calculation by the gamma method. Results: Plans with collimator 45° were little superior to plans with collimator 0°. Single arc achieved lower homogeneous indices in the planning target volume compared with dual arc, but also requiring less monitor units per fraction dose than dual arc. Single arc needs less monitor units for the treatment; the measured delivery time was reduced from 4 min of dual arc to 2.5 min of single arc. All the plans could achieve good pass rate with gamma method. Conclusion: Some of the parameters have been found as the appropriate sets which can be used as standard for volumetric modulated arc therapy of rectal cancer.