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Purpose: To develop and validate a computational method to simulate craniocaudal respiratory motion in a VMAT treatment plan. Methods: Three 4DCTs of the QUASAR respiratory motion phantom were acquired with a 2cm water-density spherical tumour embedded in cedar to simulate lung. The phantom was oscillating sinusoidally with an amplitude of 2cm and periods of 3, 4, and 5 seconds. An ITV was contoured and 5mm PTV margin was added. High and a low modulation factor VMAT plans were created for each scan. An in-house program was developed to simulate respiratory motion in the treatment plans by shifting the MLC leaf positions relative to the phantom. Each plan was delivered to the phantom and the dose was measured using Gafchromic film. The measured and calculated plans were compared using an absolute dose gamma analysis (3%/3mm). Results: The average gamma pass rate for the low modulation plan and high modulation plans were 91.1% and 51.4% respectively. The difference between the high and low modulation plans gamma pass rates is likely related to the different sampling frequency of the respiratory curve and the higher MLC leaf speeds in the high modulation plan. A high modulation plan has a slower gantry speed and therefore samples the breathing cycle at a coarser frequency leading to inaccuracies between the measured and planned doses. Conclusion: A simple program, including a novel method for increasing sampling frequency beyond the control point frequency, has been developed to simulate respiratory motion in VMAT plans by shifting the MLC leaf positions.

Purpose: To report cranio-spinal irradiation (CSI) planning experience, compare dosimetric quality and delivery efficiency with Tomotherapy from different institutions, and to investigate effect of planning parameters on plan quality and treatment time. Methods: Clinical helical tomotherapy IMRT plans for thirty-nine CSI cases from three academic institutions were retrospectively evaluated. The planning parameters: field width (FW), pitch, modulation factor (MF), and achieved dosimetric endpoints were cross-compared. A fraction-dose-delivery-timing index (FDTI), defined as treatment time per fraction dose per PTV length, was utilized to evaluate plan delivery efficiency. A lower FDTI indicates higher delivery efficiency. We studied the correlation between planning quality, treatment time and planning parameters by grouping the plans under specific planning parameters. Additionally, we created new plans using 5cm jaw for a subset of plans that used 2.5cm jaw to exam if treatment efficiency can be improved without sacrificing plan quality. Results: There were significant dosimetric differences for organ at risks (OARs) among different institutions (A,B,C). Using the lowest average MF (1.9±0.4) and 5cm field width, C had the highest lung, heart, kidney, liver mean doses and maximum doses for lens. Using the same field width of 5cm, but higher MF (2.6±0.6), B had lower doses to the OARs in the thorax and abdomen area. Most of A's plans were planned with 2.5cm jaw, the plans yielded better PTV coverage, higher OAR doses and slightly shorter FDTI compared to institution B. The replanned 5cm jaw plans achieved comparable PTV coverage and OARs sparing, while saving up to 44.7% treatment time. Conclusion: Plan quality and delivery efficiency could vary significantly in CSI planning on Tomotheapy due to choice of different planning parameters. CSI plans using a 5cm jaw, with proper selection of pitch and MF, can achieve comparable/ better plan quality with shorter delivery time compared to 2.5cm jaw plans.

Purpose: A new development in linac-based intracranial radiosurgery is treatment of multiple targets using single isocenter VMAT, which dramatically reduces treatment time while maintaining high conformality and dose gradient. Our purpose is to translate this technique to extracranial SBRT of oligometastases and address additional challenges such as interplay from motion and setup uncertainties. Methods: We propose SIDO (Single Isocenter with Distinct Optimizations) planning in which all VMAT arcs share an isocenter but each arc treats only one target. Setup uncertainties from rotations and deformations are mitigated by applying a couch translation between VMAT arcs, while interplay is minimized by using dynamic conformal arcs (DCAs) as the starting point for VMAT optimization. We compared SIDO to single (VMAT) and dual (VMAT & DCA) isocenter plans for phantom and patient (N=2) cases. Dose statistics included conformity index (CI), gradient index (GI), and modulation factor (MLC opening & total MU). Finally, we determined likelihood of needing a translational shift between SIDO arcs to correct rotational uncertainties using CBCTs from eleven previous SBRT cases. Results: For phantoms with target separations of 5–15cm, CI was 0.87±0.02, 0.89±0.05, and 0.91±0.02, for SIDO, single, and dual-isocenter VMAT respectively. GI was 6.83±1.0, 7.45±1.0 and 5.94±0.7 respectively. SIDO conformity did not trend with increasing distance between PTVs, while gradient decreased for all planning techniques. Using DCAs as the starting point of VMAT optimization decreased modulation, with a 39.4% reduction in monitor units. SIDO had 4.9% and 39% less MU than single and dual-isocenter VMAT respectively. A translational shift between SIDO arcs would be required in 10–28% of cases due to rotational uncertainties to stay within a 5mm margin, for targets separated by ≥10cm. Conclusion: SIDO for extracranial oligometastases has comparable dosimetry to traditional VMAT with low modulation similar to DCAs. Future work will quantify decrease in dosimetric interplay.

Purpose: To investigate the effect on PTV target coverage of liver VMAT SBRT treatment plans by varying the IMRT modulation factor (MF) and length of delivery time in the presence of induced respiratory motion. Methods: Four different VMAT SBRT plans (MFs 2.1 to 3.5) were created for a liver phantom within the treatment planning system (Eclipse, Varian Oncology Systems). For each plan, the dose prescription was set to 5 different values (1000cGy to 5000cGy in 5 fractions); these variations force a change in the treatment delivery time due to reduce gantry speed required to deliver higher doses. To simulate respiratory motion, an in-house program was developed that shifts the positions of the MLC leaves at each control point according to population-based respiratory curves. This was done for 4 respiratory traces of comparable peak-to-peak amplitudes and breathing cycle lengths, with one trace exhibiting a baseline drift. A PTV margin of 5mm was used for all treatment plans. Results: A two-way repeatedmeasures ANOVA looked at changes in PTV V95 and GTV V95 coverage, based on IMRT factor and treatment time. The GTV V95 coverage and mean liver dose showed no significant difference (p values >0.05). The PTV V95 coverage showed a significant decrease with increasing IMRT factor and decreasing treatment time (p values < 0.01 for both). The patient curve with baseline drift exhibited a decrease in PTV coverage with increasing delivery time. Conclusion: Decreased PTV coverage was seen with increasing modulation factor and decreasing treatment times for patients with regular breathing curves. With the advent of faster treatment times it is recommended that lower modulation factors for SBRT treatment plans be utilized to mitigate the possibility of interplay. Additionally, longer treatment times could be advantageous to blur out the interplay effect for SBRT treatments. Further investigation is required in patient plans.

Purpose: To extend a previously described technique of modeling 2‐D fluence from any MLC leaf sequence, and develop an independent and accurate calculation of modulation factors required for pre‐treatment QA of IMRT plans. This calculation is intended to replace ion chamber measurements and thus offers a quicker and more stream‐lined verification process. Methods: In‐house software developed in MATLAB was used to calculate each IMRT field's modulation factor, defined as the ratio of the dose with MLC to the corresponding open field dose (without MLC) at a given reference point. For IMRT fields, the program uses the leaf sequence file for each dynamic field to calculate 2‐D maps of the primary fluence modulation and the timings of leaf openings. Fluence modeling includes beam horn characterization and head scatter effects. Commissioning values of leaf transmission and the dynamic leaf gap for our Varian Millenium MLCs are used to incorporate the effects of transmission and rounded leaf‐end design on the fluence modulation. Interleaf leakage and tongue‐and‐groove effects are modeled based on the timing map and the shape (including penumbra) of the interleaf leakage and tongue‐and‐groove characterized from 2‐D EPIDimages. Once calculated, the IMRT fluence is convolved with a Monte‐Carlo (EGSnrcMP) pencil‐beam kernel to generate a dose at the reference point depth, and divided by the open field dose to obtain the calculated modulation factor. Results: Results from 18 IMRT plans (121 fields), including treatments from pelvis, prostate, head and neck, and anal canal cancer, show excellent agreement between calculated and ion‐chamber measured modulation factors. The mean discrepancy is −0.3%, with standard deviation of 1.2%, where calculation times are 55 seconds per field on one PC core. Conclusions: An accurate and streamlined calculation method for determining modulation factors has been developed to replace tedious ion chamber measurements needed for pre‐treatment IMRT QA.

Purpose: The purpose of this study is to come up with the most desirable and achievable target dose and OAR sparing using helical TomoTherapy planning system based on the earlier conducted studies. Methods: Patient having mesothelioma with mediastinal nodes was planned using helical TomoTherapy with Gross tumor volumes (GTVs) outlined were investigated: The goal was (1) keeping the prescribed dose to the targets while reducing the dose to the OARs and (2) escalating the target dose while maintaining the original level of healthy tissue sparing. Results: The primary lesion and nodal mass were kept at 98.7% of the dose and planning target volume (PTV) margins of 3 mm inner and 1cm outer with the planned mean of 94% volume to 54 Gy. Mean lung dose is 15.6 Gy, V5Gy:33%, V20Gy:30.1%, with contralateral lung V5Gy:0.87% Heart V45:26.2%, PTV V54Gy:90.5%, Pitch: 0.43 and Modulation factor: 1.66 Conclusion: Based on many studies on the comparison of 3DCRT, IMRT and TomoTherapy, it is recommended that helical TomoTherapy provide better target coverage and sparing of OARs. Our data proved better PTV coverage. We planned a target dose of 54Gy but our institute would recommend re-evaluation at 45Gy. Helical TomoTherapy is a promising technique in the multimodality treatment of malignant pleural mesothelioma. Unlike other studies where low doses received by the contralateral lung appear to be the limiting factor, we found using helical TomoTherapy the contralateral to be minimal V5Gy:0.87%.

Purpose: To determine the out of field response of Microstar ii OSLDs as a function of field modulation and distance in VMAT plan delivery. This work has potential application in fetal dose monitoring or measurements on cardiac pacemakers Methods: VMAT plans were created in Eclipse and optimized to varying degrees of modulation. Three plans were chosen to represent low, medium and high degrees of modulation (modulation factors as defined by MU/cGy). Plans were delivered to slabs of solid water with dimensions 60cm length, 30cm width, and 10cm height. For each modulation factor, 2 OSLDs were placed at 1cm depth with out of field distances of 1, 2, 3, 5, 8 and 10cm and the plan delivered isocentrically to a depth of 5cm. This technique was repeated for a Farmer Chamber by incrementing the table by the appropriate distance. The charge readings for the Farmer Chamber were converted to dose and the ratios taken as functions of modulation factors and distances out of field Results: Examination of the results as a function of out of field distance shows a trend of increasing OSLD/Farmer Chamber ratios for all modulation factors. The slopes appear to be roughly equivalent for all modulation factors investigated. Results as a function of modulation showed a downward trend for all out of field distances, with the greatest differences seen at 5cm and 8cm Conclusion: This study demonstrates that the response of OSLD dosimeters change as a function of out of field distance and modulation. The differences seen are within the stated accuracy of the system for the out of field distances and modulations investigated. Additional investigation is warranted to see if the OSLD response changes appreciably with longer out of field distances or wider ranges of modulation

Purpose: Important limitations for dose painting are due to treatment planning and delivery constraints. The purpose of this study was to determine the sensitivity of dose conformity to optimization parameters using the clinical TomoTherapy Hi‐Art treatment planning system (TPS). Materials and Methods: Uptake data from a patient who underwent a [ 61 Cu ]Cu‐ATSM PET/CT (surrogate of hypoxia) scan was retrospectively extracted for treatment planning. Extracted data was reformatted from voxel‐based to level‐based for compatibility with the Hi‐Art TPS. Optimized treatment plans were generated with physical objectives for each prescription sub‐volume modifying pitch, jaw width, modulation factor and iteration number. Effects of variations in plan parameters were simulated varying one physical parameter while keeping the other parameters unchanged. Avoidance structures were not used. The conformity of treatment plans to their non‐uniform prescriptions was evaluated via quality‐volume histograms (QVH) and percent receiving planned dose within 2 percent of prescription (Q0.98–1.02). Results: In general, the conformity of treatment plans to dose prescriptions was found to be adequate for delivery of dose painting plans. The conformity was better as the jaws decreased in width from 2.5 cm to 1.0 cm (Q0.98–1.02:64% vs 73%) and as the pitch increased from 0.122 to 0.443 (Q0.98–1.02:71% vs 73%). The effects on dose conformity of modulation factor and iteration number seem to be insensitive to change. Treatment delivery times varied from 5–11 minutes for changes in field width (2.5 cm to 1.0 cm) and from 11–25 minutes for changes in pitch (0.443 to 0.122). Conclusions: This investigation demonstrates the ability of the helical tomotherapy to create and deliver plans with non‐uniform dose distributions. Results indicate that agreement in prescription dose and planned dose distributions for all treatment plans are sensitive to physical parameter changes such as pitch and field width but insensitive to changes in modulation factor and iteration number.

Purpose: To determine the feasibility of reducing Tomotherapy treatment delivery time by planning with increased pitch and reduced modulation factor (MF). Method and Materials: Twelve patients with Tomotherapy treatment sites (head/neck, prostate, prostate/nodes, lung) and fraction size of 1.8–2.5 Gy originally planned with a low pitch (< 0.3) and fast gantry period (< 20 s) were replanned. The first replan increased the pitch to 0.43 and used the original MF (MF0). If the first replan was acceptable and the gantry period increased, the patient was replanned again with pitch = 0.43 and a lower MF to reduce treatment time. This was repeated until the plan (DVH) became unacceptable or the gantry period reached its minimum (15 s). If the first replan was not satisfactory or if the gantry period did not increase, the MF was increased by 0.5 to improve the plan and to test if the treatment time was still reduced. Results: For all patients, we were able to produce a DVH comparable to the original DVH using pitch = 0.43 and MF0. When comparing the original DVH to the accelerated plan DVH that most closely matched it, treatment times were reduced by 13–66%. Comparing these accelerated plans to the original plans showed the PTV median dose increase 0.4±0.5 Gy, insignificant changes in target coverage and homogeneity index, an average change in critical structure median dose of −0.8±1.3 Gy, and a maximum critical structure median dose increase of 1.3 Gy. Conclusion: Faster treatment times can be achieved by planning with increased pitch and reduced modulation. Treatments planned with pitch = 0.43 and MF0 showed little or no plan degradation and most had faster treatment times. Further reduction in treatment time was possible by reducing the MF. Conflict of Interest: Some of the authors have financial interest in TomoTherapy, Inc.

Purpose: To investigate the influence of pitch and jaw width selection on helical tomotherapy treatment plans for head and neck. Method and Materials: Helical tomotherapy plans using a combination of pitch and jaw width settings were developed for three patients previously treated for head and neck cancer. Three jaw widths (5, 2.5 and 1 cm) and six pitches (0.86, 0.7, 0.43, 0.34, 0.287 and 0.215) were used with a (maximum) modulation factor setting of 4. The 18 plans per patient were generated using an identical optimization procedure (e.g., number of iterations, objective weights and penalties, etc.), based on recommendations from TomoTherapy. The plans were compared using isodose plots, dose volume histograms, dose homogeneity indices, conformity indices, radiobiological models, and treatment times. Results: Smaller pitches and jaw widths showed better target dose homogeneity and sparing of normal tissue, as expected. The conformity index had a maximum for pitches between 0.287 and 0.43. The tumor control probabilities were greater than 99% for the 2.5‐ and 1‐cm jaw widths for all pitches except 0.86. The treatment time increased inversely proportional to the jaw width. Conclusion: Our study indicates that the smaller jaw widths (2.5 and 1 cm) and pitches near the range of 0.3–0.4 produce better plans for helical tomotherapy head and neck treatment. However, because of the large increase in treatment time with only slight improvement in sparing of the critical structures for the 1‐cm jaw width, the 2.5‐cm jaw width, currently used in our clinic, remains our preference. Supported in part by a research agreement with TomoTherapy, Inc.

Purpose: The planning process for helical tomotherapy plans is governed by three basic planning parameters: field length, pitch ratio, and modulation factor. The purpose of this work is to evaluate the influence of each parameter on the achievable plan. Method and Materials: Using commercial treatment planning software (TomoTherapy, Inc.) the same head and neck case was re‐planned by the same person using different planning parameters. The standard settings (field length of 25 mm, pitch of 0.287 and modulation factor of 2.5) were used to generate a baseline plan. Using the standard settings for two parameters the third parameter was varied for different plans. Three field lengths (10, 25, 50 mm), four pitch ratios (0.2, 0.287, 0.5, 1) and seven modulation factors were used (1.5, 2, 2.5, 4, 6, 8, 10). The plans were evaluated in terms of treatment time, target coverage, conformity, and sensitive structure sparing. Results: Increasing the pitch ratio leads to an increase in the volume of unspecified tissues that receives prescription dose (less conformal plans) and an increase in the dose to sensitive structures. The treatment time did not significantly reduce with increasing pitch (there are fewer gantry rotation but the gantry rotation period increases). Similarly, an increased modulation reduces the dose to unspecified tissue and sensitive structures. However the cost is an increased treatment time. Using a larger field length increases the dose to unspecified tissue and sensitive structures while reducing the treatment time. All plans achieved acceptable target coverage and homogeneity. Conclusions: Changing the planning parameters mainly affects plan conformity. It appears that with respect to our standard settings a slight reduction of the pitch or a slight increase in the modulation factor should improve plan quality with a modest increase in treatment time. Conflict of Interest: One co‐author is employed by TomoTherapy, Inc.

Purpose: To perform an inter‐comparison of TPS results for different IMRT delivery systems. Methods: Patient data with prostate cancer was selected. Same geometries and energy were employed for fixed gantry angle plans. Treatment includes simultaneous irradiation, prostate to 82Gy and seminal vesicles to 64Gy (41 fractions). Plans were normalized to achieve 82Gy minimum dose to prostate. Plan 1 : step&shoot with 1cm leaf‐width (MLC‐Optifocus), TPS Konrad v2.2 (Siemens). Plan 2: same plan 1 but with TPS iPlan v4.1 (BrainLAB). Plan 3: step&shoot with 0.25cm leaf‐width (mMLC‐Moduleaf, Siemens), TPS iPlan v4.1. Plan 4: dynamic with 0.5cm leaf‐width (MLC‐Millenium), TPS Eclipse v8.1 (Varian). Plan 5: Tomotherapy TPS v3.1.4.7. Longitudinal field size 2.5cm, pitch 0.277 and modulation factor 1.783, leaf width 6.25mm Results: Total numbers of MU were 508, 488, 570, 577 and 3219 for Plan 1 to 5. D98% (V82Gy) for PTV prostate were 81.6Gy (95.7%), 80.7Gy (94.9%), 81.8Gy (97.4%), 80.1Gy (84.6%) and 79.9Gy (82.6%) for Plan 1 to 5. V64Gy for PTV seminal vesicle were 98.2%, 99.9%, 100%, 92.1% and 97.8% for Plan 1 to 5. Rectum EUD (V40Gy) were 65.1Gy (56.6%), 62.7Gy (53.1%), 61.9Gy (44.3%), 63.3Gy (55.7%) and 61.9Gy (39%) for Plan 1 to 5. Rectum wall EUD was 66.4Gy, 65.2Gy, 64.8Gy, 64.9Gy and 65.2Gy for Plan 1 to 5. Bladder EUD (V65Gy) were 47.9Gy (27.4%), 47.4Gy (24.8%), 46.6Gy (24.6%), 46.2Gy (24.4%) and 45.8Gy (20.1%) for Plan 1 to 5. Femoral head D10% were 31.8Gy, 34.5Gy, 33.9Gy, 39.3Gy and 26.1Gy for Plan 1 to 5. Conclusions: MUs are lower with step&shoot MLC. MicroMLC produces slightly better conformation of PTVs. Better sparing of rectum and bladder with mMLC and Tomotherapy. For rectum wall EUD shows that mMLC produce the better results. Better sparing of femoral head is obtained with Tomotherapy. All techniques give the OAR's similar EUD (+/−2Gy), except for femoral heads.

Purpose: To analyze the sensitivity of a plan‐class specific correction factor to dose distribution in nonstandard field dosimetry. Methods: Ten different TomoTherapy‐based IMRT fields were created on a water‐filled PMMA cylindrical phantom at the center of which the absorbed dose to water was measured. Dose distribution of each IMRT field was evaluated using three parameters: modulation factor (MF), modified conformal index (COIN) and homogeneity index (HI). The relative dose in each IMRT field was measured using a PTW micro liquid ion chamber. Based on the new dosimetry formalism, the plan‐class specific correction factor was measured for two 0.6 cm3 Farmer‐type chambers (Exradin A12 and NE2571) and a 0.06 cm3 Exradin A1SL chamber. We investigated the dependence of the measured correction factor on the three plan evaluation parameters. Results: Uncertainty on the measurement of the plan‐class specific correction factor was 0.2–0.4 % and 0.2–0.7 % for the Farmer‐type chambers and Exradin A1SL, respectively. When the dose homogeneity was better than 5 % along with values of MF and modified COIN smaller than 2.0 and 0.025, respectively, the correction factor of the IMRT field was not different from unity by more than 0.7 %. For more heterogeneous IMRT fields, the correction factor deviated from unity by up to 2.4 % for the Farmer‐type chambers because of increasing residual gradient effects. For the Exradin A1SL, while it was closer to unity, the correction factor was highly variable in different IMRT fields due to a more significant impact of positioning uncertainties on the response of the smaller chamber. Conclusions: We have shown that the plan‐class specific correction factor can be characterized as a function of plan evaluation parameters, especially for Farmer‐type chambers. This work provides a recipe based on quantifying dose distribution to accurately select air‐filled ionization chamber correction factors for nonstandard fields.

Purpose: To compare plan quality between fixed‐gantry IMRT plans generated by a treatment planning system (TPS) based on biological constraints (Monaco, Elekta CMS) and rotational IMRT plans produced by a TPS based on dose‐volume constraints (TomoTherapy) for prostate irradiation. Methods: Five previously treated TomoTherapy (Ver. 3.1.4.7) plans with a prescription dose of 7560 cGy in 42 fractions to prostate PTV were selected. TomoTherapy plans were generated using 2.5 cm field width, a typical pitch of 0.43, and a mean modulation factor of 2.20. Monaco (Ver. 2.03) plans were generated for these five cases using 9 equispaced beams, 2 mm grid spacing, 2 mm beamlet width, and a Monte Carlo algorithm for final dose calculation with 3% variance. Because greater PTV dose heterogeneity was anticipated, Monaco plans that did not meet a “ 110% of the prescribed dose” criterion were rejected. Two sets of Monaco plans were generated for 58‐ and 160‐leaf Siemens MLC beam models. Various plan quality metrics were compared by the paired t‐test. Results: In all five cases, it was possible to obtain Monaco plans that met the PTV dose homogeneity criterion and equaled or exceeded the TomoTherapy plans in terms of rectal and bladder sparing. A small, but statistically significant, improvement in rectal V45 (−1.3%) and V70 (−2.4%) and in bladder V45 (−3.7%) was observed in Monaco 58‐leaf MLC plans compared to the TomoTherapy plans. In Monaco 160‐leaf MLC plans, a trend toward further improvement in critical structure sparing and PTV dose homogeneity was observed. Conclusions: Fixed‐gantry IMRT prostate plans generated by the Monaco TPS have identical or slightly improved critical structure sparing in comparison with the TomoTherapy plans, but are characterized by greater, albeit clinically acceptable, heterogeneity in target volumes.

Purpose: To develop a methodology for creating voxel‐based dose painting plans that are clinically deliverable using a commercialized, structure‐based treatment planning system (TPS). Methods and Materials: Uptake data from head and neck patients who underwent [61Cu]Cu‐ATSM (hypoxia surrogate) PET/CT scans were retrospectively extracted for treatment planning. Non‐uniform voxel‐based prescriptions were converted to structure‐based prescriptions for compatibility with the TomoTherapy Hi‐ART II™ TPS. Optimized treatment plans were generated using fixed pitch, jaw width, and modulation factor delivery parameters. The effects of structure importance, prescription point normalization, iteration number, DVH volume, Min/Max dose, and dose penalty were investigated by varying these constraints one at a time in each plan. Avoidance structures were constrained to clinical tolerances. Quality‐volume histograms (QVH) as percent receiving within 5 percent of the prescription (Q0.95–1.05) were used to evaluate treatment plan conformity. Results: Conformity of the plans to their non‐uniform dose prescription was insensitive to the optimization parameters when the prescription DVH constraint and the dose normalization point were set to the mean PTV volume and dose. Plan conformity decreased as the prescription DVH constraint (Q0.95–1.05: 88% vs. 84%) or the normalization point (Q0.95–1.05: 88% vs. 85%) deviated from the means. Plan conformity was also negatively impacted by increasing integral boost doses from 10 Gy to 30 Gy (Q0.95–1.05: 97% vs 78%), decreasing iteration number from 1000 to 100 (Q0.95–1.05: 86% vs 80%), and applying conventional planning strategies (Q0.95–1.05: 88% vs 59%). Conclusions: This investigation demonstrates the ability of a structure‐based TPS to create voxel‐based dose painting plans using a relaxed set of optimization parameters and a dose normalization point set to the mean PTV dose. Under these conditions, plan conformity is dominated by few optimization parameters. However, tightening constraints on target structures via conventional planning strategies results in decreased plan conformity.

Purpose:IMRT has become a standard technique for the treatment of many tumors. It is often important to know the out‐of‐field dose associated with such treatments, such as for fetal dose evaluation, or estimating organ doses during epidemiologic study of second cancer risk. While evaluating such doses is routine for conventional therapies (e.g., Task Group 36 data), no simple technique is available to estimate doses from IMRT treatments. This work develops and evaluates such a technique. Method and Materials: A simple equation was developed to calculate the out‐of‐field dose (Dimrt) from an IMRT treatment based on treatment parameters and the out‐of‐field dose from conventional therapy (Dconv) with the same jaw setting (which is readily estimated from such sources as TG‐36). The accuracy of this equation was tested by comparing out‐of‐field doses calculated with the developed equation to those calculated for IMRT treatments using Monte Carlo. Results: The following equation was developed to estimate the out‐of‐field dose from IMRT: Dimrt=Dcon[P×CF + (L+C)×MF], where P, L, and C are the fraction of the out‐of‐field dose originating from patient scatter, head leakage, and collimator scatter respectively, CF is the fraction of the open field covered by the CIAO field (continuous irradiated area opening), and MF is the IMRT MU modulation factor. These values are tabulated for typical treatments. Compared to the out‐of‐field dose from IMRT, doses calculated with this equation generally agreed within 25%. Larger variations were sometimes observed, particularly within 5cm of the edge of the treatment field where large variations in the out‐of‐field dose are expected due to its dependence on the particular treatment plan and planning objectives. Conclusion: A simple and suitably accurate equation was derived to calculate the out‐of‐field dose from IMRT treatments. This technique represents a viable approach for estimating the out‐of‐field dose for clinical or epidemiological applications.

Purpose: To investigate whether the overshoot effect or “ballooning” of isodose lines into the rectum observed in tomotherapy patient treatment plans is caused by the sharp inclination of the posterior border of the prostate. Method and Materials: Cylindrical target and OAR volumes were segmented in a solid water phantom and placed adjacent to one another in an anterior‐posterior orientation at the center of the phantom. Individual treatment plans were created for the following angular orientations with respect to the y‐axis (IEC coordinates): Target0/OAR0: 0° above y‐axis, Target15/OAR15: 15° above y‐axis, Target30/OAR30: 30° above y‐axis and Target45/OAR45: 45° above y‐axis. Separation between the target and OAR remained fixed at 5 mm for all angles. Treatment plans were generated with a field width of 2.5cm, pitch of 0.287, and modulation factor of 2.5. For all plans, dose prescription was 98% of the target volume to receive at least 6.0Gy. Four additional treatment plans were created in which the target extended into the OAR. Results: None of the phantom plans showed any overshoot of the isodose lines into the adjacent OAR. In the phantom plans with the target extended into the OAR, the effect was not clearly seen however a slight indication of minimal ballooning was indeed observed. Additionally, it was noted that as the inclination angle increased the mean dose to the OAR increased. Conclusion: This study served to investigate the dependency of the target inclination on the isodose ballooning effect seen in prostate patients. Our phantom study results show that the angle of inclination does not produce a ballooning of the isodose lines but does lead to an increase in mean dose to the OAR as the angle is incremented. Future studies will investigate whether the extension of the target volume into the OAR is the cause.

Purpose:Intensity modulated radiation therapy(IMRT) can be delivered using several different techniques. The present study compares dosimetric characteristics of three commercial IMRT planning and delivery systems, i.e., helical tomotherapy (TOMO), sequential tomotherapy with MIMiC (MIMiC) and dynamic multileaf collimated IMRT (DMLC‐IMRT). Method and Materials:IMRT plans for three common clinical cases, head‐and‐neck (HN), brain and prostate cancers, were generated using respective RTOG protocols 9406, 9803 and H0022 as guidelines. For each case, multiple (more than four) plans were produced for each technique using different strategies by adjusting constraints and/or parameters such as beam width, modulation factor, the number of fields, etc. Dosimetric indices were used as figures‐of‐merit to evaluate the plans. These indices include coverage index (CI), overflow index (OI), sparing index (SI), and overall conformal index (COIN), where COIN=CI×OI×SI. The plans for each technique were compared among themselves to determine the best plan of each technique which in turn was used for the comparison of different techniques. Results: For the HN case, the COINs for TOMO, MIMiC and DMLC‐IMRT were 0.333, 0.064 and 0.305, respectively. TOMO had highest SI, but DMLC‐IMRT had highest CI and OI. MIMiC plan performance was poorer than TOMO and DMLC‐IMRT for the HN case. The COINs for TOMO, MIMiC and DMLC‐IMRT were 0.471, 0.616 and 0.685 for the brain case, and 0.304, 0.487 and 0.554 for the prostate case, respectively. DMLC‐IMRT had the highest COIN for both the brain and prostate cases. TOMO had best coverage for the brain case while DMLC‐IMRT had best coverage for the prostate case. Conclusion:IMRT plan optimization is a process to balance and compromise between target coverage and normal tissue sparing. The three techniques may result in different plans for similar constraints. Using different strategies, all three techniques produced clinically acceptable treatment plans for all cases.

Purpose: To determine the fetal dose arising from a typical helical tomotherapy IMRT treatment delivered at anatomical sites distant from the embryo/fetus. Method and Materials: An anthropomorphic phantom (Rando™ slices 1–25) with the pelvis section replaced by a plastic water™ phantom was used to develop a treatment sinogram with the Hi‐Art™ device using typical parameters: 6 MV photons, 2.5 cm field width, pitch of 0.3, and modulation factor of 2. Target dimensions were 8 cm (W) × 7 cm (H) × 11 cm (L) and was centrally located in the abdominal region of Rando™, which was planned and treated to a dose of 5 Gy. Fetal dose measurements were made at a midplane depth of 10 cm in the plastic water using TLD and a calibrated diode. Four sites in Rando™ were treated using the same sinogram allowing the fetal dose to be determined as a function of distance from the treatment site center, which varied from 23 cm to 75 cm. To express results as a percent of the delivered dose for the same sinogram, an approximate correction was made to account for differences in tissue thicknesses at the various treatment sites. Results: For the same sinogram delivery (of approximately 5 Gy), fetal dose varied from 3.53 cGy for a lower abdominal treatment at a distance of 23 cm to 0.23 cGy for a brain/head treatment at a distance of 75 cm. A power law fit indicates fetal dose is proportional to the distance to the −2.3 power. Conclusion: Typical helical tomotherapy treatments at distant sites may yield fetal doses approaching 1 percent of delivered dose; and fetal dose is reduced by more than the square of the distance to the treatment site. Conflict of Interest: A collaborative research relationship is currently being developed with TOMOTHERAPY, Inc.