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Purpose The objective of this study was to investigate the influence of tumor motion on dose delivery in stereotactic body radiotherapy (SBRT) for lung cancer, using fixed field intensity- modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). Methods and Materials For each of 10 patients with stage I/II non-small-cell pulmonary tumors, a respiration-correlated four-dimensional computed tomography (4DCT) study was carried out. The internal target volume was delineated on the maximum intensity projection CT, which was reconstructed from the 4DCT dataset. A 5-mm margin was used for generation of the planning target volume. VMAT and five-field IMRT plans were generated using Pinnacle 3 SmartArc and direct machine parameter optimization, respectively. All plans were generated for an Elekta Synergy linear accelerator using 6-MV photons. Simulation was performed to study the interplay between multileaf collimator (MLC) sequences and target movement during the delivery of VMAT and IMRT. For each plan, 4D dose was calculated using deformable image registration of the 4DCT images. Target volume coverage and doses to critical structures calculated using 4D methodology were compared with those calculated using 3D methodology. Results For all patients included in this study, the interplay effect was found to present limited impact (less than 1% of prescription) on the target dose distribution, especially for SBRT, in which fewer fractions (three fractions) are delivered. Dose to the gross tumor volume (GTV) was, on average, slightly decreased (1% of prescription) in the 4D calculation compared with the 3D calculation. The motion impact on target dose homogeneity was patient-dependent and relatively small. Conclusions Both VMAT and IMRT plans experienced negligible interplay effects between MLC sequence and tumor motion. For the most part, the 3D doses to the GTV and critical structures provided good approximations of the 4D dose calculations.

Purpose: Helical tomotherapy (HT) and volumetric modulated arc therapy (VMAT) are arc-based approaches to IMRT delivery. The objective of this study is to compare VMAT to both HT and fixed field IMRT in terms of plan quality, delivery efficiency, and accuracy. Methods: Eighteen cases including six prostate, six head-and-neck, and six lung cases were selected for this study. IMRT plans were developed using direct machine parameter optimization in the Pinnacle{sup 3} treatment planning system. HT plans were developed using a Hi-Art II planning station. VMAT plans were generated using both the Pinnacle{sup 3} SmartArc IMRT module and a home-grown arc sequencing algorithm. VMAT and HT plans were delivered using Elekta's PreciseBeam VMAT linac control system (Elekta AB, Stockholm, Sweden) and a TomoTherapy Hi-Art II system (TomoTherapy Inc., Madison, WI), respectively. Treatment plan quality assurance (QA) for VMAT was performed using the IBA MatriXX system while an ion chamber and films were used for HT plan QA. Results: The results demonstrate that both VMAT and HT are capable of providing more uniform target doses and improved normal tissue sparing as compared with fixed field IMRT. In terms of delivery efficiency, VMAT plan deliveries on average took 2.2 min for prostate andmore » lung cases and 4.6 min for head-and-neck cases. These values increased to 4.7 and 7.0 min for HT plans. Conclusions: Both VMAT and HT plans can be delivered accurately based on their own QA standards. Overall, VMAT was able to provide approximately a 40% reduction in treatment time while maintaining comparable plan quality to that of HT.« less

Intensity-modulated arc therapy (IMAT) is a rotational IMRT technique. It uses a set of overlapping or nonoverlapping arcs to create a prescribed dose distribution. Despite its numerous advantages, IMAT has not gained widespread clinical applications. This is mainly due to the lack of an effective IMAT leaf-sequencing algorithm that can convert the optimized intensity patterns for all beam directions into IMAT treatment arcs. To address this problem, we have developed an IMAT leaf-sequencing algorithm and software using graph algorithms in computer science. The input to our leaf-sequencing software includes (1) a set of (continuous) intensity patterns optimized by a treatment planning system at a sequence of equally spaced beam angles (typically 10 deg. apart), (2) a maximum leaf motion constraint, and (3) the number of desired arcs, k. The output is a set of treatment arcs that best approximates the set of optimized intensity patterns at all beam angles with guaranteed smooth delivery without violating the maximum leaf motion constraint. The new algorithm consists of the following key steps. First, the optimized intensity patterns are segmented into intensity profiles that are aligned with individual MLC leaf pairs. Then each intensity profile is segmented into k MLC leaf openings using amore » k-link shortest path algorithm. The leaf openings for all beam angles are subsequently connected together to form 1D IMAT arcs under the maximum leaf motion constraint using a shortest path algorithm. Finally, the 1D IMAT arcs are combined to form IMAT treatment arcs of MLC apertures. The performance of the implemented leaf-sequencing software has been tested for four treatment sites (prostate, breast, head and neck, and lung). In all cases, our leaf-sequencing algorithm produces efficient and highly conformal IMAT plans that rival their counterpart, the tomotherapy plans, and significantly improve the IMRT plans. Algorithm execution times ranging from a few seconds to 2 min are observed on a laptop computer equipped with a 2.0 GHz Pentium M processor.« less

A new leaf-sequencing approach has been developed that is designed to reduce the number of required beam segments for step-and-shoot intensity modulated radiation therapy (IMRT). This approach to leaf sequencing is called continuous-intensity-map-optimization (CIMO). Using a simulated annealing algorithm, CIMO seeks to minimize differences between the optimized and sequenced intensity maps. Two distinguishing features of the CIMO algorithm are (1) CIMO does not require that each optimized intensity map be clustered into discrete levels and (2) CIMO is not rule-based but rather simultaneously optimizes both the aperture shapes and weights. To test the CIMO algorithm, ten IMRT patient cases were selected (four head-and-neck, two pancreas, two prostate, one brain, and one pelvis). For each case, the optimized intensity maps were extracted from the Pinnacle{sup 3} treatment planning system. The CIMO algorithm was applied, and the optimized aperture shapes and weights were loaded back into Pinnacle. A final dose calculation was performed using Pinnacle's convolution/superposition based dose calculation. On average, the CIMO algorithm provided a 54% reduction in the number of beam segments as compared with Pinnacle's leaf sequencer. The plans sequenced using the CIMO algorithm also provided improved target dose uniformity and a reduced discrepancy between the optimized and sequencedmore » intensity maps. For ten clinical intensity maps, comparisons were performed between the CIMO algorithm and the power-of-two reduction algorithm of Xia and Verhey [Med. Phys. 25(8), 1424-1434 (1998)]. When the constraints of a Varian Millennium multileaf collimator were applied, the CIMO algorithm resulted in a 26% reduction in the number of segments. For an Elekta multileaf collimator, the CIMO algorithm resulted in a 67% reduction in the number of segments. An average leaf sequencing time of less than one minute per beam was observed.« less

Purpose: Surface guided radiation therapy (SGRT) uses stereoscopic video images in combination with patterns projected onto the patient's surface to dynamically capture and reconstruct a 3D surface map. In this work, we used a C-RAD Catalyst HD system (C-RAD) to evaluate intrafraction motion in the delivery of lung SBRT. Methods: The surface acquired from the 4DCT images from our preliminary cohort of eight lung cancer patients treated with SBRT were matched to the surface images acquired prior to each treatment. Additionally, a CBCT image set was acquired. A linear regression model was established between the external and internal motion of tumor during pretreatment and used to predict the CBCT deviation during treatment. The shifts determined from CBCT and the shifts from surface map imaging were compared and assessed using Bland-Altman method. For intrafraction motion, we assessed the percentage of mean errors that fell outside of the threshold of 2 mm, 3 mm, and 5 mm along the translational directions. The required PTV margin was quantified over the course of treatment. The correlation between intrafraction treatment time and mean error of 3D displacement was evaluated using the Pearson coefficient, r Results: A total of 7971 data points were analyzed. Deviations of 2mm, 3mm, and 5mm were observed less than 7%, 2 %, and 0 % of the time along the translational direction. CBCT and Catalyst showed close agreement during patient positioning. Furthermore, the calculated PTV margins were less than our clinical tolerance of 5 mm. Using the Pearson coefficient r,the mean error of 3D displacement showed significant correlation with treatment time (r=0.69, p= 0.000002). Conclusion: SGRT can be used to ensure accurate patient positioning during treatment without an additional delivery of dose to the patient. This study shows that importance of treatment time as a consideration during the treatment planning process.

Purpose: In this study, we evaluated the performance of an Elekta linac in the delivery of gated radiotherapy. We examined whether the use of either a short gating window or a long beam hold impacts the accuracy of the delivery Methods: The performance of an Elekta linac in the delivery of gated radiotherapy was assessed using a 20cmX 20cm open field with the radiation delivered using a range of beam-on and beam-off time periods. Two SBRT plans were used to examine the accuracy of gated beam delivery for clinical treatment plans. For the SBRT cases, tests were performed for both free-breathing based gating and for gated delivery with a simulated breath-hold. A MatriXX 2D ion chamber array was used for data collection, and the gating accuracy was evaluated using gamma score. Results: For the 20cmX20cm open field, the gated beam delivery agreed closely with the non-gated delivery results. Discrepancies in the agreement, however, began to appear with a 5-to-1 ratio of the beam-off to beam-on. For these tight gating windows, each beam-on segment delivered a small number of monitor units. This finding was confirmed with dose distribution analysis from the delivery of the two VMAT plans where the gamma score(±1%,2%/1mm) showed passing rates in the range of 95% to 100% for gating windows of 25%, 38%, 50%, 63%, 75%, and 83%. Using a simulated sinusoidal breathing signal with a 4 second period, the gamma score of freebreathing gating and breath-hold gating deliveries were measured in the range of 95.7% to 100%. Conclusion: The results demonstrate that Elekta linacs can be used to accurately deliver respiratory gated treatments for both free-breathing and breath-hold patients. The accuracy of beams delivered in a gated delivery mode at low small MU proved higher than similar deliveries performed in a non-gated (manually interrupted) fashion.

Purpose: Clinical implementation of gated lung SBRT requires tools to verify the accuracy of the target positioning on a daily basis. This is a particular challenge on Elekta linacs where the XVI imaging system does not interface directly to any commercial gating solution. In this study, we used the Elekta's intra-fractional imaging functionality to perform the pretreatment CBCT verifications and evaluated both the image quality and gating accuracy. Methods: To use intrafraction imaging tools for pretreatment verifications, we planned a 360-degree arc with 1mmx5mm MLC opening. This beam was designed to drive the gantry during the gated CBCT data collection. A Catphan phantom was used to evaluate the image quality for the intra-fractional CBCT. A CIRS lung phantom with a 3cm sphereinsert and a moving chest plate were programmed with a simulated breathhold breathing pattern was used to check the gating accuracy. A C-Rad CatalystHD surface mapping system was used to provide the gating signal. Results: The total delivery time of the arc was 90 seconds. The uniformity and low contrast resolution for the intra-fractional CBCT was 1.5% and 3.6%, respectively. The values for the regular CBCT were 1.7% and 2.5%, respectively. The spatial resolution was 7 line-pairs/cm and the 3D spatial integrity was less than 1mm for the intra-fractional CBCT. The gated CBCT clearly demonstrated the accuracy of the gating image acquisition. Conclusion: The intra-fraction CBCT capabilities on an Elekta linac can be used to acquire pre-treatment gated images to verify the accuracy patient positioning. This imaging capability should provide for accurate patient alignments for the delivery of lung SBRT. This research was partially supported by Elekta

Purpose Intensity-modulated arc therapy (IMAT) is an arc-based approach to intensity-modulated radiotherapy (IMRT) that can be delivered on a conventional linear accelerator using a conventional multileaf collimator. In a previous work, we demonstrated that our arc-sequencing algorithm can produce highly conformal IMAT plans. Through plan comparisons, we explored the ability of IMAT to serve as an alternative to helical tomotherapy. Methods and Materials The IMAT plans were created for 10 patients previously treated with helical tomotherapy. Treatment plan comparisons, according to the target dose coverage and critical structure sparing, were performed to determine whether similar plan quality could be achieved using IMAT. Results In 8 of 10 patient cases, IMAT was able to provide plan quality comparable to that of helical tomotherapy. In 2 of these 8 cases, the use of non-axial coplanar or non-coplanar arcs in IMAT planning led to significant improvements in normal tissue sparing. The remaining 2 cases posed particular dosimetric challenges. In 1 case, the target was immediately adjacent to a spinal cord that had received previous irradiation. The second case involved multiple target volumes and multiple prescription levels. Both IMAT and tomotherapy were able to produce clinically acceptable plans. Tomotherapy, however, provided a more uniform target dose and improved critical structure sparing. Conclusions For most cases, IMAT can provide plan qualities comparable to that of helical tomotherapy. For some intracranial tumors, IMAT's ability to deliver non-coplanar arcs led to significant dosimetric improvements. Helical tomotherapy, however, can provide improved dosimetric results in the most complex cases.

Purpose: KV cone‐beam CTimaging(CBCT) was used to analyze setup errors when treating patients with non‐small cell lungcancer(NSCLC). For patients where an off‐line correction strategy was adopted, CBCT was also used to determine the patient specific planning target volume (PTV). Method and Materials: Eighteen patients with NSCLC were treated with CBCTimage‐guidedradiotherapy using an Elekta Synergy linac. 4D‐CT imaging was used for treatment planning. Patients were scanned in the supine position during normal relaxed free‐breathing and were immobilized with a wing‐board. The internal target volume (ITV) was determined using the maximum‐intensity‐projection (MiP) CT data to account for organ motion. A 5‐mm PTV margin was used to account for setup error. On‐line setup correction was based on the daily CBCTimaging prior to treatment. The systematic and random errors were analyzed retrospectively. The results were then fit to our CBCT off‐line correction strategy where a setup correction was made at the fifth treatment fraction to correct for the systematic error. The patient specific PTV margin was also analyzed.Results: The systematic and random errors were 5.7 ± 3.2 mm, 4.2 ± 2.6 mm and 4.8 ± 3.1 mm for anterior‐posterior (AP), medial‐lateral (ML) and superior‐inferior (SI) directions respectively. The 3D displacement vectors were 4.1 mm ± 3.1 mm (systematic ± random). If the patients were treated with the off‐line correction strategy, a patient specific PTV margin ranging from 7.0 to 17.4 mm would be needed. Conclusions: The use of a 5 mm PTV margin on the ITV determined by MiP dataset combined with daily CBCT on‐line correction is adequate to avoid geometric miss of the tumor for NSCLC patients. A larger patient specific PTV margin, ranging from 7.0 to 17.4 mm, would be needed if an off‐line correction is used.

Purpose: We have tested the capabilities of an innovative surface‐mapping system that utilizes deformable image registration tools (C‐RAD Catalyst). Its 3D positioning stability has been verified as well as its respiratory capabilities. Methods: The Catalyst daily QA phantom was used to test the stability of the system. A MatriXX ion‐chamber array in a solid water phantom was mounted on a motion platform which can provide longitudinal 1D motion.The vertical‐motion plate (respiratory plate) of the platform was used to mimic the chest movement in respiratory cycles. The respiratory plate was tracked by the Catalyst surface‐mapping system. A static 10×10 field was first delivered to establish the baseline. Gated deliveries using different gating windows (from 10% to 90% duty cycle) were also performed and the results were recorded. A comparison between the Catalyst surface tracking signal and ABC data was performed for a breast cancer patient on‐treatment. Results: Analysis of the experiment data indicates that the Catalyst is stable and reliable in detecting surface position and movement. With 50 numbers of phantom setups, the mean deviation of the shifts and rotations along all three directions was 0.6mm and 0.02 degrees, respectively. The gating capacity study suggested that respiratory gating using Catalyst system on an Elekta linac can be delivered safely and accurately. In fact, in gated beam delivery with the Catalyst system, dose distribution, dose profiles, gamma passing rates all show improvements as the gating window gets smaller. Gamma passing rates increase from 84.7% at non‐gating to 95.6% at 10% gating window. The Catalyst system also correctly recognizes the respiratory pattern of a patient under our ABC gated treatment delivery. Conclusion: Based on a phantom study and clinical observations, we have found that Catalyst system is robust in surface tracking and positioning. It also can serve as a respiratory gating solution. Funded by Elekta

Purpose: The objective of this study is to investigate the impact of the interplay effect between multi‐leaf collimator(MLC) sequences and tumor motion in the delivery of lungradiotherapy using both volumetric modulated arc therapy (VMAT) and fixed‐field intensity‐modulated radiotherapy(IMRT).Methods and Materials: For 10 patients with Stage I/II non‐small‐cell pulmonary tumors, a respiratory‐correlated four dimensional computed tomography (4DCT) study was acquired. The internal target volume was determined based on the maximum intensity projection 4DCT dataset, and a 3 to 5 mm margin was used for generation of the planning target volume. VMAT and 5‐field IMRT (step‐and‐shoot) plans were generated using Pinnacle3 SmartArc and direct machine parameter optimization (DMPO), respectively. All plans were generated for an Elekta Synergy linear accelerator using 6‐MV photons. A model was developed to simulate the interplay between MLC sequences and target movement during the delivery of VMAT and IMRT, respectively. The 4D dose distributions were calculated using a non‐rigid deformable image registration algorithm. Target volume coverage and doses to critical structures calculated using 4D methodology were compared with those calculated using 3D methodology. Results: For all patients included in this study, the interplay effect was found to present limited impact (less than 0.5%) on the target dose distribution, especially for stereotactic body radiation therapy, in which fewer fractions (3∼5) are delivered. The dose to the GTV was slightly decreased on average (1% of prescription) in the 4D calculation compared with the 3D calculation. The motion impact on target dose homogeneity was patient dependent and relatively small. Negligible effects were observed on the doses to critical structures. Conclusions: Both VMAT and IMRT plans experienced negligible interplay effects between MLC sequence and tumor motion. For the most part, the 3D doses to the GTV and critical structures provided good approximations of the 4D dose calculations.

Purpose: In this work, we report on our initial testing, commissioning, and clinical implementation of the Elekta's VMAT planning solution in the Monaco treatment planning system. Monaco VMAT was released clinically in February 2010. Method and Materials: Monaco is an IMRT‐only treatment planning system that was recently updated to include an inverse planning module for VMAT. VMAT plans are produced using a two‐step process. First, optimized fluence maps are determined from each beam direction. Next, the optimized maps are translated into a deliverable VMAT sequence. VMAT plans are delivered using a sweeping window technique where the leaves move unidirectionally across the field in alternating directions over the course of each arc. We have tested the Monaco VMAT system for a variety of treatment sites including intracranial, prostate, and head‐and‐neck. Initially five cases have been planned and verified. For each case, comparisons were made with both fixed field IMRT and VMAT plans produced using the Philips SmartArc planning module. Plan verifications were performed using phantom irradiations with the IBA MatriXX 2D ion chamber array embedded in a MULTIcube phantom. Results: On average, single arc Monaco VMAT plans delivered in 4 minutes 24 seconds. Plans verified with an average of 99.6% of the points passing a gamma analysis using a 3%/3mm distance to agreement criteria. For complex head and neck cases, Monaco produced VMAT plans delivered using a hybrid technique combining rotational and fixed field delivery. The hybrid delivery technique provided greater flexibility in shaping the dose distributions but significantly lengthened the deliver times. Conclusion: Elekta's Monaco VMAT planning tools provides a full inverse planning solution for VMAT. All of our plans have delivered accurately and the commissioning process has been completed. We anticipate commencing Monaco VMAT treatments in March 2010. Research sponsored in part through a grant from Elekta.

Purpose: Volume Metric Arc Therapy (VMAT) has received considerable attention due to its ability to provide improved plan quality and delivery efficiency relative to fixed field IMRT. However, due to the limitation of leaf motion and number of control points, the VMAT plan is not always better than fixed field IMRT. This work investigated the cases that IMRT is superior to VMAT on plan quality. Material and Methods: The fixed field IMRT and VMAT plans are produced using CMS XiO and Pinnacle3 SmartArc, respectively. The RT doses are exported to a third party software where the dose volume histogram (DVH) and isodose lines are compared. Results: Fifty clinical cases are investigated. The treatment sites include head‐and‐neck, partial‐brain, lung, pancreas, prostate and pelvis. Three out of total 50 cases show that the IMRT plan quality is better than that of VMAT. The first case is a partial brain. Both IMRT and VMAT plans have similar target coverage. However, the IMRT plan has lower mean doses to the whole brain, which can be attributed to fewer gantry angles used in IMRT. The second case is a mesothelioma, in which large and sharp shrink between target contours are found. The IMRT plan is better on the target conformality. It is more difficult for the VMAT plan to create sharp shrink of isodose line to follow the target contour. The third case is a pelvis case in which again sharp corner exists in target contours. IMRT is superior on the target conformality. Conclusion: Though for most cases, the VMAT plans are better than fixed field IMRT, there are some situations where IMRT can provide better comformality and lower dose to the volume not included in the organ in risks list. Research supported by Elekta.

Purpose: In this study, we compare the plan quality, treatment delivery efficiency and accuracy of VMAT and fixed‐field IMRT. VMAT plans were created using both anatomy‐based and fluence‐based inverse planning techniques. Method and Materials: Nine cases including 3 prostate, 3 pancreas, 2 head and neck (HN) and 1 brain were selected for this study. The patients were previously treated with step‐and‐shoot IMRT plans developed in the CMS Xio treatment planning system (TPS). For each case, two VMAT plans were generated retrospectively. The first VMAT plan was generated using ERGO++ R1.7 TPS (Elekta), which uses an anatomy‐based inverse planning method. The second VMAT plan was generated by converting the optimized fluence maps calculated by the Pinnacle3 TPS into deliverable arcs using an in‐house arc sequencer. VMAT plans were verified on an Elekta Synergy and plan comparisons were made in terms of delivery efficiency and accuracy. Results: For pancreatic, prostate and brain cases, anatomy‐based VMAT provided comparable target coverage and sparing of OARs as compared with fluence‐based VMAT and fixed‐field IMRT. For HN, fluence‐based VMAT improved target coverage and sparing of OARs compared with anatomy‐based VMAT and IMRT. For the 9 cases tested, the mean delivery time for the fluence‐based VMAT was 20% less than the anatomy‐based VMAT and 78% less than IMRT. The number of monitor units for the anatomy‐based VMAT was approximately 10% less than fluence‐based VAMT and 56% less than IMRT. The average passing rates for the plan QAs were 98%, 99% and 91% for IMRT, fluence‐based VMAT, and anatomy‐based VMAT, respectively. Conclusion: VMAT is able to reduce treatment times by 70% while producing equivalent or better dose distributions as fixed‐field IMRT. For challenging HN cases, VMAT using a fluence‐based inverse planning method improves the dose distribution quality as compared with IMRT and anatomy‐based VMAT. Research supported by Elekta

Purpose: To evaluate the effect of VMAT sequencer, MLC leaf motion range and MU difference for adjacent control points in VMAT plan, and gantry interval used for dose calculation on the treatment delivery accuracy. Material and Methods: VMAT plans with leaves moving across the field in alternate open‐close pattern for every 10 degrees of rotation were created on Pinnacle SmartArc 8.1v and Ergo++ V1.7.1, where VMAT DICOM RT plans were generated with different sequencers. Various MLC motion range and MU with different ratios were assigned to the adjacent control points. The plans were delivered on Elekta Synergy. Measurements with Matrixx ion‐chamber array were analyzed. Gamma passing rate were evaluated using dose calculated by interpreting the MU and MLC shapes between control points with different gantry intervals. Results: DICOM plans transferred from SmartArc had higher gamma passing rate than those from Ergo++. Passing rate decreased as the MU ratio increased for Ergo++ because its VMAT sequencer averaged the MU from two adjacent control points. Less than 2% difference was observed between dose calculated with 1 and 2 degree gantry interval. Compared to plans with 2.5cm MLC motion, passing rate for 5.0cm plans decreased by 5–15%. Verification results degraded as MU ratio increased from 5:5 to 1:9 if dose was calculated using the gantry interval of 2 degrees or larger. Gamma pass rate decreased as the calculation interval increased from 2 to 10 degrees for VMAT plans were calculated assuming MLC leaves were stationary between control points. Conclusion: Smaller MLC leaf motion range can improve VMAT delivery accuracy. A sequencer capable of keeping original MU for each control point should be used to generate VMAT DICOM RT plan. Fine gantry interval should be used for dose calculation to show the VMAT plan that reflects the delivered dose accurately. Research supported by Elekta.

Purpose: We have evaluated a technique for incorporating intrafraction organ motion into IMRTtreatment planning using Monte Carlo based dose engine that builds the pattern of respiratory‐induced anatomical displacement into the dose calculation. Method and Materials: For the lungcancer patients included in this study, a patient specific 3D tumor trajectory is derived using 4D CTimages by modeling the tumor as a rigid body and by performing a parametric fit to the center‐of‐mass of the tumor volumes. The patient specific tumor trajectories were then used to create the optimized plans. The optimized plans were compared with plans produced using traditional margin expansion. In addition, measurements were performed with a moving phantom programmed for sinusoidal motion to verify the accuracy of this planning approach. Results: For the three lung patients in this study, significantly improved normal tissue sparing was observed in the plans that incorporated intrafraction organ motion. On average, the motion‐based plans provided a 40.8% reduction in the volume of the involved lung receiving 80% or more of the prescribed dose. Verification measurements performed with a moving phantom demonstrated a significant improvement in the agreement between the planned and measured doses. Conclusion: In contrast to traditional margin expansion, our proposed technique resulted in a significant sparing of critical structures while providing similar target coverage. In addition, the technique obviates the need for gating or tracking.

Purpose: The Calypso® 4D Localization System™ (Calypso Medical) uses non‐ionizing ACelectromagnetic radiation to localize and track small wireless devices (called Beacon® transponders) implanted in or near a patient's tumor. We report on the first clinical experience with the use of the system for localizing and tracking the lumpectomy cavity during external‐beam accelerated partial breast irradiation (EB APBI). Method and Materials: The study included patients treated receiving EB APBI on an IRB approved protocol. Thirteen patients were implanted with both gold markers (GM) and beacon® transponders and two patients were implanted with beacon® tranponders alone. For patients in whom MRI follow‐up was anticipated, two removable interstitial breast catheters were inserted and afterloaded with gold markers and transponders. The catheters were removed post radiation therapy. Initial alignment was performed using lasers. For patients with gold markers, orthogonal images were used to obtain the necessary shift. The shift values were compared to the shift predicted under electromagnetic guidance. During treatment, Calypso was used to track the target motion. Results: Fifteen patients have been studied, and 93 treatment fractions were analyzed. The catheters and transponders overall showed good stability with inter‐transponder distance changes of less than 2 mm. Calypso based setup can be performed in less than 2 minutes. An average residual setup error of 10.29 mm was determined using gold markers. For the 63 fractions analyzed, the difference between the residual setup error determined by the GM and the Calypso system on average was 1.5 mm. Tracking showed regular motion in the range of 2–3 mm with occasional deeper breaths exceeding 4–5 mm. Conclusion: Results show excellent agreement between gold markers and electromagnetic guidance in EB APBI with electromagnetic guidance providing a more rapid setup and real time tracking during delivery. Research sponsored by Calypso Medical.

Purpose: Helical Tomotherapy and Volumetric Modulated Arc Therapy (VMAT) are arc‐based approaches to IMRTdelivery. The objective of this study is to compare VMAT and helical tomotherapy in terms of plan quality, delivery efficiency and accuracy. Method and Materials: Twelve cases including 4 prostate, 4 head & neck, and 4 lung were selected for this study. Tomotherapy plans were developed using a Hi‐Art II planning station. VMAT plans were generated using both the Pinnacle3 SmartArc IMRT module and our home‐grown arc‐sequencing algorithm. The same set of CTimages and contours were used for both VMAT and tomotherapy plans. The percentage volume of the PTV that received 95% of the prescribed dose (V95) was used to compare the target coverage. VMAT and tomotherapy plans were delivered using Elekta's PreciseBeam VMAT® control system and TomoTherapy HI_ART II system, respectively. VMAT plan QA was performed using the IBA MatriXX system, and an ion‐chamber and films were used for tomotherapy plan QA. Results: For prostate cases, VMAT provided slightly improved V95 (98.1% vs. 97.6%) and lower dose to the rectum and the bladder. Tomotherapy attained slightly higher target coverage and uniformity in HN cases, with improved V95 (98.9% vs. 98.3%) and reduced standard deviation of the PTV dose (1.2 Gy vs. 1.4 Gy). In lung cases, VMAT provided similar target coverage with improved sparing of the cord and total lung. In terms of delivery efficiency, VMAT plans on average took 2.2 minutes for prostate and lung cases, and 5.4 minutes for HN cases. These values increase to 4.8 and 7.6 minutes for tomotherapy plans. Both VMAT and tomotherapy plans can be delivered accurately based on their own QA standard. Conclusion: VMAT is able to provide ∼40% reduction in treatment time while maintaining comparable plan quality to that of helical tomotherapy. Research supported by Elekta.

Purpose: With the commercial introduction of deliverycontrol systems for Volumetric Modulated Arc Therapy (VMAT), the need has arisen for reliable and efficient techniques for performing patient specific VMAT quality assurance (QA). In this work, we have studied three patient specific QA techniques for VMAT: ion chamber with film, 2D diode array, and a 2D ion chamber array. Materials and Methods: The three QA techniques we have utilized are: (1) a stack of solid water slabs with an inserted ion chamber and film sandwiched between two slabs; (2) a 2D diode array (MapCHECK™ device inserted into a MapPHAN™ phantom); and (3) a 2D ion chamber array (MatriXX™ inserted into a MULTICube™ Phantom). Ten VMAT plans were delivered to all three QA systems on an Elekta Synergy linac.Results: With the highest spatial resolution among all three systems, film measurements can provide very good QA results when analyzed in relative mode. Absolute dose comparisons were performed for both the MapCHECK™ and MatriXX™ systems. The average passing rate in gamma analysis were 95.0% and 98.5% using 3%/3mm criteria for the above two systems, respectively. The slightly lower passing rate for MapCHECK™ QA may be attributed to the angular and dose rate dependence of the diode response. It is also observed that the MapCHECK™ QA is more sensitive to the tongue‐&‐groove effect when diodes fall between two leaves. The MatriXX™ system provides slightly higher QA passing rates. However it may be less sensitive to large dose variation within small regions due to its 7.62mm detector grid size. Conclusions: All three systems can be used for VMAT plan QA provided users are attentive about the strengths and limitations of the QA device. Research sponsored by Elekta Corporation.

Purpose: Volumetric Modulated Arc Therapy (VMAT) is an arc‐based technique that utilizes dynamically modulated arcs to deliver intensity‐modulated radiation therapy(IMRT)treatments on a conventional linear accelerator. In this work, we evaluate VMAT as a treatment technique for patients with carcinomas of the head‐and‐neck. Method and Materials: Five complex head and neck cancer patients, with multiple prescription levels, were selected for this study. Fully inverse planned VMAT plans were optimized for these patients using our homegrown arc‐sequencing software. The software uses simulated annealing to optimize the aperture shapes and weights while minimizing the differences between the optimized (ideal) and sequenced intensities. The optimized plans were compared with step‐and‐shoot IMRT plans generated using the Pinnacle3treatment planning system. VMAT plan verifications have also been performed using Elekta's Precise Beam Infinity™ control system which has been installed on an Elekta Precise linear accelerator in our clinic. Results: Using our arc‐sequencing tool, VMAT can be used to create highly conformal head‐and‐neck treatment plans. As compared with traditional fixed‐field plans, VMAT was able to reduce the average parotid dose from 85.3 cGy to 73.6 cGy per fraction. Additionally, the average number of monitor units was reduced from 1058.3 to 502.3 per fraction. Initial delivery tests demonstrate that using VMAT complex head‐and‐neck deliveries can be completed in under 6 minutes. Conclusion: VMAT should serve as an important tool in the delivery of radiation therapy for head‐and‐neck carcinomas. By utilizing the dosimetric advantages of rotational IMRT, VMAT can provide more uniform target doses and reduced critical structure doses as compared with fixed field IMRT.Conflict of Interest: Research partially sponsored by Elekta Corporation.

Purpose:Linear accelerator vendors have recently introduced deliverycontrol systems with the ability to deliver Volumetric Modulated Arc Therapy (VMAT). These systems can deliver VMAT using either a single‐arc or a multi‐arc approach. The goal of this study is to compare single‐arc and multi‐arc delivery techniques in terms of both plan quality and delivery efficiency. Method and Materials: Two prostate cases and three head‐and‐neck cases were included in this study. For each case, single‐arc and three‐arc VMAT plans were generated using our home‐grown arc sequencing algorithm that converts optimized fluence maps into deliverable arcs. Each VMAT plan was evaluated using Pinnacle's superposition dose calculation, and DVH comparisons were made between the two sets of VMAT plans. VMAT deliveries using Elekta's PreciseBeam Infinity™ control system were performed for each patient to compare the delivery efficiency. Results: For the two prostate cases, the single‐arc and three‐arc VMAT plans resulted in similar target dose coverage and organ at risk (OAR) sparing. For the three head‐and‐neck cases, the three‐arc plans provided improved target dose coverage as compared with the single‐arc plans with the average standard deviation in the target dose reduced from 7.06 to 5.82 cGy. The target volume covered by 95% of the prescribed dose also increased from 97.5% to 98.5% for the three‐arc VMAT plans. For single‐arc VMAT delivery, the average delivery time was approximately 3.2 minutes for prostate cases and 5 minutes for head‐&‐neck cases. While for three‐arc VMAT delivery, these values increased to 4.5 and 5.6 minutes. Conclusion: For simple cases such as prostate, single arc delivery provides comparable plan quality and improved delivery efficiency. However, for more complex cases, multi‐arc VMAT plans are preferable due to the ability to achieve improved dose conformity while maintaining acceptable treatment times. This work was sponsored in part through a grant from Elekta.

Purpose: The Calypso 4D tracking system can detect real‐time intra‐fractional tumor motion based on RF signals from the implanted beacons. The initial clinical experience on tracking prostate motion during the treatment is reported. We performed a series of tests to evaluate the calibration accuracy and stability of the system. Material and Methods: The first FDA approved clinical Calypso 4D tracking system was installed and used for this work. With three beacons implemented in the prostate, the system reports the target location in a real time fashion. Seven patients with prostate cancer were localized/tracked using this system. The vender provided a monthly calibration phantom and an in‐house developed stereotactic cone based alignment tool that were used to test the calibration accuracy of the Calypso 4D tracking system. The system stability was also studied using the monthly calibration phantom. Results: For the seven patients treated so far, the average initial setup shifts from the external tattoos were 1.1, 2.2 and 4.0 mm along lateral, longitudinal and vertical directions, respectively. For one patient, the observed maximum shifts along the above three axes were 4.6, 10.9 and 29.2 mm. The range of target motion during treatment was found to be 3.4 (Right‐Left), 9.9 (Superior‐Inferior) and 12.3 (Anterior‐Posterior) mm. The average patient setup time was less then 5 minutes. The daily QA of the system takes around 5 minutes on average. The vender provided system calibration procedure is based on room lasers with 2 mm error tolerance, which may potentially decrease the overall system accuracy. Our stereotactic cone based alignment tool can achieve sub‐millimeter accuracy. Repeated monthly calibration procedures showed that the system stability is within 1 mm. Conclusion: Calypso system provides efficient and accurate patient setup for prostate treatment. The current calibration procedure can be improved to achieve better accuracy.

Purpose: A common practice in lungradiotherapy is the correction of setup errors based on the initial and weekly 2D portalimages. We have evaluated the effectiveness of this correction strategy using data collected from daily kV cone‐beam CT(CBCT)imaging.Material and Method: Eighteen lungcancer patients immobilized with a wing board were treated on an Elekta Synergy LINAC using our daily on‐line IGRT protocol. Daily CBCTimaging was performed and setup variations were corrected before treatment. A total of 282 CBCTimages were collected and the daily setup errors along the anterior‐posterior, superior‐inferior and right‐left axes were calculated using volumetric image registration. With this data we simulated the weekly correction strategy where patient setup error is adjusted once every five fractions based on weekly 2D portalimaging after the initial setup. 3D shift vectors for the whole treatment course were calculated. The frequencies with which the 3D shifts exceeded 5 mm, 10 mm, 15 mm and 20 mm were used to evaluate the effectiveness of this correction strategy. Results: For these eighteen patients, the setup errors along right‐left, superior‐inferior and anterior‐posterior axes range from −1.17 to 1.34 cm, −1.29 to 1.30 cm and −1.71 to 1.21 cm respectively. The maximum value of the 3D shift vectors was 1.78 cm. Out of a total of 282 treatments, the 3D shift vectors were within 5 mm for 144 fractions (51.1%), within 10 mm for 250 fractions (88.7%), and within 15 mm for 278 fractions (98.6%). Four fractions had errors greater than 15 mm. Conclusion: Daily CBCT data can be used to evaluate different correction strategies. For the lung patients, the setup errors can be large along R‐L, S‐I and A‐P directions. A large margin is warranted to ensure adequate tumor coverage if weekly correction scheme is used.

Purpose: For treatment planning, clinicians typically compensate for respiratory motion by applying treatment margins that ensure the clinical target volume (CTV) always remains in the treatment field. We have investigated a more sophisticated approach where the pattern of organ motion is directly incorporated into both the dose calculation process and the optimization of IMRT treatment plans, and we have evaluated the degree to which plan quality can be improved by accounting for intra‐fraction organ motion during IMRT optimization. Method and Materials: In this study, we employed a convolution/superposition based dose calculation that incorporates organ motion. For each photon history, the isocenter is randomly sampled from a probability distribution defined by the respiration‐induced anatomical displacement. The resulting blurred pencil beam dose distributions have been incorporated into our IMRT plan optimizations. Comparisons were made between plans optimized using a motion‐based dose calculation and those optimized using a static dose calculation. Tests were performed for both a concave target in a phantom and a lungcancer patient. Results: For the phantom case, the mean dose to the adjacent organ‐at‐risk was reduced from 67.3% to 48.4% of the prescribed dose when motion was included in the optimization. For the lung patient, the volume of the right lung receiving greater than 80% of the prescribed dose was reduced from 14.1% to 7.2% when organ motion was included in the optimization. Conclusion: The motion‐based optimization accounts for the fact that the CTV will not be at all locations with equal probability and is thus more sophisticated than the application of patient specific margins. Consequently, significant additional sparing of critical structures can be achieved by incorporating intra‐fraction organ motion into IMRT optimization. Additionally, the agreement between the predicted and delivered doses is improved because the impact of organ motion has been accounted for in the dose calculation.