Original Volumetric Modulated Arc Therapy Plans Research Articles

Dosimetric potential of knowledge-based planning model trained with HyperArc plans for brain metastases.

Dosimetric potential of knowledge-based RapidPlan planning model trained with HyperArc plans (Model-HA) for brain metastases has not been reported. We developed a Model-HA and compared its performance with that of clinical volumetric modulated arc therapy (VMAT) plans. From 67 clinical stereotactic radiosurgery (SRS) HyperArc plans for brain metastases, 47 plans were used to build and train a Model-HA. The other 20 clinical HyperArc plans were recalculated in RapidPlan system with Model-HA. The model performance was validated with the 20 plans by comparing dosimetric parameters for normal brain tissue between clinical plans and model-generated plans. The 20 clinical conventional VMAT-based SRS or stereotactic radiotherapy plans (CL-VMAT) were reoptimized with Model-HA (RP) and HyperArc system (HA), respectively. The dosimetric parameters were compared among three plans (CL-VMAT vs. RP vs. HA) in terms of planning target volume (PTV), normal brain excluding PTVs (Brain-PTV), brainstem, chiasm, and both optic nerves. In model validation, the optimization performance of Model-HA was comparable to that of HyperArc system. In comparison to CL-VMAT, there were no significant differences among three plans with respect to PTV coverage (p>0.17) and maximum dose for brainstem, chiasm, and optic nerves (p>0.40). RP provided significantly lower V20Gy , V12Gy , and V4Gy for Brain-PTV than CL-VMAT (p<0.01). The Model-HA has the potential to significantly reduce the normal brain dose of the original VMAT plans for brain metastases.

Usability and necessity of a novel hybrid radiation therapy technique based on volumetric modulated arc therapy (VMAT) in stage III lung cancer treatment

PurposeIt is occasionally difficult to deliver a sufficient radiation dose using existing 3-dimensional conformal radiation therapy (3DCRT), intensity modulated radiation therapy (IMRT), or volumetric modulated arc therapy (VMAT) techniques in stage III lung cancer patients. To address this problem, we developed a new hybrid VMAT (hVMAT) technique. This study aimed to demonstrate the usability of the new hVMAT technique in stage III lung cancer treatment. Methods and materials32 consecutive patients with stage III lung cancer who had been treated with hVMAT were included. The hVMAT plan was made by adding two static beams to the original VMAT plan. To evaluate the superiority of the hVMAT plan, we compared the three techniques (3DCRT vs. VMAT vs. hVMAT) using the conformity index (CI), conformation number (CN), homogeneity index (HI), and the dosimetric parameters of organs at risk (OARs). ResultsThe mean V20 of the lung from the hVMAT plans (27.2%) was significantly lower than 3DCRT plans (40.5%) and VMAT plans (35.0%). Additionally, the hVMAT plans showed the best results for other lung dose parameters (V5, V10, mean lung dose) among the three plans, while maintaining an irradiated dose at an acceptable level for the spinal cord, esophagus, and heart. In addition, hVMAT also showed non-inferior results for CI, CN, and HI compared with the VMAT plans. ConclusionThis novel hybrid technique can improve all dosimetric parameters of lung while maintaining the acceptable dose for other OARs. Therefore, the new hVMAT could be a useful planning technique for treating stage III lung cancer and it might be a solution for patients who cannot receive a sufficient dose with the existing 3DCRT or VMAT plans.

Correlation of the gamma passing rates with the differences in the dose-volumetric parameters between the original VMAT plans and actual deliveries of the VMAT plans

PurposeThe aim of this study was to investigate the correlations of the gamma passing rates (GPR) with the dose-volumetric parameter changes between the original volumetric modulated arc therapy (VMAT) plans and the actual deliveries of the VMAT plans (DV errors). We compared the correlations of the TrueBeam STx system to those of a C-series linac.MethodsA total of 20 patients with head and neck (H&N) cancer were retrospectively selected for this study. For each patient, two VMAT plans with the TrueBeam STx and Trilogy (C-series linac) systems were generated under similar modulation degrees. Both the global and local GPRs with various gamma criteria (3%/3 mm, 2%/2 mm, 2%/1 mm, 1%/2 mm, and 1%/1 mm) were acquired with the 2D dose distributions measured using the MapCHECK2 detector array. During VMAT deliveries, the linac log files of the multi-leaf collimator positions, gantry angles, and delivered monitor units were acquired. The DV errors were calculated with the 3D dose distributions reconstructed using the log files. Subsequently, Spearman’s rank correlation coefficients (rs) and the corresponding p values were calculated between the GPRs and the DV errors.ResultsFor the Trilogy system, the rs values with p < 0.05 showed weak correlations between the GPRs and the DV errors (rs<0.4) whereas for the TrueBeam STx system, moderate or strong correlations were observed (rs≥0.4). The DV errors in the V20Gy of the left parotid gland and those in the mean dose of the right parotid gland showed strong correlations (always with rs > 0.6) with the GPRs with gamma criteria except 3%/3 mm. As the GPRs increased, the DV errors decreased.ConclusionThe GPRs showed strong correlations with some of the DV errors for the VMAT plans for H&N cancer with the TrueBeam STx system.

Enhanced optimization of volumetric modulated arc therapy plans using Monte Carlo generated beamlets.

A treatment planning system (TPS) produces volumetric modulated arc therapy (VMAT) plans by applying an optimization process to an objective function, followed by an accurate calculation of the final, deliverable dose. However, during the optimization step, a rapid dose calculation algorithm is required, which reduces its accuracy and its representation of the objective function space. Monte Carlo (MC) routines, considered the gold standard in accuracy, are currently too slow for practical comprehensive VMAT optimization. Therefore, we propose a novel approach called enhanced optimization (EO), which employs the TPS VMAT plan as a starting point, and applies small perturbations to nudge the solution closer to a true objective minimum. The perturbations consist of beamlet dose matrices, calculated using MC routines on a distributed-computing framework. DICOM files for clinical VMAT plans files are exported from the TPS and used to generate input files for the EGSnrc MC toolkit. Beamlet doses are calculated using the MC routines, each corresponding to a single multileaf collimator leaf from a single control point traveling 0.5cm in or out of the field. A typical VMAT plan requires 5000 to 10000 beamlets, which may be calculated overnight. This results in a ternary-valued objective function, which may use the same clinical objectives as the original VMAT plan. A simple greedy search algorithm is applied to minimize this function and determine the optimal set of ternary variables. The resulting modified control point parameters are imported into the TPS to calculate the final, deliverable dose, and to compare the EO plan with the original. EO was evaluated retrospectively on seven VMAT plans (two adult brain, one pediatric brain, two head and neck, and two prostate). Additionally, the use of stricter objectives was investigated for two of the cases: the left cochlea planning organ at risk (OAR) volume objective for the pediatric brain case, and the rectum objective for a prostate case. EO produced improved objective scores (by 6% to 60%) and dose-volume histograms (DVH) for the brain plans and the head and neck plans. For each of these plans, the target dose minimum and homogeneity were preserved, while one or more of the OAR DVH's was reduced. Although EO also reduced the objective scores for the prostate plans (by 46% and 79%), their absolute score and DVH improvements were not substantial. The stricter objective on the pediatric brain case resulted in lower dose to the OAR without compromising the target dose. However, the rectum dose in the prostate case could not be improved without reducing dose homogeneity to the planning target volume, suggesting that VMAT prostate cases may already be highly optimized by the TPS. We have developed a novel approach to improving the dose distribution of VMAT plans, which relies on MC calculations to provide small modifications to the control points. This method may be particularly useful for complex treatments in which a certain OAR is of concern and it is difficult for the treatment planner to obtain an acceptable solution with the TPS. Further development will reduce the beamlet computation time and result in more sophisticated EO treatment planning methods.

Dosimetric impact of baseline drift in volumetric modulated arc therapy with breath holding.

We investigated the change of dose distributions in volumetric modulated arc therapy (VMAT) under baseline drift (BD) during breath holding. Ten VMAT plans recalculated to a static field at a gantry angle of 0° were prepared for measurement with a 2D array device and five original VMAT plans were prepared for measurement with gafchromic films. These measurement approaches were driven by a waveform reproducing breath holding with BD. We considered breath holding times of 15 and 10 s, and BD at four speeds; specifically, BD0 (0 mm/s), BD0.2 (0.2 mm/s), BD0.3 (0.3 mm/s), and BD0.4 (0.4 mm/s). The BD was periodically reproduced from the isocenter along the craniocaudal direction and the shift during breath holding (ShiftBH) ranged 0-6 mm.The dose distribution of BD0.2, BD0.3 and BD0.4 were compared to that of BD0 using gamma analysis with the criterion of 2%/2 mm. The mean pass rates of each ShiftBH were 99.8% and 98.9% at 0 mm, 96.8% and 99.4% at 2 mm, 94.9% and 98.6% at 3 mm, 91.5% and 98.4% at 4 mm, 70.8% and 94.1% at 4.5 mm, and 55.0% and 83.6% at 6 mm for the array and film measurements, respectively. We found significant differences in ShiftBH above 4 mm (ρ < 0.05). Hence, it is recommended that breath holding time should be shortened for patients to preserve the reproducibility of dose distributions.

A sliding-window approach for improved VMAT dose calculation accuracy

The continuous delivery of volumetric modulated arc therapy (VMAT) plans is usually approximated by discrete apertures at evenly-spaced gantry angles for dose calculation purposes. This approximation can potentially lead to large dose calculation errors if the gantry angle spacings are large and/or there are large changes in the MLC apertures from one control point (CP) to the next. In this work, we developed a sliding-window (SW) method to improve VMAT dose calculation accuracy. For any 2 adjacent VMAT CPs ni and ni + 1, the dose distribution was approximated by a 2-CP SW IMRT beam with the starting MLC positions at CP ni and ending MLC positions at CP ni + 1, with the gantry angle fixed in the middle of the 2 VMAT CPs. Therefore, a VMAT beam with N CPs was approximated by a SW plan with N-1 SW beams. To validate the method, VMAT plans were generated for 10 patients in Pinnacle using 4° gantry spacing. Each plan was converted to a SW plan and dose was recalculated. Another VMAT plan, with 1° gantry spacing, was created by interpolating the original VMAT beam. The original plans were delivered on an Elekta Versa HD and measured with ArcCHECK. For both the isodose distribution and DVH, there were significant differences between the original VMAT plan and either the SW or the interpolated plan. However, they were indistinguishable between the SW and the interpolated plans. When compared with measurement, the average passing rates of the original VMAT plans were 87.3 ± 2.8% and 93.1 ± 1.0% for the 5 HN and 5 spine SBRT cases, respectively. On the other hand, the passing rates for both the VMAT1 and SW plans were above 95% for all the 10 cases studied. The dose calculation times of the original VMAT plans and the SW plans were very similar. We conclude that the proposed SW approach improves VMAT dose calculation accuracy without increase in dose calculation time.

Sparing Cardiac Substructures With Optimized Volumetric Modulated Arc Therapy and Intensity Modulated Proton Therapy in Thoracic Radiation for Locally Advanced Non-small Cell Lung Cancer

PurposeIncreasing radiation dose to the heart is associated with worse survival in stage III non-small cell lung cancer. We sought to evaluate the ability of optimized volumetric modulated arc therapy (VMAT) and intensity modulated proton therapy (IMPT) to spare cardiac substructures. We also wanted to determine how a cardiac optimization treatment planning algorithm influences dose distribution to other thoracic organs at risk (OARs). Methods and MaterialsCardiac substructures were retrospectively contoured for all patients with stage III non-small cell lung cancer who were treated at our institution with VMAT to 60 Gy in 2-Gy fractions. The structures included valves, atrioventricular node, coronary arteries, chambers, and great vessels. New cardiac-optimized VMAT plans were created to spare these structures while preserving planning target volume coverage and maintaining standard dose constraints to OARs. Dosimetry variables for the new cardiac-optimized VMAT plans were compared via paired t test with the original VMAT plans. IMPT plans were also created, and the cardiac-optimized VMAT plans were then similarly compared with the IMPT plans. ResultsTwenty-six patients who were treated from July 2013 to September 2017 were included. Compared with the original VMAT plans, statistically significant improvements were demonstrated for all cardiac structures for the new cardiac-optimized VMAT plans while maintaining or improving appropriate lung, esophagus, and spinal cord constraints and planning target volume coverage goals. Compared with cardiac-optimized VMAT, IMPT demonstrated additional statistically significant improvements for some cardiac dosimetry metrics while maintaining or improving other thoracic OAR constraints. ConclusionsVMAT is now widely available, and high-quality VMAT plans that incorporate cardiac substructures into the optimization process can provide overall improvements in dose to OARs and, in particular, substantial sparing of critical cardiac structures. IMPT provides some incremental dosimetric improvements beyond cardiac-optimized VMAT, the clinical significance of which remains uncertain.

Modulation indices and plan delivery accuracy of volumetric modulated arc therapy.

PurposeWe evaluated the performance of various modulation indices (MI) for volumetric modulated arc therapy (VMAT) to predict plan delivery accuracy.MethodsThe specific indices evaluated were MI quantifying the mechanical uncertainty (MI t), MI quantifying the mechanical and dose calculation uncertainties (MI c), MI for station parameter optimized radiation therapy (MISPORT), modulation complexity score for VMAT (MCS v), leaf travel modulation complexity score (LTMCS), plan averaged beam area (PA), plan averaged beam irregularity (PI), plan averaged beam modulation (PM), and plan normalized monitor unit (PMU) to predict VMAT delivery accuracy. By utilizing 240 VMAT plans generated with the Trilogy and TrueBeam STx, Spearman's rank correlation coefficients (r) were calculated between the MIs and measures of conventional methods.ResultsFor the Trilogy system, MI c showed the highest r values with gamma passing rates (GPRs) (r = −0.624 with P < 0.001 for MapCHECK2 and r = −0.655 with P < 0.001 for ArcCHECK). For TrueBeam STx, MI c also showed the highest r values with GPRs (r = −0.625 with P < 0.001 for the MapCHECK2 and r = −0.561 with P < 0.001 for the ArcCHECK). The MI t and MI c showed the highest r values to the MLC position errors for the Trilogy and TrueBeam STx systems (r = 0.770 with P < 0.001 and r = 0.712 with P < 0.001, respectively). The PA showed the highest percent of r values (P < 0.05) to differences in the dose‐volume parameters between original VMAT plans and actual deliveries for the Trilogy systems (30.9%). Both the MI t and MI c showed the highest percent of r values (P < 0.05) to differences in the dose‐volume parameters between original VMAT plans and actual deliveries for the TrueBeam STx systems (31.8%).ConclusionTo comprehensively review the results, the MI c showed the best performance to predict the VMAT delivery accuracy.

Reliability of the gamma index analysis as a verification method of volumetric modulated arc therapy plans

BackgroundWe investigate the gamma passing rate (GPR) consistency when applying different types of gamma analyses, linacs, and dosimeters for volumetric modulated arc therapy (VMAT).MethodsA total of 240 VMAT plans for various treatment sites, which were generated with Trilogy (140 plans) and TrueBeam STx (100 plans), were retrospectively selected. For each VMAT plan, planar dose distributions were measured with both MapCHECK2 and ArcCHECK dosimeters. During the planar dose distribution measurements, the actual multileaf collimator (MLC) positions, gantry angles, and delivered monitor units were recorded and compared to the values in the original VMAT plans to calculate mechanical errors. For each VMAT plan, both the global and local gamma analyses were performed with 3%/3 mm, 2%/2 mm, 2%/1 mm, 1%/2 mm, and 1%/1 mm. The Pearson correlation coefficients (r) were calculated 1) between the global and the local GPRs, 2) between GPRs with the MapCHECK2 and the ArcCHECK dosimeters, 3) and between GPRs and the mechanical errors during the VMAT delivery.ResultsFor the MapCHECK2 measurements, strong correlations between the global and local GPRs were observed only with 1%/2 mm and 1%/1 mm (r > 0.8 with p < 0.001), while weak or no correlations were observed for the ArcCHECK measurement. Between the MapCHECK2 and ArcCHECK measurements, the global GPRs showed no correlations (all with p > 0.05), while the local GPRs showed moderate correlations only with 2%/1 mm and 1%/1 mm for TrueBeam STx (r > 0.5 with p < 0.001). Both the global and local GPRs always showed weak or no correlations with the MLC positional errors except for the GPRs of MapCHECK2 with 1%/2 mm and 1%/1 mm for TrueBeam STx and the GPR of ArcCHECK with 1%/2 mm for Trilogy (r < − 0.5 with p < 0.001).ConclusionsThe GPRs varied according to the types of gamma analyses, dosimeters, and linacs. Therefore, each institution should carefully establish their own gamma analysis protocol by determining the type of gamma index analysis and the gamma criterion with their own linac and their own dosimeter.

A planning study for evaluation of feasibility and benefit of generalized equivalent uniform dose-assisted optimization in volumetric-modulated arc therapy for recurrent head and neck cancer

Background: Re-irradiation is effective but challenging on balancing the tumor control and late complications for recurrent head and neck cancer (HNC). The aim of this study was to investigate the feasibility of incorporating the biologically based generalized equivalent uniform dose (gEUD) objective function in the volumetric-modulated arc therapy (VMAT) treatment planning optimization process and to evaluate its benefit in such a clinical dilemma. Methods: Five recurrent HNC cases were selected. Respective original VMAT plan which fulfilled acceptable constraints was created using dose-volume (DV) optimization for each case in the first phase. Based on the DV plan, multiple corresponding plans incorporating the added single Max EUD objective for the spinal cord (SC) with stepwise stricter constraint for further biological optimization were generated for dosimetric comparison. Evaluated parameters compared between DV and DV-gEUD plans included target coverage, conformity index, homogeneity index, and sparing of organs at risk (OARs). Results: When adopting SC_gEUD90% and SC_gEUD80% as Max EUD constraint of SC, DV-gEUD optimization compared to DV optimization yielded 17.1% and 20.3%, reduction of the average D max of SC, respectively (both P value <0.05), with a non-inferior target coverage, similar conformity and slightly poorer homogeneity. The planning target volume/OAR geometry might be a factor impacting the extent of benefit provided by this gEUD-assisted optimization process. Conclusions: The method we recommended as gEUD-assisted optimization is feasible and efficient to produce better plan quality in the re-irradiation setting using VMAT for HNC to diminish the risk of myelopathy. More investigations are awaited for a better understanding of the correlation between the ɑ parameter and the OAR radiobiology.

Comparison of gastric-cancer radiotherapy performed with volumetric modulated arc therapy or single-field uniform-dose proton therapy

Background: Proton-beam therapy of large abdominal cancers has been questioned due to the large variations in tissue density in the abdomen. The aim of this study was to evaluate the importance of these variations for the dose distributions produced in adjuvant radiotherapy of gastric cancer (GC), implemented with photon-based volumetric modulated arc therapy (VMAT) or with proton-beam single-field uniform-dose (SFUD) method.Material and methods: Eight GC patients were included in this study. For each patient, a VMAT- and an SFUD-plan were created. The prescription dose was 45 Gy (IsoE) given in 25 fractions. The plans were prepared on the original CT studies and the doses were thereafter recalculated on two modified CT studies (one with extra water filling and the other with expanded abdominal air-cavity volumes).Results: Compared to the original VMAT plans, the SFUD plans resulted in reduced median values for the V18 of the left kidney (26%), the liver mean dose (14.8 Gy (IsoE)) and the maximum dose given to the spinal cord (26.6 Gy (IsoE)). However, the PTV coverage decreased when the SFUD plans were recalculated on CT sets with extra air- (86%) and water-filling (87%). The added water filling only led to minor dosimetric changes for the OARs, but the extra air caused significant increases of the median values of V18 for the right and left kidneys (10% and 12%, respectively) and of V10 for the liver (12%). The density changes influenced the dose distributions in the VMAT plans to a minor extent.Conclusions: SFUD was found to be superior to VMAT for the plans prepared on the original CT sets. However, SFUD was inferior to VMAT for the modified CT sets.

Impact of pitch angle setup error and setup error correction on dose distribution in volumetric modulated arc therapy for prostate cancer

In volumetric modulated arc therapy (VMAT) for prostate cancer, a positional and rotational error correction is performed according to the position and angle of the prostate. The correction often involves body leaning, and there is concern regarding variation in the dose distribution. Our purpose in this study was to evaluate the impact of body pitch rotation on the dose distribution regarding VMAT. Treatment plans were obtained retrospectively from eight patients with prostate cancer. The body in the computed tomography images for the original VMAT plan was shifted to create VMAT plans with virtual pitch angle errors of ±1.5° and ±3°. Dose distributions for the tilted plans were recalculated with use of the same beam arrangement as that used for the original VMAT plan. The mean value of the maximum dose differences in the dose distributions between the original VMAT plan and the tilted plans was 2.98±0.96%. The value of the homogeneity index for the planning target volume (PTV) had an increasing trend according to the pitch angle error, and the values of the D 95 for the PTV and D 2ml, V 50, V 60, and V 70 for the rectum had decreasing trends (p<0.05). However, there was no correlation between differences in these indexes and the maximum dose difference. The pitch angle error caused by body leaning had little effect on the dose distribution; in contrast, the pitch angle correction reduced the effects of organ displacement and improved these indexes. Thus, the pitch angle setup error in VMAT for prostate cancer should be corrected.

Gamma-index method sensitivity for gauging plan delivery accuracy of volumetric modulated arc therapy

PurposeThe aim of this study was to investigate the sensitivity of the gamma-index method according to various gamma criteria for volumetric modulated arc therapy (VMAT). MethodsTwenty head and neck (HN) and twenty prostate VMAT plans were retrospectively selected for this study. Both global and local 2D gamma evaluations were performed with criteria of 3%/3 mm, 2%/2 mm, 1%/2 mm and 2%/1 mm. In this study, the global and local gamma-index calculated the differences in doses relative to the maximum dose and the dose at the current measurement point, respectively. Using log files acquired during delivery, the differences in parameters at every control point between the VMAT plans and the log files were acquired. The differences in dose–volumetric parameters between reconstructed VMAT plans using the log files and the original VMAT plans were calculated. The Spearman's rank correlation coefficients (rs) were calculated between the passing rates and those differences. ResultsConsiderable correlations with statistical significances were observed between global 1%/2 mm, local 1%/2 mm and local 2%/1 mm and the MLC position differences (rs = −0.712, −0.628 and −0.581). The numbers of rs values with statistical significance between the passing rates and the changes in dose–volumetric parameters were largest in global 2%/2 mm (n = 16), global 2%/1 mm (n = 15) and local 2%/1 mm (n = 13) criteria. ConclusionLocal gamma-index method with 2%/1 mm generally showed higher sensitivity to detect deviations between a VMAT plan and the delivery of the VMAT plan.

Texture analysis on the fluence map to evaluate the degree of modulation for volumetric modulated arc therapy.

Texture analysis on fluence maps was performed to evaluate the degree of modulation for volumetric modulated arc therapy (VMAT) plans. A total of six textural features including angular second moment, inverse difference moment, contrast, variance, correlation, and entropy were calculated for fluence maps generated from 20 prostate and 20 head and neck VMAT plans. For each of the textural features, particular displacement distances (d) of 1, 5, and 10 were adopted. To investigate the deliverability of each VMAT plan, gamma passing rates of pretreatment quality assurance, and differences in modulating parameters such as multileaf collimator (MLC) positions, gantry angles, and monitor units at each control point between VMAT plans and dynamic log files registered by the Linac control system during delivery were acquired. Furthermore, differences between the original VMAT plan and the plan reconstructed from the dynamic log files were also investigated. To test the performance of the textural features as indicators for the modulation degree of VMAT plans, Spearman's rank correlation coefficients (rs) with the plan deliverability were calculated. For comparison purposes, conventional modulation indices for VMAT including the modulation complexity score for VMAT, leaf travel modulation complexity score, and modulation index supporting station parameter optimized radiation therapy (MISPORT) were calculated, and their correlations were analyzed in the same way. There was no particular textural feature which always showed superior correlations with every type of plan deliverability. Considering the results comprehensively, contrast (d = 1) and variance (d = 1) generally showed considerable correlations with every type of plan deliverability. These textural features always showed higher correlations to the plan deliverability than did the conventional modulation indices, except in the case of modulating parameter differences. The rs values of contrast to the global gamma passing rates with criteria of 2%/2 mm, 2%/1 mm, and 1%/2 mm were 0.536, 0.473, and 0.718, respectively. The respective values for variance were 0.551, 0.481, and 0.688. In the case of local gamma passing rates, the rs values of contrast were 0.547, 0.578, and 0.620, respectively, and those of variance were 0.519, 0.527, and 0.569. All of the rs values in those cases were statistically significant (p < 0.003). In the cases of global and local gamma passing rates, MISPORT showed the highest correlations among the conventional modulation indices. For global passing rates, rs values of MISPORT were -0.420, -0.330, and -0.632, respectively, and those for local passing rates were -0.455, -0.490 and -0.502. The values of rs of contrast, variance, and MISPORT with the MLC errors were -0.863, -0.828, and 0.795, respectively, all with statistical significances (p < 0.001). The correlations with statistical significances between variance and dose-volumetric differences were observed more frequently than the others. The contrast (d = 1) and variance (d = 1) calculated from fluence maps of VMAT plans showed considerable correlations with the plan deliverability, indicating their potential use as indicators for assessing the degree of modulation of VMAT plans. Both contrast and variance consistently showed better performance than the conventional modulation indices for VMAT.

SU-E-T-405: Robustness of Volumetric-Modulated Arc Therapy (VMAT) Plans to Systematic MLC Positional Errors

Purpose: To evaluate the dosimetric impact of systematic MLC positional errors (PEs) on the quality of volumetric-modulated arc therapy (VMAT) plans. Methods: Five patients with head-and-neck cancer (HN) and five patients with prostate cancer were randomly chosen for this study. The clinically approved VMAT plans were designed with 2–4 coplanar arc beams with none-zero collimator angles in the Pinnacle planning system. The systematic MLC PEs of 0.5, 1.0, and 2.0 mm on both MLC banks were introduced into the original VMAT plans using an in-house program, and recalculated with the same planned Monitor Units in the Pinnacle system. For each patient, the original VMAT plans and plans with MLC PEs were evaluated according to the dose-volume histogram information and Gamma index analysis. Results: For one primary target, the ratio of V100 in the plans with 0.5, 1.0, and 2.0 mm MLC PEs to those in the clinical plans was 98.8 ± 2.2%, 97.9 ± 2.1%, 90.1 ± 9.0% for HN cases and 99.5 ± 3.2%, 98.9 ± 1.0%, 97.0 ± 2.5% for prostate cases. For all OARs, the relative difference of Dmean in all plans was less than 1.5%. With 2mm/2% criteria for Gamma analysis, the passing rates were 99.0 ± 1.5% for HN cases and 99.7 ± 0.3% for prostate cases between the planar doses from the original plans and the plans with 1.0 mm MLC errors. The corresponding Gamma passing rates dropped to 88.9 ± 5.3% for HN cases and 83.4 ± 3.2% for prostate cases when comparing planar doses from the original plans and the plans with 2.0 mm MLC errors. Conclusion: For VMAT plans, systematic MLC PEs up to 1.0 mm did not affect the plan quality in term of target coverage, OAR sparing, and Gamma analysis with 2mm/2% criteria.

SU‐E‐T‐28: A Treatment Planning Comparison of Volumetric Modulated Arc Therapy Vs. Proton Therapy for Post‐Mastectomy Radiotherapy

Purpose:The delivery of post‐mastectomy radiotherapy (PMRT) can be challenging for patients with left‐sided breast cancer due to the PTV size and proximity to critical organs. This study investigates the use of protons for PMRT in a clinically‐representative cohort of patients, and quantitatively compares volumetric modulated arc therapy (VMAT) to proton therapy to have an evidence‐based rationale for selecting a treatment modality for these patients.Methods:Eight left‐sided PMRT patients previously treated at our clinic with VMAT were identified for the study. PTVs included the chest wall and regional lymph nodes. Passively scattered (PS) and intensity modulated proton therapy (IMPT) plans were constructed using the Eclipse proton planning system. The resulting plans were compared to the original VMAT plan on the basis of PTV coverage; dose homogeneity index (DHI) and conformity index (CI); dose to organs at risk (OAR); tumor control probability (TCP), normal tissue complication probability (NTCP) and secondary cancer complication probability (SCCP). Differences were tested for significance using the paired Student's t‐test (p&lt;0.01).Results:All modalities produced clinically acceptable PMRT plans. The comparison demonstrated proton treatment plans provide significantly lower NTCP values for the heart and the lung while maintaining significantly better CI and DHI. At a prescribed dose of 50.4 Gy (RBE) in the PTV, the calculated mean NTCP value for the patients decreased from 1.3% to 0.05% for the whole heart (cardiac mortality) and from 3.8% to 1.1% for the lungs (radiation pneumonitis) for both proton therapy plans from VMAT plans. Both proton modalities showed a significantly lower SCCP for the contralateral breast compared to VMAT.Conclusion:All three plans (VMAT, PS, and IMPT) provide acceptable treatment plans for PMRT. However, proton therapy shows a significant advantage over VMAT with regards to sparing OARs and may be more advantageous for patients with unfavorable anatomy.

TH‐E‐BRE‐04: An Online Replanning Algorithm for VMAT

Purpose:To develop a fast replanning algorithm based on segment aperture morphing (SAM) for online replanning of volumetric modulated arc therapy (VMAT) with flattening filtered (FF) and flattening filter free (FFF) beams.Methods:A software tool was developed to interface with a VMAT planning system ((Monaco, Elekta), enabling the output of detailed beam/machine parameters of original VMAT plans generated based on planning CTs for FF or FFF beams. A SAM algorithm, previously developed for fixed‐beam IMRT, was modified to allow the algorithm to correct for interfractional variations (e.g., setup error, organ motion and deformation) by morphing apertures based on the geometric relationship between the beam's eye view of the anatomy from the planning CT and that from the daily CT for each control point. The algorithm was tested using daily CTs acquired using an in‐room CT during daily IGRT for representative prostate cancer cases along with their planning CTs. The algorithm allows for restricted MLC leaf travel distance between control points of the VMAT delivery to prevent SAM from increasing leaf travel, and therefore treatment delivery time.Results:The VMAT plans adapted to the daily CT by SAM were found to improve the dosimetry relative to the IGRT repositioning plans for both FF and FFF beams. For the adaptive plans, the changes in leaf travel distance between control points were &lt; 1cm for 80% of the control points with no restriction. When restricted to the original plans' maximum travel distance, the dosimetric effect was minimal. The adaptive plans were delivered successfully with similar delivery times as the original plans. The execution of the SAM algorithm was &lt; 10 seconds.Conclusion:The SAM algorithm can quickly generate deliverable online‐adaptive VMAT plans based on the anatomy of the day for both FF and FFF beams.

Dosimetric effect of respiratory motion on volumetric‐modulated arc therapy‐based lung SBRT treatment delivered by TrueBeam machine with flattening filter‐free beam

The purpose of this study is to investigate the interplay effect between dynamic MLC movement and tumor respiratory motion in volumetric‐modulated arc therapy (VMAT)‐based lung SBRT treatment delivered by the flattening filter‐free (FFF) beam of a Varian TrueBeam machine. Six lung cancer patients with tumor motions ranging between 0.5–1.6 cm were recruited in this study. All patients underwent 4D‐CT scan with audiocoaching. A two‐arc VMAT plan was retrospectively generated using Varian's Eclipse planning system for each patient. To explicitly describe the interplay effect, the contributions of each control point in the original static VMAT plans to each respiratory phase were calculated, and then ten new VMAT plans corresponding to different respiratory phases were generated and imported back into Eclipse planning system to calculate the radiation dose based on the CT images of related respiratory phase. An in‐house 4D dose calculation program with deformable registration capacity was used to calculate the accumulative 4D dose distribution of the targets. For all patients, the PTV coverage dropped significantly with increased respiratory motion amplitude. However, V100 and D90 of the GTV and GTV+5mm, which mimic the target with setup error of less than 5 mm, were either unchanged or slightly increased up to 1.2%, and the variations of their minimum doses were less than 3.2%. Our results indicated that for VMAT‐based lung SBRT treatment delivered by FFF beam of TrueBeam machine, the impact of interplay effects on target coverage is insignificant, as long as a sufficient margin was given.PACS number: 87.53.Ly

Comparison of Dose Delivery Between Original Arc and Split Short Arcs Volumetric Modulated Arc Therapy

Delivery time for volumetric modulated arc therapy (VMAT) for treatment of tumors that can move with respiration is usually more than 2-3 minutes, longer than is possible for a single breath-hold. Original arc VMAT delivery was divided into split short arcs VMAT (SSVMAT) delivery, in order to facilitate breath-holding. This study aimed to evaluate dose delivery of SSVMAT compared to standard arc VMAT. Dose accuracies of five SSVMAT plans (3 liver, 2 lung) were evaluated. Breath-hold interval times were designed to be 10 s, 15 s, 20 s, 30 s, and 40 s. Original VMAT plans were divided into a SSVMAT plan for each of these interval times. Interruption after irradiation during each interval time was repeated for a whole VMAT delivery. Deliveries were performed with a beam modulator. A two-dimensional ionization chamber array system (2D array) was used for dose measurement. Measurement sampling time of the 2D array was set to 50 ms, and a sensitivity correction that depended on gantry angle was applied. The dose difference on the central coronal plane between SSVMAT delivery and the original VMAT delivery was analyzed. The dose at the isocenter was also compared using the center chamber of the 2D array. Pass rate was calculated from the dose difference with a criterion of 1% for a region that delivered a dose higher than 10% of the maximum dose. Mean differences in doses at the isocenter between VMAT and SSVMAT were not significant (0.5 ± 0.5% for 10 s interval, 0.2 ± 0.3% for 15 s interval, 0.2 ± 0.2% for 20 s interval, 0.0 ± 0.1% for 30 s interval, and 0.0 ± 0.1% for 40 s interval, p > 0.05). Pass rates based on dose difference were 85 ± 17% for 10 s interval, 99.9 ± 0.2% for 15 s interval, 100 ± 0% for 20 s interval, 100 ± 0% for 30 s interval, and 100 ± 0% for 40 s interval. SSVMAT with the breath-hold interval of 10 s had a significantly reduced pass rate (p = 0.017). In the present study, a practical irradiation method that can be used to divide VMAT delivery into feasible breath-hold intervals was developed. Analysis of dose verification revealed that SSVMAT deliveries with intervals of 15 s or longer provided accurate doses.

Extended PACS: digital imaging and integration with PACS on pathology

It is occasionally difficult to deliver a sufficient radiation dose using existing 3-dimensional conformal radiation therapy (3DCRT), intensity modulated radiation therapy (IMRT), or volumetric modulated arc therapy (VMAT) techniques in stage III lung cancer patients. To address this problem, we developed a new hybrid VMAT (hVMAT) technique. This study aimed to demonstrate the usability of the new hVMAT technique in stage III lung cancer treatment.32 consecutive patients with stage III lung cancer who had been treated with hVMAT were included. The hVMAT plan was made by adding two static beams to the original VMAT plan. To evaluate the superiority of the hVMAT plan, we compared the three techniques (3DCRT vs. VMAT vs. hVMAT) using the conformity index (CI), conformation number (CN), homogeneity index (HI), and the dosimetric parameters of organs at risk (OARs).The mean V20 of the lung from the hVMAT plans (27.2%) was significantly lower than 3DCRT plans (40.5%) and VMAT plans (35.0%). Additionally, the hVMAT plans showed the best results for other lung dose parameters (V5, V10, mean lung dose) among the three plans, while maintaining an irradiated dose at an acceptable level for the spinal cord, esophagus, and heart. In addition, hVMAT also showed non-inferior results for CI, CN, and HI compared with the VMAT plans.This novel hybrid technique can improve all dosimetric parameters of lung while maintaining the acceptable dose for other OARs. Therefore, the new hVMAT could be a useful planning technique for treating stage III lung cancer and it might be a solution for patients who cannot receive a sufficient dose with the existing 3DCRT or VMAT plans.