Total MU Research Articles

MO‐F‐108‐07: Impact of Additional Intensity Modulation On Arc‐Based Stereotactic Body Radiotherapy (SBRT) for Lung Cancer

Purpose: A single‐isocenter multi‐segment dynamic conformal arc technique (SiMs‐arc) has been used in our department in the past three years for efficient planning and delivery of over 200 lung SBRT treatments. The purpose of this study is to investigate the impact of additional intensity‐modulation made available by Varian RapidArc on the dosimetric quality and delivery efficiency of arc‐based lung SBRT. Methods: Four patients previously treated with lung SBRT using SiMs‐arc were selected. SiMs‐arc plans were generated with the isocenter located in the geometric center of patient axial plane (to allow for collision‐free gantry rotation around the patient) and six 60°‐arc segments spanning from 1° to 359°. Two RapidArc plans, one using the same arc geometry as SiMs‐arc and the other using typical partial arcs (210°) with the isocenter inside PTV, were generated for each patient. All plans, generated using Eclipse V10.0, were normalized with PTV V100 to 95%. PTV coverage, dose to organs‐at‐risk and total MUs were compared to assess the impact of additional intensity‐modulation provided in RapidArc. Results: RapidArc plans produced higher PTV D99 (by 1.0%–3.1%) and minimum dose (by 2.4% to 9.8%); better PTV conformality index (by 1%–8%); and less volume of 50% dose outside 2cm from PTV (by 0–20.8cc) than the corresponding SiMs‐arc plans. No significant dose differences were observed for lungs, trachea, chest wall and heart. RapidArc using partial‐arcs had lowest maximum dose to spinal cord. RapidArc plans required 1.5 to 1.91 times more MUs than SiMs‐arc plans to deliver the same dose. Conclusion: Additional intensity‐modulation enlisted by RapidArc produces modest dosimetric improvements over conformal arcs for lung SBRT, but requires more MUs (by a factor > 1.5) to deliver. The dosimetric improvements, most notably in PTV minimum dose and in dose conformality for irregularly shaped PTVs, may outweigh the increased MUs in using RapidArc.

WE-G-108-09: Modulation Severity as a Predictor for Patient-Specific IMRT QA Delivery Failure

Purpose: The goal of this work was to determine if highly modulated fields contribute to decreased passing rates for patient-specific IMRT delivery QA. Methods: Data for 10 Varian Eclipse-planned patients in each of 3 categories were collected: IMRT, carriage shift, and RapidArc. Total plan modulation was calculated as the ratio of total MU to dose per fraction. Applied modulation, in cGy, was determined for each field as the ratio of total MU per field to total plan modulation. The difference between applied modulation and dose from the planning system on a per beam basis was calculated and the mean values found for each patient and each category. Limits were applied based on the category mean plus 3 standard deviations for 2 beams as well as maximum limits for a single beam. Data were evaluated with respect to these limits for 889 patients. The individual results were compared with the IMRT delivery QA data collected with the IBA Matrixx using absolute 3% dose and 3mm DTA acceptance criteria. Results: Modulation limits were exceeded for IMRT plans in 64%, 44% and 22% of cases and for RapidArc plans in 43%, 22% and 2% of cases for delivery QA passing rates of 92%, 95% and 98%, respectively. The mean passing rate for delivery QA was approximately 97% (range 90.4–100%). Conclusion: There is a definite trend indicating lower delivery QA passing rates with severe modulation for both modalities. However, given an arbitrary acceptable passing rate of 95% with the modulation test failing 44% and 22% of the time for IMRT and RapidArc, respectively, modulation may not be a strong predictor of delivery QA acceptability. The cause(s) of severe modulation should be evaluated on a case-by-case basis with appropriate time and attention allotted for all phases of the QA process.

A comparison of subject room dust with home vacuum dust for evaluation of dust-borne aeroallergens

BackgroundAssessment of indoor allergen is valuable in exposure research and evaluation of allergic individuals. Collection methods range from grab vacuum samples to filtration devices located in the breathing range of an individual. For practical purposes, many research studies use analysis of collected house dust to evaluate allergen reservoirs. ObjectiveTo test the hypothesis that house dust collected from the family vacuum is equivalent to house dust collected by a technician following standard protocol. MethodsHomes from a healthy homes demonstration project (n = 41) were sampled using a specific Department of Housing and Urban Development–suggested protocol in the bedroom of the child with asthma and a simple grab procedure from the family vacuum. Samples were evaluated for the presence of 5 allergens, Bla g2, Can f1, Der f1, and Der p1 combined as total mite, Fel d1, and Mus m1. Samples were also evaluated for total antigenic protein from 4 fungal taxa, including Alternaria, Aspergillus, Cladosporium, and Penicillium. ResultsAll of the allergens and antigens tested showed good correlation between the 2 collection methods. Fungal antigens ranged up to 92,651 nanograms per gram of dust for Aspergillus, and allergens ranged up to 17,928 nanograms per gram of dust for Can f1. The best correlation was for Cladosporium (r = 0.91), and the weakest was for dust mite (r = 0.34). ConclusionAllergens and antigens tested from samples collected by protocol and by grab sampling from the home vacuum were highly positively correlated. Grab samples taken from the family vacuum may be a good surrogate for evaluating home allergen exposure.

Sensitivity analysis of physics and planning SmartArc parameters for single and partial arc VMAT planning

We investigate the sensitivity of various physics and planning SmartArc parameters to generate single and partial arc VMAT plans with equivalent or better plan quality as IMRT. Patients previously treated with step‐and‐shoot IMRT for several treatment sites were replanned using SmartArc. These treatment sites included head and neck, prostate, lung, and spine. Effect of various physics and planning SmartArc parameters, such as continuous vs. binned dose rate, dynamic leaf gap, leaf speed, maximum delivery time, number of arcs, and control point spacing, were investigated for Elekta Axesse and Synergy linacs. Absolute dose distribution was measured by using the ArcCHECK 3D cylindrical diode array. For all cases investigated, plan metrics such as conformity indices and dose homogeneity indices increased, while plan QA decreased with increasing leaf speed. Leaf speed had a significant impact on the segment size for low dose per fractionation cases. Constraining leaf motion to a lower speed not only avoids tiny large leaf travel and low‐dose rate value, but also achieves better PTV coverage (defined as the volume receiving prescription dose) with less total MUs. Maximum delivery time, the number of arcs, and the spacing of control points all had similar effects as the leaf motion constraint on dose rate and segment size. The maximum delivery time had a significant effect on the optimization, acting as a hard constraint. Increasing the control point spacing from 2 to 6 degrees increased the PTV coverage, but reduced the absolute dose gamma passing rate. Plans generated using continuous and binned dose rate modes did not show any difference in the quality and the delivery for the Elekta machines. Dosimetric analysis with a 3D cylindrical QA phantom resulted in 93.6%–99.3% of detectors with a gamma index 3%/2 mm <1 for all cases.PACS number: 80

SU-E-T-596: Choroidal Metastases Irradiation with Volumetric Modulated Arc Radiotherapy.

Intraocular metastases are present in up to 10% of patients with systemic malignancies. With tumor cells spreading hematogenously, the posterior choroid is frequently involved because of its rich vasculature. We treated a woman who developed bilateral choroidal metastases from breast cancer. For the patient, we designed three radiotherapy techniques. The study was to find the dosimetric differences between them. For the patient with bilateral choroidal metastases, three-dimensional conformal radiotherapy (3D-CRT), inverse intensity-modulated radiotherapy (IMRT) and volumetric modulated arc radiotherapy (V-MAT) plans were designed. The prescription dose was 60 Gy in 30 fractions. The dose distribution in the target, the dose to the organs at risk, total MU and delivery time were compared. In 3D-CRT, IMRT and V-MAT plans, the volume of target in 100% prescription dose were 92.5%, 93.3% and 92.0% respectively. V-MAT plan showed the best conformity and homogeneity in target. IMRT plan showed a better homogeneity than 3D-CRT plan. In 3D- CRT, IMRT and V-MAT plans, the max doses on left lens were 975.3 cGy, 739.9cGy, 870.2 cGy respectively. The max doses on right lens were 929.9 cGy, 802.9 cGy and 889.0 cGy respectively. V-MAT plan showed the lowest value for the max dose on left optic nerve and the lower value for the right one compared to 3D-CRT plan. The MUs were 386, 973, 709 and the delivery times were 10 min, 14min and 1.6 min respectively in 3D-CRT, IMRT and V-MAT. V-MAT could significantly reduce the delivery time. V-MAT plan showed the best conformity and homogeneity in target. IMRT showed the lowest value in the max doses to lenses though the delivery time was longest. Overall, V-MAT showed a similar or better dose distribution, and most importantly, improved treatment delivery efficiency. With the results, V-MAT technology could be expected to choroidal metastases.

SU-E-T-440: Dosimetry Parameter Comparison between Field-In-Field Technique and Conventional Physical Wedge Technique for Whole Breast Radiation Treatment.

To compare an advantage/disadvantage between Field-in-Field (FIF) technique and conventional physical wedge (PW) technique for a whole breast (WB) tangential field irradiation. Total 86 patients were included in this study. 46 of them were left breast cancer cases. FIF/PW plans were created by Eclipse (V7.3, Varian) with 6MV or 6MV mixed with 18MV. Plans had a same ioscenter location, beam setup and normalized isodose line selection for each case. Varian 21EX Linac with 120 MLC was used for beam delivery. Two plans were compared by PTV encompassed by 95% isodose line (V95), dose inhomogeneity (DI), dose received by 10% volume of lung (D10), mean lung dose (MLD), dose received by 5% volume of heart (D5), Mean heart dose (MHD), total MU, maximum dose in the plan and the number of field needed for each fraction. Comparing with PW plan, FIF plan showed an average percentage improvement of V95 was 0.1±1.6, DI was 0.6±5.0, MLD was 1.5±4.2, D5 was 2.0±8.8, MHD was 3.2±4.6. However, D10 increased by 1.4%±0.050. FIF lowered an average daily MU by 28.5%±0.080, maximum dose by 0.5%±0.018, and increased number of treatment field by 1.50±0.356. There were 12 cases treated with mixed beam in PW technique vs 10 in FIF technique. The advantages of FIF technique included: (1) Reduce radiation contamination to contra lateral breast and Linac room induced activity by remove the PW, lower MU and diminish a higher energy. (2) Time saving was not only from less MU but also from not need go into the treatment room for a wedge adjustment. (3) Reduced the therapist work load. (4) Regular MU 2nd check was applied because there was not FIF merge involved in the treatment field. With a MLC to shape the field and treatment record/verification system to control the treatment, increasing number of treatment field didn't show as a problem.

SU-E-T-591: Volumetric-Modulated Arc Radiotherapy for Carcinomas of the Pituitary Adenomas: A Treatment Planning Comparison with Fixed Field IMRT.

A treatment planning study was performed to compare volumetric-modulated arc radiotherapy against conventional fixed field IMRT on pituitary adenomas cancer patients. CT datasets of 10 patients affected by carcinoma of the pituitary adenomas were included and four plans were generated for each case: fixed five-Fields and seven-Fields IMRT, single (RA1)- and double (RA2)-modulated arcs with the RapidArc technique. The radiation dose was prescribed as 45 Gy in 25 fractions. DVHs were adopted for the statistics of above parameters, as well as conformal index (CI), homogeneity index (HI), dose-volumetric parameters of normal tissues, total accelerator output MUs and total treatment time. Target coverage resulted basically equivalent among four plans. CI and HI were 0.84±0.04, 0.84±0.03, 0.86±0.03, 0.85±0.04 and 9.03±1.05 8.39±1.55, 10.02±1.30, 10.59±1.30, respectively. The V20Gy (in %) and V40Gy (in %) of brainstem were 30.48±16.84, 10.23±7.47, 21.34 ±12.2, 26.4±12.1 and 5.06±4.37, 4.27±4.49, 3.97±4.45, 4.49±4.56, repectively. The max dose(Gy)of Optic nerve-right and Optic nerve-left were 31.87±12.04, 33.23±12.65, 28.96 ±13.42, 28.68±12.63 and 33.15±12.51, 34.15±11.51, 30.37 ±12.41,30.09±11.34 .Concerning the healthy brain tissue: V20Gy (in %) was 7.11 ± 2.1,7.92 ±2.7, 8.12 ± 1.8 and 8.0 ± 2.0, respectively. The number of computed MU/fraction were 580 ± 69 (5-IMRT), 598 ± 112 (7- IMRT),474 ±43(RA1), and 477 ± 77 (RA2) . In our study, rapidarc showed improvement in organ at risk sparing with uncompromised target coverage .Rapidarc could lead to the less MU and shortor delivery time compared to IMRT.

SU-E-T-559: Evaluation of Flatening Filter Free (FFF) Beams in Radiotherapy of Head and Neck Cancer.

To investigate th applicability of the flattening filter free (FFF) to fractionated radiotherapy of head and neck cancer. Twelve patients previously treated with IMRT were selected. A SIB protocol was chosen so that 66/54 Gy were prescribed to the gross tumors/elective nodal volumes in 30 fractions. Photon beams of 6X(6X-FFF) were used for both IMRT (@ 9 gantry angles) and VMAT (single and dual full arcs). Consistent optimization and acceptance criteria were used for all plans, with prescription dose to cover 90% of PTV66 and hot spots limited to 110% and within PTV66. Both plan quality and deliver-ability were compared. All plans met clinical objectives. The IMRT plan quality is similar or slightly better than VMAT, except that PTV66 conformity index is significantly better in 2-arc VMAT (1.18) than IMRT (1.32). The total MUs for IMRT are 1680/2090 for 6X/6X-FFF, a 24% increase. However, the total beam-on time (BOT) under treatment automation is 430/400 seconds for 6X(@300 MU/min)/6X-FFF (@400 MU/min), a 6% decrease. Increasing dose rate to the maximum (600/1400 for 6X and 6X-FFF) can reduce BOT by up to 50%, but at a cost of increased MU (up to 40%) and degraded plan quality. Significant reduction in MU and BOT are realized in VMAT, with ∼600 MU for both 6X and 6X-FFF and either single or dual-arcs. The BOT for VMAT is governed by the gantry speed and approximately 1 min for single arc and 2 min for dual arcs. Plan quality is similar between IMRT and 2-arc VMAT for both 6X and 6X-FFF. Total MU for IMRT and delivery time is significantly higher for IMRT than VMAT, but not much different between flat and non-flat beams. For standard fractionated radiotherapy of HN cancer, there is no advantage for the non-flat beam. The recommended choice is 2-arc VMAT.

MO-D-BRB-08: BEST IN PHYSICS (THERAPY) - A Real Time Dose Monitoring and Dose Reconstruction Tool for Patient Specific VMAT QA and Delivery.

To develop a real time dose monitoring and dose reconstruction tool to identify and quantify sources of errors during patient specific VMAT delivery and QAMethods: The VMAT delivery monitor tool called Linac Data Monitor (LDM) has been developed that connects to the linac in clinical mode and displays, records and compares real-time machine parameters to the planned parameters. A new quantity called integral error keeps a running total of leaf overshoot and undershoots errors in each leaf pair multiplied by leaf width and the amount of time during which error exists in MU delivery. Another tool reconstructs pinnacle format delivered plan based on the saved machine logfile and recalculates actual delivered dose in patient anatomy. Delivery characteristics of various standard and hypofractionation VMAT plans delivered on Elekta Axesse and Synergy linacs were quantified. The MLC and gantry errors for all the treatment sites were 0.00±0.59mm and 0.05±0.31°, indicating a good MLC gain calibration. Standard fractionation plans had a larger gantry error than hypofractionation plans due to frequent dose rate changes. On average the MLC errors were negligible but larger errors of 4-6 mm and 2.5° were seen when dose rate varied frequently. Large gantry errors occurred during the acceleration and deceleration process, and correlated well with MLC errors (p<0.0001). PTV mean, minimum, maximum dose discrepancy were 0.87±0.21%, 0.99±0.59% and 1.18±0.52%. The other OAR doses were within 2.5% except a few that showed up to 5.6% discrepancy in maximum dose. Realtime displayed normalized total positive integral error (normalized to the total MUs) correlated linearly with MLC and gantry errors (p<0.001) and dosimetric discrepancy (PTVmean: p<0.01; PTVmax: p<0.067 and PTVmax: p<0.046). Errors may exist during complex VMAT planning and delivery. LDM is capable of detecting and quantifying mechanical and dosimetric errors at various stages of planning and delivery.

SU-D-BRA-03: Simultaneous MV-KV Imaging for Intra-Fractional Motion Management during Volume Modulated Arc Therapy (VMAT) Delivery on the Varian TrueBeam.

To evaluate a MV-kV intra-fractional imaging technique for use during volume modulated arc therapy (VMAT) with the Varian TrueBeam. MV-kV image pairs were acquired intra-fractionally during VMAT delivery. kV images (11 fps) were acquired throughout delivery using a standard pre-programmed imaging template. MV images (9.5 fps) were acquired simultaneously by deploying the EPID and passively collecting the resulting images using Varian proprietary software, iTools Capture. Localization accuracy was evaluated by imaging a Rando phantom implanted with 3 fiducials while moving the couch according to XML- programmed trajectories simulating typical prostate and respiratory motion. VMAT delivery was done using a single 360 degree arc in TrueBeam Developer mode. The effect on accuracy of total MU and gantry speed was studied. To improve image quality, MV frame averaging was performed and the MV and kV images were then registered to their corresponding DRRs using in-house registration software. From these 2D registrations, the 3D position at each MV-kV acquisition point was determined. Between 130 and 390 MV-kV pairs were acquired for each delivery. The mean difference between planned couch and measured fiducial 3D positions with prostate motion was less than 0.03 cm in each direction (SD 0.03 cm). Neither gantry speed nor MU significantly impacted accuracy. for respiratory motion, the mean difference between planned and measured position was less than 0.04 cm. Standard deviation averaged 0.06 cm but increased to 0.12 cm with large instantaneous motion and less MV dose per frame. MV frame averaging and inaccuracies in MV image gantry angle determination also affected accuracy, particularly with significant motion. With high quality MV imaging, MV-kV localization techniques can be highly accurate, even in the presence of significant motion. As clinical MV-kV methods become available, such techniques can provide an efficient and accurate method for monitoring intra-fractional motion. This work was partially supported through a research agreement with Varian Medical Systems, Palo Alto, CA.

Investigating the effect of dose rate and maximum allowable MLC leaf velocity in dynamic IMRT

The purpose of this study is to analyze the effect of various dose rates (DR) and maximum allowable MLC leaf velocities (MLV) in dynamic Intensity Modulated Radiotherapy (IMRT) planning and delivery of head and neck patients. Five head and neck patients were retrospectively included in this study. The initial dynamic IMRT 'reference plans' were created for all these patients, using a DR of 400 MU/min and MLV of 2.5 cm/s. Additional plans were generated by varying the DR and MLV values. The DR value was varied from 100 to 600 MU/min, in increments of 100 MU/min, for a MLV of 2.5 cm/s. Also the MLV was varied from 0.5 to 3 cm/s, in increments of 0.5 cm, for a DR of 400 MU/min. In order to maintain the prescribed dose to the PTV, the DR was allowed to vary ('beam hold or DR modulation' during delivery) when the MLV was changed and the MLV was allowed to vary when the DR was changed. The mean doses to the PTV as well as parotids, maximum dose of spinal cord and total MU were recorded for analysis. The effect of DR and MLV on treatment delivery was analyzed using the portal dosimetry for all the above plans. The predicted portal dose fluences of the TPS were compared with the measured EPID fluences using gamma evaluation criteria of 2% dose difference and 2 mm distance to agreement. A small proportional increase in OAR doses with DR was observed. Increases to MLV value resulted in decreases of the OAR doses and this effect was considerable for values below 1.5 cm/s. DR and MLV both resulted in no appreciable dose variation to the target. The total MU to deliver the plan increases with increasing DR and decreasing MLV. When comparing portal images derived from the treatment plans with portal images obtained by delivering the treatments, it was observed that the treatments was most reliably delivered when the DRs were set to lower values and when the MLVs were set to higher values.

A study of segment weight optimization with the CMS XiO step‐and‐shoot IMRT technique for prostate cancer

The aim of this study was to compare IMRT optimization in the CMS XiO radiotherapy treatment planning system, with and without segment weight optimization. Twenty‐one prostate cancer patients were selected for this study. All patients were initially planned with step‐and‐shoot IMRT (S‐IMRT). A new plan was then created for each patient by applying the segment weight optimization tool (SWO‐IMRT). Analysis was performed on the (SWO‐IMRT) and (S‐IMRT) plans by comparing the total number of segments, monitor units, rectal and bladder dose. The study showed a statistically significant reduction in the total number of segments (mean: 25.3%; range: 16.8%–31.1%) with SWO‐IMRT as compared to S‐IMRT (p<0.0001). Similarly, a mean reduction of 3.8% (range: 0.4%–7.7%) in the total MU was observed with SWO‐IMRT (p<0.0001). The study showed an average rectal dose decrease of 13.7% (range: 7.9%–21.4%) with SWO‐IMRT (p<0.0001). We also observed a statistically significant reduction of 26.7% (range: 16.0%–41.4%; p < 0.0001) in the mean dose to the posterior one‐third rectum and an overall reduction in mean bladder dose of 2.2% (range: 0.1%–6.1%) for SWO‐IMRT (p<0.0001). This study shows that the segment weight optimization method significantly reduces the total number of segments and the dose to the rectum for IMRT prostate cancer. It also resulted in fewer monitor units for most of the prostate cases observed in this study.PACS numbers: 85.55.ne; 87.55.de; 87.55.kd

Corrigenda

BJOG: An International Journal of Obstetrics & GynaecologyVolume 118, Issue 13 p. 1689-1689 Free Access Corrigenda This article corrects the following: Is the operative delivery rate in low-risk women dependent on the level of birth care? A randomised controlled trial S Bernitz, R Rolland, E Blix, M Jacobsen, K Sjøborg, P Øian, Volume 118Issue 11BJOG: An International Journal of Obstetrics & Gynaecology pages: 1357-1364 First Published online: July 12, 2011 First published: 11 November 2011 https://doi.org/10.1111/j.1471-0528.2011.3225.xAboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat In the following article1 the number of patients stated for ‘other’ and ‘Not included at onset of labour’ were incorrectly stated as 519 and 1770 in Figure 1. These should have read 522 and 1773, respectively. Additionally, in the final paragraph of the discussion the text incorrectly read: ‘Only small non-significant differences in total operative delivery rate were found (total operative delivery rate, MU versus SU P = 0.57 and MU versus SU P = 0.44) when it should have read: ‘Only small non-significant differences in total operative delivery rate were found (total operative delivery rate, MU versus NU P = 0.57 and MU versus SU P = 0.44). We apologise for these errors. Reference 1 Bernitz S, Rolland R, Blix E, Jacobsen M, Sjøborg K, Øian P. Is the operative delivery rate in low-risk women dependent on the level of birth care? A randomized controlled trial. BJOG 2011; 118: 1357– 64. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Volume118, Issue13December 2011Pages 1689-1689 ReferencesRelatedInformation

Evaluation of volumetric modulated arc therapy for cranial radiosurgery using multiple noncoplanar arcs

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

SU‐E‐T‐791: Applications of RapidArc in Cervical Esophageal Cancer Radiation Therapy: A Comparison with Fixed‐Field IMRT in Dosimetry and Implementation

Purpose: Comparing fixed‐field intensity‐modulated radiotherapy (f‐IMRT) with intensity‐modulated arc therapy (IMAT) treatment plans in dosimetry and practical application for cervical esophageal carcinoma.Methods: For ten cervical esophageal carcinoma cases, f‐IMRT plan (7 fixed‐fields) and two IMAT plans, namely RA (co‐planar 360° arcs) and RAx (co‐planar 360° arcs without sectors from 80° to 110°, and 250° to 280°). DVHs were adopted for the statistics of above parameters, as well as conformal index (CI), homogeneity index (HI), dose‐volumetric parameters of normal tissues, total accelerator output MUs, and total treatment time.Results: There were differences between RAx and f‐IMRT as well as RA in PTV parameters such as HI, V95% and V110%, but not in CI. RAx reduced lung V5 from (50.9±9.8)% in f‐IMRT and (51.4±10.8)% in RA to (49.3±10.4)% in RAx (P&lt;0.05). However, lung V30, V40, V50 and MLD increased in RAx. There was no difference in the mean heart dose in three plans. Total MU was reduced from 1174.8±144.6 in f‐IMRT to 803.8±122.2 in RA and 736.2±186.9 in RAx (P&lt;0.05) Conclusions: Compared with fIMRT, IMAT reduced low dose volumes of lung and total MU on the basis of meeting clinical requirements.

SU‐E‐T‐855: The Correlation Analysis of IMRT Dosimetry Errors and Parameters of Beams

Purpose: To study the relationships of IMRT plansˈ beam characteristic parameters, furthermore in order to research the effect of parameters settings on the absolute dose accuracy. Methods: Forty‐three esophageal carcinoma cases were planned with the Pinnacle 7.6c TPS using appropriate optimization parameters and dose constraints. All the plans were transplanted into solid water phantom. DOSE1 dosimeter and 0.13cc ion Farmer Chamber (IBA Corp) were used to measure the absolute dose and obtained the dosimetry errors later. The parameters of the 43 casesˈ statistics were collected, and their relations were analyzed. Then we summarized the dosimetry errors, made comparison and analyzed the relativity between the beam parameters and absolute dose verification errors. Results: 1. The describing parameters of beams feature were related. (1) Beam numbers were significantly correlated with beam segment numbers and total MUs, both p‐values were equal to 0.000, but were not distinctly connected with average beam segment numbers and average MUs. (2) Beam segment numbers were prominently interrelated with average beam segment numbers, total MUs and average MUs. (3) Average beam segment numbers were negatively correlated with average MUs and the statistical correlation coefficient was −0.358, yet had no relationship with total MUs. (4) The relationship between total MUs and average MUs was significantly correlated, and the p‐value was 0.003. 2. There was an obvious interrelation between dose accuracy and parameter settings, all of the p‐values were almost 0.000. Dose deviation was positively correlated with the total number of fields, segments, MUs and average segment numbers, while was negatively connected with average MUs.Conclusions: For the purpose of decreasing the absolute dose verification errors, the quantity of beam angles and beam segments should be reduced, considered the aspect of the designing of treatment plans.

SU-E-T-632: Stereotactic Radiosurgery for Multiple Brain Metastases Using Flattening Filter-Free Beam and Comparison Study with the Regular 6MV

Purpose: To investigate the benefits of using TrueBeam 6MV flattening filter-free (6F) mode in treating multiple intracranial metastases. To compare MU and dosimetric characteristics of the RapidArc (RA) plans using 6F and the regular 6 MV beam (6X). Methods: We have selected 10 patients with 2–12 brain metastases treated by IMRS. They also had RA plans done on Varian-23IX under ARIA-V8.6 in previous study. All these RA plans have been re-planned for TrueBeam which requires ARIA-V8.9. The prescribed dose is 2000 cGy and normalized to 98% of PTV. Two RA plans have been generated for each patient: 6X (600 MU/min) and 6F (1400 MU/min). The gantry arc-spans, collimator angles, and field sizes are the same in both plans for each patient. For fair comparison, the optimization objectives, priorities, even the timing of priority changes are kept the same at best in both planning process due to the adaptive nature of RapidArc optimization. Results: For dosimetric evaluation and comparison, the homogeneity, conformity, and gradient indices are calculated for both plans. It is found all the dosimetric indices in 6F plans are comparable with the corresponding 6X plans in each patient. The maximal dose to brainstem, chiasm, eyes, and optical nerves and the mean dose to normal brain are calculated. In the comparison study of critical organ doses, it is found that they are comparable or slightly better in 6F plans compared to 6X. Total MUs in 6F plans are about 10–20% more than MUs in 6X plan of each patient, with a group average of 16% in 6F plans. Conclusion: The dosimetric characteristics of 6F plans are found to be comparable with 6X plans in radiosurgery treatment of multiple brain lesions. Counting only 10– 20% more MUs if using 6F, the treatment delivery is much faster and thus greatly shortens treatment time.

SU‐E‐T‐836: Dosimetric Comparison of Intensity‐Modulated Radiotherapy and Volumetric Modulated Arc Radiotherapy for the Treatment of Multiple Intracranial Metastases

Purpose: To evaluate the dosimetric differences of intensity‐modulated radiotherapy (IMRT) and volumetric modulated arc radiotherapy (V‐MAT)for multiple intracranial metastases. Methods: Ten patients with multiple intracranial metastases were included in this analysis. Three treatment techniques were designed for each patient: intensity‐modulated radiotherapy with seven fixed field(IMRT), volumetric modulated arc radiotherapy with single 358° coplanar arc(V‐MAT1) and volumetric modulated arc radiotherapy with double 358° coplanar arcs(V‐MAT2) . Dose prescription was 60Gy to PTV in 30 fractions. All plans were normalized to the mean dose to PTV.The dose distribution in the target,the dose to the organs at risk,total MU and delivery time in three techniques were compared. Results: The best conformity and homogeneity in PTV were observed in V‐MAT2 plans,no statistically significant differences were observed between IMRT and V‐MAT1.For the maximum dose in lens,eyes and brainstem,the two V‐MAT plans were significantly lower than IMRT plan(p&lt;0.05). For the maximum dose in optic nerves,the V‐MAT2 plans were statistically lower than IMRT plan(p&lt;0.05).For the mean dose to normal brain tissue, no statistically differences were observed in the three techniques. V‐MAT2 plans showed the highest values in the normal brain tissue volume receiving 5Gy and 10 Gy doses.In the normal brain tissue volume receiving 20Gy,30Gy and 40 Gy doses,IMRT and V‐MAT2 plans showed the highest and lowest values respectively. Compared with IMRT plans,the average monitor units reduced 29% and 24% in V‐MAT 1 and V‐MAT2 plans and the delivery time was significantly shorter in V‐MAT plans. Conclusions: Comparing to IMRT plans, V‐MAT plans showed similar or better in the target dose distribution,reduced irradiation doses on organs in risk and moreover significantly decreased the monitor units and delivery time.

SU‐E‐T‐837: Comparison Between Step&amp;shoot and Dynamic MLC Based IMRT for Prostate Treatment

Purpose: To compare step&amp;shoot and dynamic MLC‐based IMRT for prostate treatment. Methods: Patient data with prostate cancer was selected. Same beam geometries and energy were used. Treatment plan includes simultaneous irradiation of prostate to 82Gy and seminal vesicles to 64Gy (41 fractions). Plans were normalized to achieve 82Gy minimum dose to prostate. Step&amp;shoot was delivered using Primus linac (Optifocus MLC, 10mm leaf‐width, 200MU/min) and Konrad v2.2 TPS (Siemens). MLC step‐size of 0.5cm, minimum of 3 MU per segment and 15 intensity levels was used. Dynamic was delivered using Clinac 21EX (Millenium MLC, 0.5cm leaf‐width, 300MU/min) and Eclipse v8.1 TPS (Varian). MLC step‐size was 0.2cm. Plans dose distributions were compared in PCRT3D TPS (TRF). Optimization and dose calculation time, treatment time, MUs, PTVs and OAR DVH analysis and treatment plan QA were compared for both techniques. Results: Optimization and dose calculation time was 220seg for S&amp;S and 360seg for dynamic. Total MU was 508 for S&amp;S and 577 for dynamic. D98% (V82Gy) for PTV prostate were 81.5Gy (95.7%) for step&amp;shoot and 80.1Gy (84.6%) for dynamic respectively. D98% (V64Gy) for PTV seminal vesicle were 64.2Gy (98.2%) for S&amp;S and 62Gy (92.3%) for dynamic respectively. Rectum, V40Gy, V65Gy and EUD were 56.6cc, 21.2cc and 65.1Gy for S&amp;S and 55.7cc, 19.7cc and 63.3Gy for dynamic respectively. Bladder V65Gy and EUD were 27.4cc and 47.9Gy for S&amp;S and 24.4cc, and 46.2Gy for dynamic. Femoral head D10% were 40.4Gy for S&amp;S and 39.3Gy for dynamic. Treatment time was 531seg for S&amp;S and 271 seg for dynamic. Both treatment plans QA are within tolerances. Conclusions: Both IMRT modalities could be used for prostate treatment. Calculation time and MU are lower with S&amp;S. D98%, V82Gy and V64Gy are slightly better using S&amp;S. OAR doses are few percent better using dynamic. Treatment time is 50% less with dynamic.

SU-E-T-800: Evaluation of Left-Sided Breast Cancer Treatment Using Rotational and Fixed-Gantry Radiotherapy

Purpose: To compare treatment planning for left‐sided breast cancer using the full spectrum of external beam delivery platforms: 3‐dimensional conformal radiotherapy (3DCRT), TomoDirect (TD), fixed‐beam intensity‐modulated radiotherapy(IMRT), and rotational therapy including both volumetric modulated arc therapy (VMAT) and Tomotherapy helical (TH). Methods: Ten left‐sided breast cancer patients with and without positive lymph nodes treated with 3DCRT or fixed‐gantry IMRT to the entire breast were re‐planned with rotational Monaco VMAT, TH and TD. The dosimetric metrics, such as PTV coverage, organ‐at‐risk sparing, DVHs, minimum/maximum/mean doses are evaluated. The VMAT plans were delivered using an Elekta Synergy beam modulator accelerator, the treatment delivery parameters such as the total MUs and delivery time from different techniques are compared. Results: For plans providing comparable PTV coverage, the VMAT plans (HI=1.13) were generally more inhomogeneous than the TH (HI=1.08) and TD (HI=1.04) plans, while TD is less conformal. For the cases with regional node involvement, averaged across the patient, the heart mean doses were 9.2 and 8.8Gy in the VMAT and TH plans, compared to 11.9 and 11.8 Gy for the 3DCRT and TD plans; the mean contra‐lateral lungdoses were 4.4 and 3.2Gy for the VMAT and TH, 3.8 and 1.3Gy for the 3DCRT and TD plans. VMAT plans require shorter delivery time compared to TH. On average, the total MUs for VMAT plans are approximately 11.6% of those TH plans Conclusions: Rotational VMAT and TH offer certain dosimetric advantages over the conventional breast treatment requiring regional nodal coverage at the cost of slightly higher doses to contra‐lateral lung and/or breast. In comparison to TH, VMAT offers more efficiently dosedelivery to achieve comparable dosimetric plans. For early stage breast cancer patients requiring no nodal coverage, 3DCRT and TD are potentially more beneficial in terms of heart, contra‐lateral lung/breast sparing.