D Mynampati

Application of AAPM TG 119 to volumetric arc therapy (VMAT)

The purpose of this study was to create AAPM TG 119 benchmark plans for volumetric arc therapy (VMAT) and to compare VMAT plans with IMRT plan data. AAPM TG 119 proposes a set of test clinical cases for testing the accuracy of IMRT planning and delivery system. For these test cases, we generated two treatment plans, the first plan using 7–9 static dMLC IMRT fields and a second plan utilizing one‐ or two‐arc VMAT technique. Dose optimization and calculations performed using 6 MV photons and Eclipse treatment planning system. Dose prescription and planning objectives were set according to the TG 119 goals. Plans were scored based on TG 119 planning objectives. Treatment plans were compared using conformity index (CI) for reference dose and homogeneity index (HI) (for D5‐D95). F or test cases prostate, head‐and‐neck, C‐shape and multitarget prescription dose are 75.6 Gy, 50.4 Gy, 50 Gy and 50 Gy, respectively. VMAT dose distributions were comparable to dMLC IMRT plans. Our planning results matched TG 119 planning results. For treatment plans studied, conformity indices ranged from 1.05–1.23 (IMRT) and 1.04–1.23 (VMAT). Homogeneity indices ranged from 4.6%–11.0% (IMRT) and 4.6%–10.5% (VMAT). The ratio of total monitor units necessary for dMLC IMRT to that of VMAT was in the range of 1.1–2.0. AAPM TG 119 test cases are useful to generate VMAT benchmark plans. At preclinical implementation stage, plan comparison of VMAT and IMRT plans of AAPM TG 119 test case allowed us to understand basic capabilities of VMAT technique. PACS number: 87.55.Qr

Spine stereotactic body radiation therapy plans: Achieving dose coverage, conformity, and dose falloff

We report our experience of establishing planning objectives to achieve dose coverage, conformity, and dose falloff for spine stereotactic body radiation therapy (SBRT) plans. Patients with spine lesions were treated using SBRT in our institution since September 2009. Since September 2011, we established the following planning objectives for our SBRT spine plans in addition to the cord dose constraints: (1) dose coverage—prescription dose (PD) to cover at least 95% planning target volume (PTV) and 90% PD to cover at least 99% PTV; (2) conformity index (CI)—ratio of prescription isodose volume (PIV) to the PTV < 1.2; (3) dose falloff—ratio of 50% PIV to the PTV (R(50%)); (4) and maximum dose in percentage of PD at 2 cm from PTV in any direction (D(2cm)) to follow Radiation Therapy Oncology Group (RTOG) 0915. We have retrospectively reviewed 66 separate spine lesions treated between September 2009 and December 2012 (31 treated before September 2011 [group 1] and 35 treated after [group 2]). The χ(2) test was used to examine the difference in parameters between groups. The PTV V(100% PD) ≥ 95% objective was met in 29.0% of group 1 vs 91.4% of group 2 (p < 0.01) plans. The PTV V(90% PD) ≥ 99% objective was met in 38.7% of group 1 vs 88.6% of group 2 (p < 0.01) plans. Overall, 4 plans in group 1 had CI > 1.2 vs none in group 2 (p = 0.04). For D(2cm), 48.3% plans yielded a minor violation of the objectives and 16.1% a major violation for group 1, whereas 17.1% exhibited a minor violation and 2.9% a major violation for group 2 (p < 0.01). Spine SBRT plans can be improved on dose coverage, conformity, and dose falloff employing a combination of RTOG spine and lung SBRT protocol planning objectives.

SU-F-T-600: Influence of Acuros XB and AAA Dose Calculation Algorithms On Plan Quality Metrics and Normal Lung Doses in Lung SBRT

PURPOSE To study the influence of superposition-beam model (AAA) and determinant-photon transport-solver (Acuros XB) dose calculation algorithms on the treatment plan quality metrics and on normal lung dose in Lung SBRT. METHODS Treatment plans of 10 Lung SBRT patients were randomly selected. Patients were prescribed to a total dose of 50-54Gy in 3-5 fractions (10?5 or 18?3). Doses were optimized accomplished with 6-MV using 2-arcs (VMAT). Doses were calculated using AAA algorithm with heterogeneity correction. For each plan, plan quality metrics in the categories- coverage, homogeneity, conformity and gradient were quantified. Repeat dosimetry for these AAA treatment plans was performed using AXB algorithm with heterogeneity correction for same beam and MU parameters. Plan quality metrics were again evaluated and compared with AAA plan metrics. For normal lung dose, V20 and V5 to (Total lung- GTV) were evaluated. RESULTS The results are summarized in Supplemental Table 1. PTV volume was mean 11.4 (±3.3) cm3 . Comparing RTOG 0813 protocol criteria for conformality, AXB plans yielded on average, similar PITV ratio (individual PITV ratio differences varied from -9 to +15%), reduced target coverage (-1.6%) and increased R50% (+2.6%). Comparing normal lung doses, the lung V20 (+3.1%) and V5 (+1.5%) were slightly higher for AXB plans compared to AAA plans. High-dose spillage ((V105%PD - PTV)/ PTV) was slightly lower for AXB plans but the % low dose spillage (D2cm) was similar between the two calculation algorithms. CONCLUSION AAA algorithm overestimates lung target dose. Routinely adapting to AXB for dose calculations in Lung SBRT planning may improve dose calculation accuracy, as AXB based calculations have been shown to be closer to Monte Carlo based dose predictions in accuracy and with relatively faster computational time. For clinical practice, revisiting dose-fractionation in Lung SBRT to correct for dose overestimates attributable to algorithm may very well be warranted.

SU‐F‐T‐589: HybridArc Planning Criteria for Brain SRS

PURPOSE To compare VMAT SRS plans, dynamic conformal arc (DCA) plans, and Brainlab iPlans capability of planning and delivering brain SRS plans by employing HybridArc. HybridArc utilizes both DCA and IMRT. Using HybridArc, the amount of DCA versus IMRT needs to be optimized. METHODS Four SRS patients with the aim of reducing brainstem dose were selected for this study. All patients were contoured in iPlan and transferred to Eclipse for VMAT planning. In iPlan, DCA plans were created for each case. Moreover, nine HybridArc plans with DCA-IMRT ratios between 9:1 through 1:9 were created with a single ring structure generated by subtracting 3 mm expansion of target from a 10 mm expansion of the target. Two static IMRT beams were used in each of the five DCA arcs for HybridArc. The dose was prescribed to DCA only and HybridArc plans and normalized so that the target volume (TV) receives 100% dose to 99.5% of the TV to achieve 120% ∼ 130% max dose within targets. Following metrics were compared: PITV, V12Gy, CGIc, CGIg, CGI, brainstem max dose, and total monitor units (MUs). RESULTS A brainstem max dose comparable with VMAT from 30% IMRT and less with 50% or more IMRT could be achieved. PITV decreased with increasing IMRT portion and begins to saturate past an IMRT portion of 30%. The CGIg index, which represents how fast the dose falls off, was better with HybridArc in all HybridArc plans. Total MUs increased with increasing IMRT but less than VMAT in all cases. CONCLUSION Overall, a lower brainstem max dose and a lower V12Gy with fewer MUs can be achieved with HybridArc. Considering all factors, it would be best to use a DCA-IMRT ratio of either 7:3 or 6:4.

SU‐E‐T‐122: Anisotropic Analytical Algorithm (AAA) Vs. Acuros XB (AXB) in Stereotactic Treatment Planning

Purpose: To evaluate dosimetric differences between superposition beam model (AAA) and determinant photon transport solver (AXB) in lung SBRT and Cranial SRS dose computations. Methods: Ten Cranial SRS and ten Lung SBRT plans using Varian, AAA _11.0 were re-planned using Acuros _XB_11.0 with fixed MU. 6MV photon Beam model with HD120_MLC used for dose calculations. Four non-coplanar conformal arcs used to deliver 21Gy or 18Gy to SRS targets (0.4 to 6.2cc). 54Gy (3Fractions) or 50Gy (5Fractions) was planned for SBRT targets (7.3 to 13.9cc) using two VAMT non-coplanar arcs. Plan comparison parameters were dose to 1% PTV volume (D1), dose to 99% PTV volume( D99), Target mean (Dmean), Conformity index (ratio of prescription isodose volume to PTV), Homogeneity Index [ (D2%-D98%)/Dmean] and R50 (ratio of 50% of prescription isodose volume to PTV). OAR parameters were Brain volume receiving 12Gy dose (V12Gy) and maximum dose (D0.03) to Brainstem for SRS. For lung SBRT, maximum dose to Heart and Cord, Mean lung dose (MLD) and volume of lung receiving 20Gy (V20Gy) were computed. PTV parameters compared by percentage difference between AXB and AAA parameters. OAR parameters and HI compared by absolute difference between two calculations. For analysis, paired t-test performed over the parameters. Results: Compared to AAA, AXB SRS plans have on average 3.2% lower D99, 6.5% lower CI and 3cc less Brain_V12. However, AXB SBRT plans have higher D1, R50 and Dmean by 3.15%, 1.63% and 2.5%. For SRS and SBRT, AXB plans have average HI 2 % and 4.4% higher than AAA plans. In both techniques, all other parameters vary within 1% or 1Gy. In both sets only two parameters have P>0.05. Conclusion: Even though t-test results signify difference between AXB and AAA plans, dose differences in dose estimations by both algorithms are clinically insignificant.

SU-E-J-202: Is Pretreatment Imaging at Each Treatment Fraction Needed in Spine SBRT to Enable Margin Reduction?

PURPOSE To study translational setup errors of Spine SBRT treatments using BodyFIX immobilization system and the potential for margin reduction using pretreatment imaging. METHODS 148 online match results of 39 Spine SBRT patients treated at our institution were reviewed. All treatments performed using Varian, TrueBeam and MI™, BodyFIX immobilization device. Prescriptions (dose-fractionation schedules) for the patients in the study are 8 Gy X 3 (n=24), 6 Gy X 4 (n=10) and 16Gy X 1 (n=5). For all patients, OBI was performed on verification day and before each treatment. Recorded shifts are from adopted couch positions after physician online review of KV image-pair and CBCT. Data analyzed by computing Mean error (M), Systematic error (Σ) and Random error (σ) for three translation directional shifts and 3D Vector length. To estimate correlation between imaging sessions, Anova-test and T-Test with alpha of 0.05 performed over the data. RESULTS Mean error (M), Systematic error (Σ) and Random error (σ) of Vertical direction are -1.81mm, 4.05mm and 2.01mm respectively. Similarly, Errors in Longitudinal direction are 0.76mm (M), 4.21mm (Σ) and 1.34mm (σ). Errors in Lateral direction are - 0.16mm (M), 3.22mm (Σ) and 1.83 mm (σ). Mean, SD and CI95% of 3D Vector lengths are 7.93mm, 4.41mm and 0.7mm respectively. Anova-test results show, P-Values for longitudinal and lateral shifts are > 0.3 and P-value of vertical shift is 0.054. This signifies mean of the each imaging session are not significantly different. T-Test on treatment sessions data of vertical shifts with respect to verification day shows, mean of Vertical shifts on verification day are different from Vertical shifts on treatment sessions. CONCLUSION Results shows immobilization with BodyFIX is reproducible in the mm range. To eliminate systematic error component and to enable margin reduction, pretreatment imaging prior to each treatment fraction is indispensable.

SU‐E‐T‐172: Portal Dosimetry of Gated VMAT; with and Without Gating

PURPOSE To study duty cycle and respiratory period influence on VMAT delivery using portal dosimetry. METHODS For this study, we selected 12 ARC fields from four different gated VMAT plans with Varian, TrueBeam 6 MV photon beam and HD120 MLC. For Planning and portal dose prediction we used Varian, Eclipse (Version 10.0) AAA and PDIP algorithms respectively. We acquired integrated portal images with and without gated arc fields delivered to EPID at SID 140cm. We simulated respiratory periods with Quasar, Motion Phantom. For gated delivery, we used planned duty cycle for patient respiratory trace, and 6 sec per breath (SPB) and duty cycles 40%, 30%, 20% and 10% with 3SPB. Eclipse, Portal dosimetry software was used for Gamma analyses of measured and predicted portal dose images. In addition, we compared with and without gated portal dose images of same arc. MLC log file analyses of delivered arcs were done with Mobius, Dose lab software. Gamma passing criteria is to have gamma<1 for greater than 90% data points. RESULTS For all arcs, predicted vs. measured dose distributions passed with criteria ΔD = 3% & DTA = 3mm, excluding few 10% duty cycle measurements. For these, pass criteria is ΔD = 5% & DTA = 4mm. Similarly, with and without gated portal dose image comparison passed the criteria of ΔD = 1% & DTA = 1mm excluding few 10% duty cycle measurements. For these pass criteria is ΔD = 2% & DTA = 2mm. For all delivered arcs, Leaf error 95 percentile is < 0. 1mm and error RMS is below 0.05mmConclusion: Except 10 % duty cycle, the gating influence on VMAT delivery is minimal. Therefore, gated VMAT QA can be done without gating. Duty cycles larger than 20% are recommended to minimize delivery errors.

SU‐E‐T‐604: 16‐MV Photon Beams Do Not Improve Plan Quality Compared to 6‐MV Photon Beams in Prostate Cancer IMRT

PURPOSE Photon energies 10 MV or higher are generally considered optimal for treatment of deep-seated pelvic targets. We performed dosimetric quality assessment of Prostate IMRT plans in patients treated with 6-MV and 16-MV Photons, to discern if 16-MV plan quality was superior to 6-MV treatment plans. METHODS From our institutional database, treatment plans of 84 patients previously treated for early stage prostate cancers were included in this retrospective study. Forty-two patients were treated with 6-MV and forty-two with 16-MV. Beam energy choice was based on linac capability, physician preference and not on patient separation. All patients were planned with a coplanar 7-F IMRT technique. The prescription dose (75.6-Gy), optimization technique and planning objectives were similar in all patients. Dose distributions were evaluated using various indices-Conformity-Index (CI), Healthy-Tissue Conformity Index (HTCI), Homogeneity-Index (HI), Gradient-Index (GI), Conformity-Number (CN), Normal-Tissue Integral Dose (NTID), Body-mass-index (BMI) and quality of coverage (QC). Rectal and Bladder dose-volume indices were evaluated per RTOG guidelines. Non-parametric Mann-Whitney test was applied in the statistical analysis and for a p-value <0.05, the null hypothesis is rejected. RESULTS Mean PTV was 197.9cc (±13.1) for the 6-MV group and 191.8cc (±10.3) for the 16-MV group. MUs per fraction were 905 (±32) for 6-MV and 862 (±41) for 16-MV plans. The CI, HTCI and GI were statistically similar between 6-MV and 16-MV plans (p=0.22). Indices HI, QC and CN all showed statistically significant improvement for 6-MV plans compared to 16-MV plans (p<0.03). NTID was slightly lower for 16-MV plans, but not statistically significant, compared to 6-MV plans. NTID correlated with BMI for 16-MV group (r=0.70) and weakly for 6-MV group (r=0.28). Rectal V65, V40 and Bladder V65 were similar between 6-MV and 16-MV plans. CONCLUSION We conclude that 16-MV photon beams do not provide additional dosimetric advantage compared to 6-MV photon beams in Prostate IMRT.

SU‐E‐T‐210: An Investigation for the Use of OSLDs for in Vivo Patient‐Specific Quality Assurance for Patients Requiring Bolus

PURPOSE For treatments that require addition of bolus, dose to the skin and superficial regions must be known for adequate plan evaluation. Dose calculation inaccuracies in these regions may occur because they are in the build-up region, as well as a Results of the IMRT/VMAT optimization algorithm ignoring the bolus during optimization. In this study, we evaluated the routine use of optically-stimulated luminescent dosimeters (OSLDs) as a means of in vivo quality assurance for superficial dosimetry in patients requiring bolus during treatment. METHODS Landauer nanoDot OSLDs were used to perform measurements on 11 patients (total 13 sites) including head and neck, anal, groin, and breast. Treatment techniques included RapidArc, IMRT, and 3D-CRT. All patients were treated with nominal energy 6MV. One OSLD was placed on the patients skin, under the prescribed bolus, for one fraction. Bolus thickness was 0.5 cm. Each OSLD was read using the Landauer MicroStar reader, which operates in cw mode with a 1-s read period, and corrected for sensitivity. Three readings were taken and the average calculated. Readings that were out of tolerance (15 percent) were repeated. OSLD readings were compared with TPS(Eclipse) calculations at a depth of 4mm (corresponding to the inherent OSLD build-up). RESULTS 10 sites passed with less than 10 percent deviation (measurement accuracy of standard nanoDot). 6 Sites showed less than 3 percent deviation, 8 sites showed less than 5 percent deviation. 1 Site showed greater than 13 percent deviation. 2 sites showed greater than 25 percent deviation, and upon repetition, yielded similar results. Large deviations correlated to treatment sites with irregular surfaces (tracheotomy, skin creases). CONCLUSION OSLDs are a quick and efficient way to perform in vivo quality assurance of doses in the superficial regions in patients requiring bolus for treatments. For simple geometries studied, TPS calculations show sufficient accuracy.

SU‐E‐T‐111: Radiochromic Film Measurements of Small Field Surface Doses from Flattened and Unflattened 6MV and 10MV Beams

Purpose: To measure surface doses from small fields from flattened and unflattened 6MV and 10MV photon beams using radiochromic film.Methods: GAFCHROMIC EBT2 film was used in a solid water phantom to make surface dose measurements from photon beams generated by a Varian TrueBeam Stx linac. Four energies were included: 6MV and 10MV, as well as two corresponding flattening filter free beams (6FFF and 10FFF). The field sizes considered where 10, 5, 4, 3, 2, 1 cm2. One 3×3 cm piece of EBT2 film was irradiated at the surface and at dmax in a solid water phantom for every energy and field size. Films were also placed at dmax and irradiated to obtain a calibration curve. All films were scanned in an EPSON V700 flatbed scanner and analyzed according to established protocols using in‐house developed software. Results: The measurements of surface dose for each field were normalized by the dose measured at dmax. Surface doses ranged from 10% for a 1 cm2 6MV field to 25% for a 10 cm2 6MV field. The ranges for other energies were 16–33%, 5–17%, and 10– 17% for 6FFF, 10MV, and 10FFF, respectively. As expected, the surface dose for both 6MV and 6FFF fields are higher than surface doses from 10MV and 10FFF fields. The surface dose relative to dmax is shown to be a monotonically increasing function of field size. Unflattened beams exhibit a higher surface dose in comparison to flattened beams and this discrepancy is larger for 6MV and 6FFF fields Conclusions: This work makes use of a simple technique for measuring surface doses from small fields relevant to SRS and SBRT, taking advantage of the high resolution offered by radiochromic film. The surface dose measured for a range of standard and small field sizes for flattened and unflattened beams has been presented.