ESTRO 36 Abstract Book

S822 ESTRO 36 _______________________________________________________________________________________________

Objective’ optimization tool in Varian Eclipse (v 13.6) incorporating the Photon Optimizer algorithm (v 13.6.23). The RA approach currently used in this study implements two complete arcs to deliver at least 95% of the prescribed dose to the Planning Target Volume (PTV) while minimizing dose to the surrounding Organs at Risk (OAR). In general, RA tends to use fewer MUs per treatment fraction than Intensity Modulated Radiation Therapy (IMRT) with an associated reduction in the risk of secondary induced cancers. The MU Objective tool offers the possibility to further decrease total Monitor Units while maintaining clinically acceptable plan quality. Material and Methods Thirty clinically approved RA plans (prostate only n=22, prostate and nodes n=8) were selected for re-optimization using the MU Objective tool. This tool allows variation of the Minimum MU, Maximum MU and Strength (S). The ‘S’ parameter weights the optimizer to reach the MU goal within the defined Min MU and Max MU limits. Based on a previous study [1], the Min MU was set to 0%, the Max MU to 50% of the total clinical plan MUs for the non-optimized RA plan and ‘S’ was set to the maximum value of 100. The prescribed doses were either 74Gy in 37 Fractions (or 60 in 20), collimator angles were 30 ⁰ and 330 ⁰ to minimize the tongue-and-groove effect, jaw tracking was enabled and all plans were treated at 6MV and 600 MU/minute maximum dose rate. The dose/volume objectives for the PTV and OAR were unchanged. Dose calculations were performed using the Anisotropic Analytic Algorithm (v 13.6.23) with a calculation grid size of 2.5 mm, taking into account inhomogeneity correction and disregarding air cavity correction. To determine the quality of the absorbed dose distributions resulting from smoothing, the Paddick Conformity Index (CI PAD ) and the International Commission on Radiation Units (ICRU) Homogeneity Index (HI) were calculated for all plans [2]. CI PAD = (TV PI ) 2 /(PI x TV) Where PI is the volume of the prescription isodose line (95%), TV PI is the target volume within the PI, and TV is the target volume. HI = (D 2% -D 98% )/D 50% Where D 50% is the dose received by 50% of the target volume and so on. Results The MU Objective tool resulted in a reduction of total prostate RA plan MUs by approximately 29%. The average ICRU HI for the prostate patients varied from 0.055 to 0.111 (σ = 0.015, CI: 0.07-0.08). The CI PAD varied overall from 0.617 to 0.860 (σ = 0.067, CI: 0.72-0.78).

Conclusion The MU Objective tool facilitates the reduction of total prostate RA plan MUs with PTV coverage and OAR sparing maintained. A lower total MU number should translate to lower leakage from the linear accelerator and less scatter within the patient. EP-1530 Validation of RayStation Fallback Planning dose-mimicking algorithm L. Bartolucci 1 , M. Robilliard 1 , S. Caneva- Losa 1 , A. Mazal 1 1 Institut Curie, Radiotherapy, Paris, France Purpose or Objective To demonstrate with end-to-end tests the ability of RayStation v5.02 (RaySearch Laboratories AB, Stockholm, Sweden) fallback planning module (RFP) to perform an accurate Helical Tomotherapy (HT) to volumetric modulated arc therapy (VMAT) plan conversion by validating the dose-mimicking algorithm used during the automatic optimization of the fallback plans. Material and Methods Thirty patient plans of various treatment sites previously treated with HT were switched to 6 MV dual-arc VMAT plans using RFP and default dose-mimicking algorithm parameters. For the purpose of this study no further optimizations were performed and delivery quality assurance (DQA) were designed for each fallback plan. DQA were delivered on a TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA) and planar/absolute dose measurements were acquired using the ArcCHECK diode array (Sun Nuclear Corporation, Melbourne, FL) with an insert containing an Exradin A1SL ionization chamber (Standard Imaging, Middleton, WI). 3D dose distributions in the patient geometry were reconstructed within 3DVH software (Sun Nuclear Corporation, Melbourne, FL) by using ArcCHECK Planned Dose Perturbation (ACPDP). Agreement between planned and delivered dose was eventually evaluated with global and local 2D/3D gamma-index analysis (3%/3mm and 2%/2mm criteria) and DHV-based comparisons were performed using the following dosimetric parameters: quality of coverage (Q=D98%/Dref), mean dose to target (MDT=Dmean/Dref) and integral dose to organs at risks (ID_OAR=∑·Di·Vi).