ESTRO 38 Abstract book

S1004 ESTRO 38

phase of the intensity modulated proton treatment, IMPT. A plan uncertainty analysis was carried out to assess the feasibility, at planning level, compared to the consolidated VMAT photonic approach. Material and Methods Twenty patients presenting advanced stage HCC, unsuitable to other loco-regional therapies, and treated with VMAT 60 Gy in 6 fractions, were selected from the institutional database for this study. They were all immobilized with thermoplastic body mask including a styrofoam block as abdominal compressor. 4D-CT was acquired to evaluate and account for any residual respiratory related motion, and CTV delineated after co- registration with MRI. CTV to PTV margin was 4 to 7 mm. IMPT plans were generated for a Varian ProBeam system using spot scanning, optimized with the Nonlinear Universal Proton Optimizer (v. 15.07) using the robust optimization tool Proton Dose Perdurbation PDP, and calculated with the Proton Convolution Superposition algorithm with a 2.5 mm grid. Two to three fields were arranged, according to the patient anatomy. The perturbation features used for the robust optimization were the CT calibration variation of ±3%, and the isocentre localisation uncertainty of 6 mm to cover the worst case scenario. VMAT plans were designed for a Varian TrueBeam linac, with two 10MV FFF partial arcs, optimized with PRO (v. 15.07) and calculated with Acuros dose calculation algorithm with a 2.5 mm grid. Quantitative metrics to compare IMPT and VMAT plans were derived from the DVHs. Results The target coverage was fully achieved by both VMAT and IMPT plans, as well as all the OAR objectives. The inclusion of the uncertainties in the IMPT optimization lead to some deterioration in the target dose homogeneity, while none of the coverage parameters or OAR objective was violated. Comparison, in the worst case scenario of 3% HU and 6 mm isocentre position errors, between robust IMPT and VMAT yield to PTV coverage in terms of D 98% of 58.7±0.6 and 58.8±0.4 Gy, respectively. For the liver deducted the PTV volume, the V <21Gy (goal <70 cm 3 ) was 1081 ±105 and 965±120 cm 3 , the integral dose was 2.8±1.2 and 7.5±2.9 Gy·cm 3 ·10 5 for IMPT and VMAT, respectively. Conclusion Robust optimization IMPT with the support of abdominal compression, can be considered a viable solution also for advanced HCC patients. EP-1851 VMAT versus 3D-CRT for the irradiation of left breast or chest wall plus supra/infraclavicular nodes M.A. Gilio 1 , R. Bagnoli 2 , M. Mignogna 2 , A. Tofani 3 , S. Linsalata 3 1 University of Pisa, Medical Physics, Pisa, Italy ; 2 Azienda USL Toscana nord ovest, Radiotherapy, Pisa, Italy ; 3 Azienda USL Toscana nord ovest, Medical Physics, Pisa, Italy Purpose or Objective The aim of the present study is to compare a dual arc VMAT technique and half beam block 3D-CRT technique for the irradiation of left breast or postmastectomy chest wall plus supra- and infraclavicular lymphnodes. The two techniques were compared in terms of target coverage, doses to OAR and total MU. Material and Methods 26 planning CT were segmented for left breast or chest wall, supra/infraclavicular nodes, heart, omolateral and controlateral lungs, spine and controlateral breast. For every CT two plans were generated: a 3D-CRT with two tangential half beam block for the breast or chest wall and two nearly opposed half beam block for the supra\infraclavicular lymphonodes, using the same isocenter for all the beams (6MV, 10MV or 15MV - Philips

targets, robust optimization should be performed with Monte Carlo to provide reliable results. EP-1849 Evaluation of plan modulation parameters on pre-treatment QA results during VMAT and SABR M. Chun 1 , J. Kim 1 , C.H. Choi 1 , J. Yoon 1 , H.J. An 1 , J. Son 1 , O. Kwon 1 , J.M. Park 1 1 Seoul National University Hospital, Department of Radiation Oncology, Seoul, Korea Republic of Purpose or Objective To rigorously demonstrate how well the plan parameters and modulation indices impact on the pre-treatment quality assurance on gamma passing rates (GPR) for patient-specific quality assurance of volumetric modulated arc therapy (VMAT) and stereotactic ablative body radiotherapy (SABR). Material and Methods A total of 332 VMAT and 339 SABR patients’ plan with TrueBeam STx linear accelerator (Varian Medical System) was selected. Two-dimensional gamma analysis were performed between calculated and measured dose distribution with 2 mm/2% criteria for VMAT, and 1 mm/2% for SABR, respectively. Modulation indices by considering the speed of MLC (MI s ), acceleration of MLC (MI a ), both MLC properties, gantry rotation, and dose rate variation (MI t ), and also modulation complexity score for VMAT (MCS v ), and leaf travel MCS (LTMCS) were obtained. Plan parameters of mean aperture, total monitor unit (MU), and the percentage of jaw tracking (%JT) were also acquired. Statistical analysis between GPR and plan modulation parameters were performed with Spearmans’ rank correlation coefficient ( r s ) with respect to single and multiple parameter regressions to find the most influencing parameters for each treatment technique. Results In VMAT plans, the r s to GPRs with MI s , MI a , and MI t were - 0.392, -0.374, and -0.375, respectively, which showed the MLC, gantry rotation, and dose rate variation largely impacted on the GPR while the majorities were with speed of MLC. MCS v and LTMCS showed r s to GPR of -0.155 and - 0.028, relatively less than MI s , MI a , and MI t . In SABR plans, MCS v and LTMCS showed r s to GPR of 0.261 and 0.310, which was higher than those with MI s , MI a , and MI t (-0.044, -0.108, and -0.102), presenting the definition of modulation complexity score highlighted more on MU in MCS v and LTMCS. The r s to GPR for MU and the %JT were - 0.299 and -0.265 in SABR while those in VMAT were -0.116, and -0.036, respectively. The highest r s to GPR in multiple parameter regression was 0.425 with MI s and %JT combination in VMAT, while those in SABR was with MU and %JT combination (0.376). Conclusion The MLC modulation mostly affected the GPR when delivering the VMAT plans, while these representing tendencies moved to total monitor unit, and the percentage of jaw tracking for SABR. EP-1850 Intensity modulated protons: feasibility for hypofractionated hepatocellular carcinoma treatment L. Cozzi 1 , T. Comito 2 , A. Fogliata 2 , C. Franzese 2 , S. Tomatis 2 , M. Scorsetti 1 1 Humanitas Research Hospital and Humanitas University, Radiation Oncology Dept and Biomedical Science Dept, Milan-Rozzano, Italy ; 2 Humanitas Research Hospital, Radiation Oncology Dept, Milan-Rozzano, Italy Purpose or Objective The radiation treatment of hepatocellular carcinoma HCC with a spot scanning protonic approach is challenging due to the respiratory induced motion of the target. In the absence of complex rescanning and gating procedures, a simple approach is here explored, based on the combination of an abdominal compression to reduce the internal motion, and a robust optimization in the planning