ESTRO 2020 Abstract book

S851 ESTRO 2020

framework. APM substitutes the IDD with a sum of up to 13 Gaussian functions. This enables an efficient integration of lung degradation effects through analytical convolution with Gaussian modulation kernels while maintaining compatibility with matRad’s biological models. Proton and carbon ion treatment planning relies on validated base data against the Syngo TPS from the Heidelberg Ion Therapy Center (HIT). For helium ions, generic machine data was fitted to MC simulations. Each plan was optimized on both physical and RBE- weighted dose with similar objectives and dose per fraction. The plans were recalculated including heterogeneity effects induced by lung-tissue. The resulting dose distributions were compared to the uncorrected dose in terms of homogeneity D 5 -D 95 , dose coverage ΔD 95 , and dose difference in the planning target volume (PTV). Results In Figure 1, heterogeneity-corrected RBE-weighted dose distributions of one selected patient are shown for all particles, and the relative difference to the uncorrected dose distributions which reveals deviations up to 11.3% for helium ions. Figure 2 shows selected quality indicators for all patients inside the PTV. When heterogeneity is considered, the mean dose decreases inside the PTV. The correction yields a larger D5-D95 (less homogeneous) and a lower D95 (less dose coverage). This holds true for both physical and RBE- weighted dose, yet no systematic differences can be identified. However, the difference is larger for helium and carbon ions than for protons.

Material and Methods The anatomy and target volume of the first treated patient were used for this study. A dose of 25 Gy in one fraction was prescribed to the planning target volume (PTV). Treatment plans were generated on Varian TrueBeamTM and 6-MV flattening filter free (FFF) beam (Eclipse planning system, V.15). Firstly, several plans (Plans #1-4) prescribed to the 75.0% isidose line were generated and compared to chose the best one. The 4 plans differed in terms of number, length arcs and couch rotations. Secondly, from the best plan, other treatment plans were generated: one with 10FFF beams, and the other 3 plans were optimized to have a prescription isodose line between 63% to 75% (corresponding to dose heterogeneity of 150% and 130%). All plans were optimized to be conformal to the PTV and meet dose constraints on the organs at risk (AAPM Task Group 101). The plans were compared by prescription isodose line, plan conformity index, as well as dose to the healthy heart. To assess the delivery efficiency, planned monitor units (MU) and estimated treatment time were evaluated. Results For Plans #1-4, the PTV coverage ranged from 96- to 98.5%; with a mean cardiac dose from 4.9-5.2Gy; MUs ranged from 7300 to 8541 for an beam-delivery-time (BDT) of 5.5, 5; 6 and 7 minutes, respectively. For the second part of the analysis, from the best geometrical conformation Plan, other 4 plans (Plans #5-8) with 10FFF approach and plans prescribed to 70, 72 and 63 isodose lines, were optimized. The PTV coverage ranged from 96- to 98.6%; with a mean cardiac dose from 4.9-5.2Gy, and MU from 6269 to 9394. CI ranged from 0.96-0.98. The BDT was ranged from 3 (for plans with 10MV-FFF beams) to 7 minutes. Conclusion Clinically acceptable plans were generated with Linac- based STAR approach. All plans were considerably more efficient in terms of MU and delivery time. The 10FFF approach was faster but it was not considered for all patients, due to the presence of cardiac device such as ICD. PO-1490 Lung degradation effects on RBE-weighted dose in proton, carbon and helium treatment plans N. Homolka 1,2,3 , H. Wieser 1,2,4 , M. Bangert 1,2 , M. Ellerbrock 2,5 , N. Wahl 1,2 1 German Cancer Research Center DKFZ, Medical Physics in Radiation Oncology, Heidelberg, Germany ; 2 Heidelberg Institute for Radiation Oncology HIRO, National Center for Radiation Research in Oncology NCRO, Heidelberg, Germany ; 3 Ruprecht Karl University of Heidelberg, Medical Faculty, Heidelberg, Germany ; 4 Ludwig Maximilian University of Munich, Faculty for Physics, Munich, Germany ; 5 Heidelberg Ion Therapy Center HIT, Department of Radiation Oncology- Heidelberg University Hospital, Heidelberg, Germany Purpose or Objective Particle treatment planning for tumors in or near lung is compromised by the inhomogeneous tissue causing a degradation of the integrated depth dose (IDD), leading to under-dosage of the target and to unwanted dose distal to the target. This effect is independent of the algorithm used for dose calculation but is caused by insufficient information about sub-CT-resolution structures. Previous studies showed that the degradation may be modeled via a “modulation power” assigned to lung-tissue. We investigate the impact of this degradation on physical and biological effective (RBE-weighted) dose distributions for clinical patient treatment plans for proton, helium, and carbon ions. Material and Methods Patient treatment plans were calculated and optimized using the treatment planning system (TPS) matRad. matRad’s pencil beam algorithm was used in combination with the analytical probabilistic modeling (APM)