ESTRO 2020 Abstract Book

S801 ESTRO 2020

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)

(EBT3) and a Euromechanics EMP 5001-424 solid water phantom are employed to verify this bolus attenuation. Bolus dimensions are 2cm thickness and 3.3cm inner diameter. Films are put on the phantom at SSD 100cm under the HDSB. The same technique explained above is used. Films are analysed with IBA OmniPRO IMRT software. Results The results obtained for HVL, TVL and CVL (figure 1) and the analysis of the film (figure 2) are shown in the following figures.

Conclusion The HDSB necessary thickness for the first HVL in an electron beam is smaller than 5mm for a 6MeV energy and the first TVL is smaller than 8mm for the same energy. With 9mm of HDSB the charge value is reduced to a 1% of Q o . After the film dosimetry is performed, it is concluded that the HDSB can be used to modulate intensity in a 6MeV electron beam. PO-1489 Treatment Planning Study for STereotactic Arrhythmia Radioablation (STAR) of Ventricular Tachycardia I. Bonaparte 1 , F. Gregucci 1 , A. Surgo 1 , R. Carbonara 1 , N. Vitulano 2 , F. Quadrini 2 , M. Grimaldi 2 , A. Di Monaco 2 , A. Fiorentino 1 1 Miulli General Regional Hospital, Radiation Oncology, Acquaviva delle Fonti-Bari, Italy ; 2 Miulli General Regional Hospital, Cardiology, Acquaviva delle Fonti- Bari, Italy Purpose or Objective STereotactic Arrhythmia Radioablation (STAR) was recently introduced for ventricular tachycardia (VT). With precise high-dose of radiation to a well define target, STAR could become more than an option in the next future. The first STAR treatment based on linear accelerator (Linac) in our department was delivered in September 2019, thus the present analysis reported the differences of treatment plans in terms of efficiency and efficacy.