ESTRO 2022 - Abstract Book

S1515

Abstract book

ESTRO 2022

For the target structures 4DDT revealed a dose calculation accuracy of 1% for D 50% considering the interplay effect. D 98% of the static plan was disturbed by up to 7% for the PTV and up to 2% for the CTV for 5 out of 6 patients. For liver patients D 33%_liver showed deviations between the two tools up to 14%, while for pancreas patients D 1%_stomach varied up to 6.5%. The two 4DDT methods agreed within 2% for D 98% and D 2% for PTV and CTV for all patients, except for one liver patient where the deviation increased up to 7% (Fig1). The γ -pass rates analysis between the p-4DDT and the f-4DCT showed an agreement above 91% for all patients, confirming the usability of p-4DDT for dose prediction (Fig2). Variations in starting phase and dose rate were negligible for the 4DDT outcome.

Conclusion The p-4DDT could be used prospectively to determine the impact of beam and organ motion for pancreatic and liver cases in scanned proton therapy. The systematic uncertainties covered by the PTV margins compensated well for the motion effects of the investigated indications preserving an excellent CTV coverage when motion was considered.

PO-1715 Assessing the dosimetric impact of intrafraction prostate motion in dose-escalated linac-based SBRT

V. Faccenda 1,2 , D. Panizza 1,3 , M.C. Daniotti 4 , S. Trivellato 1 , P. Caricato 1,2 , R. Lucchini 3,5 , S. Arcangeli 3,5 , E. De Ponti 1,3

1 ASST Monza, Medical Physics Department, Monza, Italy; 2 University of Milan, Department of Physics, Milan, Italy; 3 University of Milan Bicocca, School of Medicine and Surgery, Milan, Italy; 4 University of Milan Bicocca, Department of Physics, Milan, Italy; 5 ASST Monza, Radiation Oncology Department, Monza, Italy Purpose or Objective The aim of this study was to investigate the impact of intrafraction prostate motion on dose metrics and the effect of beam gating and motion correction in dose-escalated linac-based SBRT. Materials and Methods A total of 56 fractions from 13 patients treated with dose-escalated SBRT using VMAT technique with FFF arcs, were examined. Real-time 3D prostate motion data were acquired using a novel electromagnetic tracking device. Beam delivery was interrupted whenever the prostate trespassed a 2-mm safety tolerance in any of the three spatial directions and table couch position corrected unless the offset was transient. Prostate trajectories with and without beam gating and motion correction events were reconstructed and analyzed with in-house C++ code. Both actually delivered treatments (case A) and non-gated treatments (case B) were simulated by incorporating the observed prostate motion for each fraction into the patient original treatment plan with an isocenter shift method. The total dose of each patient was then estimated by accumulating the motion inclusive dose distributions recalculated with Monaco Monte Carlo TPS from all fractions. Target and organs at risk (OARs) parameters were derived from reconstructed DVHs and compared to planned values. In addition, all dosimetric parameters were compared with protocol dose constraints. Results Average values of mean prostate displacements in case A were -0.2 mm [-1.5 – 0.8], 0.1 mm [-1.4 – 1.5], and -0.3 mm [-1.7 – 1.4] in lateral, longitudinal, and vertical directions, respectively. The same values in case B were -0.3 mm [-3.1 – 0.8],