ESTRO 2022 - Abstract Book

S1514

Abstract book

ESTRO 2022

Conclusion From these results, we conclude: 1) reference plan VaD may not be representative of the delivered dose in conventional WFs; dose-response relationships based on reference VaD may be confounded by dosimetric deviations from reference, and 2) the effect of dose mapping on rectum VaD should be carefully considered, especially if accumulated dose is to be used for plan adaptation.

PO-1714 parameter vs logfile based 4D proton dose tracking for small movers

F. Lebbink 1,2 , S. Stocchiero 3 , E. Engwall 4 , M. Stock 1 , D. Georg 3 , B. Knäusl 1,3

1 MedAustron Ion Therapy Centre, Medical Physics, Wiener Neustadt, Austria; 2 Medical University of Vienna, Department of Radiation Oncology, Wien, Austria; 3 Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria; 4 RaySearch Laboratories AB, Physics, Stockholm, Sweden Purpose or Objective The motion compensation strategy in particle therapy depends on the anatomic region, motion amplitude and underlying beam delivery technology. The prerequisite for improving existing treatment concepts for moving targets is the quantification of the interplay effect between organ motion and beam delivery and its impact on the dose distribution and hence treatment delivery accuracy. While retrospective logfile based analysis gives insight into the patient’s breathing and beam delivery time structure, a prospective 4D dose prediction allows adaptation on a patient specific basis. Materials and Methods Dose distributions of 3 pancreas and 3 liver cancer patients with motion amplitudes below 4mm were analysed. All patients were treated with scanned pulsed proton beams delivered by a synchrotron. Dose prescription was 5x7.5 Gy(RBE) for pancreas and 15x4.68 or 10x5 Gy(RBE) for liver. The treatment planning system RayStation8B (MCv4.2) (RaySearch) was used employing robust optimisation for mitigating different organ fillings. Treatment accuracy was determined using: (1) file based 4D dose tracking (f-4DDT) considering the time structure from accelerator logfiles and surface scanner breathing patterns (C-Rad) for each fraction; (2) parameter based 4D dose tracking tool (p-4DDT). Input parameters encompassed the averaged dose rate extracted from the accelerator logfiles over all fractions, scanning speed as well as constant breathing cycle length. The p-4DDT method was used additionally to investigate the influence of the starting phase and dose rate. Both methods considered the given time structures for tracking the static dose on 8 4DCT phases. This dose distribution was mapped onto the planning CT using deformable image registration and accumulated for all fractions. DVH parameters and γ -pass rates with a 2%/2mm criteria were used for dosimetric evaluation.

Results