ESTRO 2023 - Abstract Book

S1749

Digital Posters

ESTRO 2023

To evaluate the impact of different types of pencil beam scanning proton accelerators and spot sizes on interplay effects and plan quality for lung cancer patients treated with SBRT. Materials and Methods Five lung cancer patients treated in our institution with photon SBRT were selected to represent varying tumor volumes, and respiratory motion amplitudes for this retrospective study. Respiratory motion amplitude ranged from 0.3 to1.0 cm with compression. For each patient, plans were created using: 1) linear accelerator proton minibeams (LPMB) ( σ : 0.9-3.9 mm); 2) linear accelerator proton beams (LPB) ( σ : 2.9-5.5 mm); and 3) cyclotron-generated proton beams (CPB) ( σ : 2.7- 7.0 mm). All plans were robustly optimized on the GTV using 4DCTs with the single-field optimization technique. For each patient, all plans were normalized to have the same dose to 99% of the GTV. Interplay effects were evaluated for ten scenarios (i.e., ten starting breathing phases) using machine generic time models and a constant breathing cycle length of 4 seconds. The energy switching time is 0.005 s for LPMB and LPB, and one second for CPB. The dose was computed on ten breathing phases, based on the spot distribution, and the mapped doses were accumulated on the reference phase through deformable registrations. The acceptance criterion for interplay effects was at least 95% of the GTV receiving the prescription dose (V100%RX>95%). Volumetric repainting (VR) was performed a number of times, based on each case, until the acceptance criterion was met. To assess plan quality in the nominal scenario, we compared the conformity index (CI), R50, and the percentage of lung volume receiving 20 Gy (V20Gy). CI is defined as the ratio of the 100% isodose volume to the GTV. R50 is defined as the 50% isodose volume divided by the GTV. Results Average CIs of 1.90±0.3, 2.08±0.4, and 2.24±0.3; average R50s of 5.86±1.5, 7.17±1.6, and 8.79±2.1; and average lung V20Gy values of 2.39±1.2, 3.04±1.4, and 3.90±1.6 were obtained for LPMB, LPB, and CPB plans, respectively. In interplay effect evaluation, the mean V100%RX values of the GTV were 94.02±6.4%, 98.03±2.8%, and 99.67±0.3%for LPMB, LPB, and CPB plans, respectively. After VR, the V100%RX values were improved (on average) by 5.61%, 1.88%, and 0.19% for LPMB, LPB, and CPB plans, respectively, leading to all plans meeting the acceptance criteria for interplay effects. For the same number of VR, delivery time was the lowest for LPB plans, while delivery times for LPMB and CPB plans were comparable.

Conclusion LPMB, with the smallest spot size, produced superior plan quality in conformity, intermediate dose spill, and normal lung tissue sparing. In the absence of repainting, proton machines with large spot sizes generated more robust plans against interplay effects. VR improved the plan robustness against interplay effects for modalities with small spot sizes and fast energy changes, preserving the low dose sparing aspect of the LPMB, even when motion was included.