ESTRO 2025 - Abstract Book

S4040

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

conservative limit without proper evidence. The aim of this study is therefore to revisit the tolerance by studying the dose coverage under different matching tolerances on retrospective patients’ data treated in the past year. Material/Methods: A retrospective review of the high-risk prostate patient who underwent proton therapy at our centre between August 2023 to March 2024 was conducted. The patients were typically treated with a four-field intensity-modulated proton therapy plan with a 3.5% range uncertainty and 5 mm setup uncertainty for robust optimization. RayStation 12A treatment planning system (TPS) was used to evaluate the dose coverage to the lymph node CTV after translating 1 mm to 8 mm in the superoinferior, anteroposterior and right-left directions. The dose coverage metric used in this study is D97%. Results: 19 prostate patients were included in this study based on the inclusion-exclusion criteria. The results of the dose coverage under translation of various magnitudes and directions are shown in Figure 1 . All the evaluated plans can achieve D97% dose coverage for the lymph node for translation up to 7 mm. For a 8 mm lymph node deviations, 4 out of 19 plans (21%) have under-coverage (<97%) of the lymph node CTV when considering motion in the SI direction, while 5 out of 19 plans (26%) has under-coverage when the lymph node CTV was translated in the AP direction. These findings suggest that the imaging protocol tolerance can be increased to 7 mm while maintaining sufficient dose coverage and achieving the clinical goal.

Conclusion: Our study provide evidence to prove that the tolerance for the imaging protocol can be increased to 7 mm. This could potentially reduce the number of re-setups, imaging dose to patient and overall shorter treatment time for the patient.

Keywords: Prostate cancer, Proton therapy, Imaging Protocol