Dosimetric impact of daily skin surface variations in kidney stereotactic ablative proton therapy

Abstract

Purpose Proton beam therapy (PBT) is a promising modality in kidney stereotactic ablative radiotherapy (SABR). However, the sensitivity of PBT to materials in the beam path enforces special considerations in patient positioning. In clinical practice, we observed significant variability in the skin surface, which may perturb delivered dose. Here, we characterize the impact of skin surface variations in kidney stereotactic body proton therapy (SBPT). Materials and methods Patients treated with kidney SBPT between 2024 and 2025 were included. Assessment plans were created using 3.0 mm positioning uncertainty in robust optimization to deliver a prescription dose of 50 Gy(RBE) in 5 fractions to 99% of the target volume (iCTV). Virtual CTs were generated for dose accumulation by mapping HU from the planning scan onto the day-of-treatment CBCT. The assessment plan was mapped to each virtual CT set. The dose to 95–100% of the target (iCTV D95-Dmin) and the volume of healthy kidney receiving 10–50 Gy (V10-V50) were compared between planned and accumulated distributions. Results Fifteen patients treated in 75 fractions were included. The depth of the tumor centroid along the beam path varied by up to 17.4 mm relative to the planned anatomy. Depth variations exceeded 5.0 mm in 22.7% of fractions, and resulted in lower iCTV D95, D98, D99, and Dmin (p < 0.01) compared to assessment plan distributions. The accumulated D99 dropped by up to 24.3%. Conclusion Skin surface variations may compromise kidney SBPT treatment quality. Further investigation into mitigation strategies, such as adaptive radiotherapy, are merited.