LINAC-based online adaptive stereotactic ablative radiotherapy for kidney tumors: In silico analysis

Daily anatomical variations can compromise dosimetric plan quality in kidney stereotactic ablative radiotherapy (SABR). The purpose of this study is twofold: (1) To characterize the impact of daily anatomical variations on delivered dose in kidney SABR. (2) To compare linear accelerator (LINAC)-based adaptive radiotherapy (ART) against target margin expansion in mitigating dose perturbations due to anatomical variations. Patients treated with 5-fraction stereotactic ablative proton therapy between 2024 and 2025 were retrospectively replanned with volumetric modulated arc therapy to deliver 50 Gy to 99% of the target volume. Cone beam computed tomography (CBCT) setup images were converted to virtual CTs (vCTs) via a commercial deformable registration-based algorithm. Dose perturbations were modeled by mapping plans with increasing target margins (0.0, 0.5, 1.0 mm) to each vCT. In parallel, plans were reoptimized on each vCT to simulate online adaptation. One-way within-subjects analysis of variance was used to compare dosimetric parameters. Ten patients were included in the study. Anatomical variations resulted in the 99% internal clinical target volume (iCTV) coverage (D99) falling below the prescribed dose in 30 (60%) of fractions when no margin was added. The addition of 0.5 mm and 1.0 mm margins raised coverage rates to 92% and 98%, respectively. The largest single-fraction drop in iCTV D99 was 2.2% (22 cGy) when no margin was added. ART achieved 100% target coverage retention at the prescribed dose level. Addition of planning margins increased kidney mean dose (p = 0.02) and volume receiving at least 40 Gy (p = 0.01). ART marginally decreased global 10 to 50 Gy isodose volumes (p > 0.05). Online adaptation improved retention of target dose coverage at the prescription level. However, perturbations due to daily anatomical variations were minimal. The clinical impact of ART may be limited to organ sparing rather than mitigating dose perturbations in the kidney SABR context.