Impact of Nuclear Halo Modeling on Secondary Cancer Risk Estimates In Proton Craniospinal Irradiation
Abstract
Purpose
To quantify the impact of proton dose calculation methodology, including nuclear halo contributions, on secondary cancer risk for near-field organs following proton craniospinal irradiation.
Methods
Lifetime attributable risk at age 70 (LAR₇₀) for lung and kidney was estimated in nineteen patients treated with proton pencil beam scanning using three dose calculation approaches: an analytical algorithm with simple halo correction (AA), TOPAS Monte Carlo simulation with constant relative biological effectiveness (MC-1.1), and TOPAS Monte Carlo simulation incorporating a variable RBE model based on dose-averaged linear energy transfer (MC-LETd). Lung LAR₇₀ was estimated using a BEIR VII–based framework with organ equivalent dose, while kidney LAR₇₀ was estimated using the RadRAT kidney model based on mean organ dose. For each patient, LAR₇₀ estimates and relative differences for MC-1.1 and MC-LETd were calculated with respect to AA.
Results
Compared with AA, Monte Carlo–based dose calculations resulted in higher estimated lifetime attributable risk at age 70 (LAR₇₀) for both lung and kidney. For lung cancer, LAR₇₀ increased from 1.07% (95% CI: 0.84–1.29) with AA to 1.28% (1.02–1.54) using MC-1.1 and 1.35% (1.08–1.61) using MC-LETd, corresponding to relative increases of 21.7% (17.8–25.6) and 28.9% (23.9–33.9), respectively. Kidney cancer LAR₇₀ increased from 0.41% (0.21–0.62) with AA to 0.46% (0.24–0.68) with MC-1.1 and 0.50% (0.27–0.74) with MC-LETd, corresponding to relative increases of 24.2% (11.5–36.9) and 40.5% (23.5–57.5). Across both organs, MC-LETd yielded the highest risk estimates, due to the elevated dose-averaged LET (>2 keV/µm) near the field edge.
Conclusion
The findings highlight the importance of accurately characterizing dose in near-field organs when estimating secondary cancer risk. Increases in estimated risk for near-field organs were non-negligible, suggesting that Monte Carlo–based dose calculation may be important for secondary cancer risk assessment in proton craniospinal irradiation.