Dosimetric impact of computational grid resolution and range pull-back on carbon-ion treatment planning

The effectiveness of carbon ion radiotherapy (C-ion RT) is influenced by the computational grid resolution and range-modulation strategies used during treatment planning. The study aimed to evaluate the dosimetric effects of grid resolution and range pull-back (shift) on tumor target coverage and organs-at-risk (OARs) sparing, establish the anatomical site-specific range safety margins or error tolerance. Dose-volume parameters for spot-scanning (SS) and uniform-scanning (US) carbon ion plans were evaluated using an in-house treatment planning system (TPS) at grid resolutions of 1 to 4 mm. A range pull-back, intentionally set between 1 and 5 mm in 1-mm increments, was introduced. Logistic regression models were employed to characterize the dose response relationships between range pull-back and dose metrics of target and OARs. US plans demonstrated relatively stable target coverage in D₉₅ and D₉₈ metrics (ΔD_95US = 0.12 %, ΔD_98US = 0.86 % when comparing 4 to 1 mm grids), whereas SS plans showed a slight decline in these coverage metrics (ΔD_95SS = 1.09 %, ΔD_98SS = 1.59 % for 4 mm versus 1 mm grids). OAR dose-volume parameters remained highly consistent at 1 to 2 mm grids but exhibited significant deviations at 3 to 4 mm grids. The range safety margins were routinely established as ±1 mm for thoracic targets, ±2 mm for head-and-neck targets, ±3 mm for abdominal targets, and ±2 to 3 mm for pelvic targets. Based on our findings, a 2 mm computational grid is recommended in most clinical scenarios of carbon ion radiotherapy, and a 1 mm grid is preferred for targets adjacent to serial OARs or small targets near critical OARs. Implementing anatomic site-specific range safety margins can reduce radiation-induced adverse events while preserving relative biological effectiveness (RBE)-weighted dose coverage within the tumor target.