A modified broad beam model for uniformly scanned carbon ion therapy accounting for field inhomogeneities

The purpose was to perform beam modeling and plan verification for uniform scanned (US) carbon ion therapy delivered by Heavy Ion Medical Machine (HIMM). As the field inhomogeneity is larger than the commonly accepted 3% gamma pass rate criteria, a simple flat broad beam model is no longer accurate. A modified broad beam model accounting for field inhomogeneity was proposed and validated. The commissioning process for automatic beam modeling was described. Characteristic lateral dose distributions were collected at different depths for each field combination. The field inhomogeneities were modeled as two-dimensional interpolations of the lateral dose data. Dose calculation was based on ray-tracing combined with an asymmetric double-sigmoid function describing the dose at field edge. Two types of plan verifications on three US nozzles were carried out: 1) without range compensators 2) with range compensators. The distance-to-agreement at distal fall-off was within 1 mm. The absolute dose calibration was within 1.9% and the mean value was 0.6 (±0.5%). The verification plans satisfied 95% pass rate based on 3mm/3% gamma analysis for all three nozzles. Comparison with literature suggested a clinical factor of 1.33. The modified broad beam model satisfied the gamma analysis requirement and could be used for commissioning carbon US beams.