Dosimetric Analysis of Analytical Carbon-ion Pencil Beam Algorithm in Treatment Planning for Head-and-Neck Tumor Radiotherapy

Carbon ion radiotherapy is expected to become a workhorse for head and neck tumors radiotherapy due to its inverted depth dose distribution and high relative biological effectiveness, which has the advantages of less damage to normal tissues and low side effects. The accuracy of dose algorithms directly affects the accuracy of clinical irradiation doses, and reducing the dose calculation error of carbon ion radiotherapy could help improve patient’s survival and local control rates. In this paper, the dose distribution of carbon ion radiotherapy for 5 head and neck patients were calculated by the analytic pencil beam algorithm and validated using Monte Carlo method. Three analysis methods were carried out for the calculated and simulated results: 1) Gamma pass rate analysis under multiple evaluation criteria combination (2%/2 mm, 3%/2 mm, 3%/3 mm, 3%/5 mm), 2) DVH and reference index analysis, 3) central axis dose curve comparison. The analysis results showed that the average gamma pass rate of the pencil beam algorithm was 90.441% under the evaluation criteria of 3%/2 mm, which almost met the dose calculation requirements of carbon ion radiotherapy for head and neck tumors. The calculated results with pencil beam algorithm are more accurate in the transverse beam direction, but there is a range deviation by which the structure inhomogeneity and cavity exited in the irradiation path. In order to reduce the impact of pencil beam dose calculation error, a more conservative angle selection to carbon-ion beam treatment of head and neck tumors is recommended. The results herein could be used to guide the design of carbon ion radiotherapy for head-and-neck tumors based on analytical pencil beam algorithm.