Advancement in Hypofractionation of Carbon Ion Radiotherapy for Prostate Cancer

Purpose: To assess the effect of alteration in dose fractionation on late radiation toxicity in prostate cancer patients treated with conformal carbon ion radiotherapy (C-ion RT) Methods and Materials: Applied carbon ion dose in 20fractions were either of 66.0Gy RBE or 63.0Gy RBE and that in 16fractions was 57.6Gy RBE.. Seventy hundreds and forty prostate cancer patients who received the C-ion RT of 20fractions/5weeks or 16fractions/4weeks were analyzed in regard to late radiation toxicity. In addition, biochemical relapse free and survival rates in patient subgroup treated with each dose-fractionation were analyzed. Results: No grade 3 or worse toxicities were observed either in the rectum or genitourinary system (GU) and the incidences of grade 2 rectum and GU toxicity in the 664 patients followed up more than 12 months were 1.9% and 4.8%, respectively. The incidence of late GU toxicity in patients treated with 16 fractions (1.5%) was lower than that of 20fractions (6.2%). Dose dependency was observed between 66.0Gy RBE and 63.0Gy RBE in patients received C-ion RT of 20fraction. Overall biochemical relapse free rate (BRF) at 5 years for 740 patients was 90.2%. Gleason\u27s score, T-stage, and initial PSA were significant prognostic factors for BRF, and T-stage was also a significant prognostic factor for overall survival rate. The 5 year-BRF of the patients treated with 16 fractions C-ion RT (87.5%) was comparable to that of 20 fractions (90.5%). Overall and cause-specific survival rates at 5 year of the entire group were 95.4% and 98.8%, respectively.Conclusion: C-ion RT of 57.6Gy RBE in 16 fractions could offer even lower incidence of GU toxicity and comparable BRF compared to that of 63.0 or 66.0Gy RBE in 20fractions. Advancement in hypofractionation could be safely achieved in the C-ion RT for prostate cancer