The clinical relative biological effectiveness and prostate‐specific antigen kinetics of carbon‐ion radiotherapy in low‐risk prostate cancer

BACKGROUND: To evaluate the clinical relative biological effectiveness (RBE) of carbon‐ion radiotherapy (C‐ion RT) for prostate cancer. METHODS: The records of 262 patients with low‐risk prostate cancer (median age, 65 [47–80] years) treated with C‐ion RT at QST Hospital, National Institutes for Quantum Science and Technology in Japan during 2000–2018 were reviewed retrospectively. Four different protocol outcomes and prostate‐specific antigen (PSA) responses were evaluated. The median follow‐up was 8.4 years. The Kaplan–Meier method was used to estimate the biochemical or clinical failure‐free rate (BCFFR). Clinical RBE was calculated using the tumor control probability model. RESULTS: The 5‐, 7‐, and 10‐year BCFFRs were 91.7%, 83.8%, and 73.2%, respectively. The 10‐year BCFFRs of patients who received C‐ion RT at 66 Gy (RBE) in 20 fractions, 63 Gy (RBE) in 20 fractions, and 57.6 Gy (RBE) in 16 fractions were 81.4%, 70.9%, and 68.9%, respectively. The PSA level and density during follow‐up were better in the patients treated with the lower fraction size. A higher PSA nadir and shorter time to PSA nadir were risk factors for biochemical or clinical failure by multivariate Cox regression. The tumor control probability analysis showed that the estimated clinical RBE values to achieve an 80% BCFFR at 10 years for 20, 16, and 12 fractions were 2.19 (2.18–2.24), 2.16 (2.14–2.23), and 2.12 (2.09–2.21), respectively. CONCLUSIONS: Using clinical data from low‐risk prostate cancer patients, we showed the clinical RBE of C‐ion RT decreased with increasing dose per fraction.