Tumor control probability analysis in carbon-ion radiotherapy for prostate cancer considering the oxygen effect

PURPOSE: Carbon ion radiotherapy (CIRT) is expected to be effective against hypoxic tumors due to its lower oxygen enhancement ratio (OER) compared to X-ray radiotherapy (XRT). OER is expected to have a significant impact on therapeutic efficacy, especially for prostate cancer, which has low oxygen partial pressure. This study aimed to derive OERs for photon and carbon-ion spread-out Bragg peak (SOBP) irradiation through cellular experiments and incorporate them into the tumor control probability (TCP) function to test consistency with clinical outcomes. METHODS: To derive the OER for photons and the OERs at three depths (proximal, center, and distal) within the carbon-ion SOBP, the dose-dependent survival rate of the prostate cancer cell line PC3 was measured under both aerobic and hypoxic conditions. TCP functions incorporating dose-dependent OERs based on clinical prescription doses were fitted to clinical outcomes of prostate cancer treated with XRT and CIRT to test the clinical relevance of the obtained OERs. RESULTS: The OERs at 10% survival for PC3 cells were 2.12 ± 0.68 for photons, and 1.70 ± 0.13, 1.65 ± 0.19, and 1.56 ± 0.13 at the proximal, center, and distal depths of the carbon-ion SOBP, respectively. The TCP functions calculated using dose-dependent OERs were not inconsistent with the clinical outcomes for both XRT and CIRT, supporting the clinical relevance of the OERs derived from cell experiments. CONCLUSION: The carbon-ion SOBP used in prostate cancer treatment has a lower OER than photons, suggesting its potential effectiveness against hypoxic tumors.