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

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.

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.

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.

The carbon-ion SOBP used in prostate cancer treatment has a lower OER than photons, suggesting its potential effectiveness against hypoxic tumors.