The purpose of this study is to determine the maximal tolerated dose (MTD) of CIRT in the treatment of localized prostate cancer and to evaluate the efficacy of such treatment at the MTD. Participants will be treated with CIRT with escalating dose regimens to evaluate the maximal tolerated dose (MTD) in terms of acute and subacute toxicity observed during and within 6 months after the completion of CIRT. Once the MTD for localized prostate cancer is determined, the MTD will be used as the recommended dose to patients fulfilling the inclusion criteria in the Phase II part of the trial.