24 Gy in One Fraction Urethral-sparing "HDR Like" SBRT for Prostate Cancer

Patients with prostate cancer who are candidates for stereotactic radiotherapy to the prostate and seminal vesicles will undergo staging exams, which will include prostate specific membrane antigen (PSMA) positron emission tomography-computed tomography (PET-CT). If the PET scan is negative and the uroflowmetry is acceptable, the patients will perform the treatment, after fiducial implantation, simulation CT and magnetic resonance (MR), in a single fraction, delivered with a high-dose-rate (HDR)-like urethral sparing technique. In 70 patients, acute and late toxicity, biochemical control, overall survival, cancer specific-survival, and quality of life (through specific questionnaires) will be evaluated.

This is an interventional prospective non-randomised single-center trial that will enroll, 13 patients for the first phase of the study, according to the optimal design of Simon. The hypothesis is that the proportion of patients free from acute cumulative G3-G4 toxicity (Common Terminology Criteria for Adverse Events-CTCAE v5.0 scale) 1 month after the end of treatment must be \< 85% to suspend treatment and \> 95% to consider the treatment as safe. The treatment will be interrupted if G3-G4 toxicities and/or biochemical recurrences are recorded within a month in 2 or more patients; otherwise the study will continue with the second phase. Another 52 patients will be added, for a total of 65 patients. The treatment consists in a single fraction of 24 Gy with the "urethral sparing HDR like" technique. Assuming a minimal drop-out, given that the primary endpoint is toxicity one month after treatment, we will enroll 5 more patients in total, thus reaching a total number of 70 patients (65 necessary + 5 for any drop-outs). In the absence of worse subsequent toxicity, and with biochemical control comparable to that in the literature as well as historical treatments in our Radiation Oncology, the scheme will be considered safe, and the enrollment will continue open label. Patients with negative lymph nodes and without distant metastases will be enrolled and treated on the prostate/prostate and tumor vesicles (depending on the stage) at a total dose of 24 Gy delivered in a single fraction, sparing the urethra and with a dose distribution similar to high dose rate brachytherapy. Androgen Deprivation Therapy (ADT) will be prescribed according to the guidelines for the stages of the disease. Short-term cortisone and alpha lytics will be prescribed, for prophylactic purposes. Follow-up visits will be performed at 1, 3, 6,12,18, 24, 36, 48, 60 months with blood tests including prostate specific antigen (PSA) and testosterone (performed every 3 months in the first two years, every 6 months in the next 3 years), imaging, when needed, and quality of life questionnaires: European Organization for Research and Treatment of cancer (EORTC) quality of life questionnaire (QLQ) C30 and QLQ-PR25, International Prostate symptoms score (IPSS), International Index of Erectile Function (IIEF)-5, Expanded Prostate Cancer Index Composite (EPIC)- 26.