The purpose of this study is to evaluate a technical feasibility of MR-guided SBRT fraction dose adaptation for low-risk, intermediate-risk and low-volume metastatic prostate cancer. .

Hypofractionated SBRT to 36.25 Gy in 5 fractions (fraction dose 7.25 Gy) for low- and intermediate risk prostate cancer is standard of care. Using higher fraction doses that might be beneficial for disease control is not safe in all patients, unless adaptive treatment is performed. In this clinical trial the feasibility of online MR-guided SBRT fraction dose adaptation (between 7 and 8 Gy) for prostate cancer will be investigated. A fraction dose to the planning target volume (PTV) will be adapted to organs-at-risk (OAR) geometry (position, shape and volume) on online planning MRI scans of the treatment day ("geometry-of-the-day"). Thus the investigators hope to maximize treatment individualization.

Inclusion Criteria:

• Written informed consent according to Swiss law and ICH/GCP regulations before any trial specific procedures;
• Males;
• Histologically confirmed adenocarcinoma of the prostate;
• Low (Grade group 1 [Gleason score ≤6], clinical stage T1-2a N0 M0, and PSA < 10 ng/ml) and intermediate (Grade group 2-3 [Gleason score 7], clinical stage T2b-2c N0 M0, PSA 10-20 ng/ml) risk prostate cancer with indications for SBRT with curative intent;
• Newly diagnosed low-volume metastatic prostate cancer with indications for SBRT, in whom metastatic disease was confirmed on a bone scintigraphic scan and soft-tissue imaging done within 3 months of starting androgen deprivation therapy (ADT);
• IPSS <15;
• Age: ≥ 18 years old;
• Karnofsky performance status ≥70;
• Patients able to tolerate long (approximately 60 minutes) treatment time;
• Patients who are willing and able to comply with scheduled visits, treatment, and other trial procedures.

Exclusion Criteria:

• Previous radical treatment for prostate cancer - either radiation therapy (external or brachytherapy), surgery, TUR-P, cryosurgery, or HIFU;
• Contraindications for MRI, e.g., patients with metal fragments or implanted devices that are not MR compatible such as certain types of pacemakers and aneurysm clips or severe claustrophobia;
• Lack of safe contraception;
• Known or suspected non-compliance, drug or alcohol abuse;
• Inability to follow the procedures of the trial, e.g. due to language problems of the participant;
• Enrolment of the investigator, his/her family members, employees and other dependent persons.

It is not yet known if there will be a plan to make IPD available.