The aim of this phase II study is to determine the early multiparametric magnetic resonance imaging response of the dominant intraprostatic lesion and correlate these findings with prostate specific antigen response in patients with intermediate to (very) high risk localized prostate cancer treated with online adaptive stereotactic radiotherapy without intraprostatic fiducial markers.

Background: Stereotactic body radiotherapy (SBRT) is an established standard of care treatment for localized prostate cancer. Recent pathological studies suggest the presence of a dominant intraprostatic lesion (DIL) within the prostate, which serve as a central driver of tumor aggressiveness and recurrence after treatment. Prostate-specific antigen (PSA) monitoring is the cornerstone of follow-up, though time to PSA nadir after SBRT can extend to 3 years.

Methods: This phase II observational study aims to evaluate the correlation between PSA response and early multiparametric magnetic resonance imaging (mpMRI) changes of the DIL(s) at 6- or 9-months following online adaptive SBRT for intermediate to (very) high risk localized prostate cancer. The primary outcome is the achievement of local control as assessed by mpMRI at 6 months post-SBRT or, if necessary, at 9 months post-SBRT (in case of image non complete response at 6 months). A complete imaging response is defined as the disappearance of all morphological and functional mpMRI changes of PIRADS (Prostate Imaging Reporting and Data System) ≥3 lesion(s) correlating with Gleason ≥7a pathology. No confirmatory biopsies will be conducted. The required sample size for this study will be 28 patients, assuming a complete response rate of 80% at 6 months, targeting an acceptable margin of error with 95% confidence, and a margin of error of 15%.

Inclusion Criteria:

• Patients with histologically confirmed localized prostate cancer who are planned for CT- or MRI-adaptive SBRT for prostate cancer;
• Presence of PIRADS ≥3 lesion(s) in a mpMRI correlating with Gleason ≥7 score at diagnosis;
• Intermediate to (very) high risk localized prostate cancer (≤ cT3b and cN0);
• ECOG performance status of 0-2;
• Age ≥ 18 years;
• PSMA PET ≤3 months is compulsory for high-risk prostate cancer (as part of clinical routine);
• Written informed consent.
• Willingness and ability to comply with schedule

Exclusion Criteria:

• Previous (≤10 years) local therapy of the prostate including transurethral resection of the prostate (TURP),
• Contraindication for MRI;
• Previous (≤10 years) radiotherapy in the pelvis;
• Lymph node metastases or distant metastases (i.e. no localized prostate cancer);
• Participation in a clinical trial which might influence the results of this project;
• Claustrophobic anxiety;
• Uncontrolled intercurrent illness;
• Relation to investigator (family or professional)

Patients referred to our tertiary referral centre for prostate cancer

Individual participant data that underlie the results reported in this trial after de-identification. Other documents may be shared include the study protocol.

Beginning 9 months and ending 36 months after article publication.

Proposals to access study data should send request to matthias.guckenberger@usz.ch. To gain access, data requesters must sign data access agreement.