Early Magnetic Resonance Imaging Response of the Dominant Intraprostatic Lesion After Online Adaptive Stereotactic Body Radiotherapy for Localized Prostate Cancer and Correlation With Prostate Specific Antigen Response

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)