PSMA-PET-Guided Intraprostatic Boost in Prostate SBRT (PROBE): A Phase 2 Trial

Area of Science:

• Oncology
• Radiotherapy
• Medical Imaging

Background:

• Prostate stereotactic radiation therapy (SBRT) utilizes dose escalation for improved outcomes.
• Accurate delineation of dominant intraprostatic lesions (DIL) is crucial for effective boost planning.
• Combined imaging modalities like ⁶⁸Ga-PSMA-PET/CT and mpMRI offer complementary information for DIL targeting.

Purpose of the Study:

• To evaluate the safety and concordance of combined ⁶⁸Ga-PSMA-PET/CT and mpMRI for planning DIL boost in prostate SBRT.
• To assess the feasibility of using PSMA-PET/CT for dose escalation in prostate cancer radiotherapy.

Main Methods:

• Phase 2 trial involving patients with intermediate- to high-risk prostate adenocarcinoma.
• Fusion of ⁶⁸Ga-PSMA-PET/CT and mpMRI for gross tumor volume (GTV) delineation.
• Semiautomatic contouring of DIL using %SUV_max thresholds and concordance metrics for GTV selection.
• 5-fraction SBRT plan incorporating prostate, pelvic nodes, GTV_union, and GTV_overlap with ADT.

Main Results:

• Strong volumetric correlation (ρ = 0.817) between GTV_MRI and GTV_PET.
• Median Dice similarity coefficient of 0.56 and Jaccard index of 0.37 indicated poor spatial concordance.
• Median %SUV_max threshold for GTV_PET contouring was 48%, inversely correlated with DIL SUV_max.
• Low rates of acute grade 2 urinary (13.3%) and GI (6.6%) toxicity, with no grade ≥3 events.

Conclusions:

• ⁶⁸Ga-PSMA-PET/CT and mpMRI-based boost volumes are volumetrically similar but spatially discordant.
• A median %SUV_max of 48% is identified for PSMA-PET GTV contouring.
• PSMA-PET guided intraprostatic boost is a safe approach for dose escalation in prostate SBRT, supported by a favorable toxicity profile.