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

PURPOSE: To assess the safety of using combined Gallium68-prostate-specific membrane antigen (PSMA)-positron emission tomography (PET)/computed tomography (CT) (68Ga-PSMA-PET/CT) and multiparametric magnetic resonance imaging (mpMRI) for planning dominant intraprostatic lesion (DIL) boost in prostate stereotactic radiation therapy (SBRT) for dose escalation (PROBE).

METHODS AND MATERIALS: Patients with intermediate- or high-risk prostate adenocarcinoma with DIL identified on mpMRI and 68Ga-PSMA-PET/CT and suitable for SBRT were enrolled in this phase 2 trial. 68Ga-PSMA-PET/CT was fused with mpMRI for gross tumor volume (GTV) delineation. Semiautomatic contouring of DIL was performed using 20% to 90% of the maximum standardized uptake value (SUVmax) (DILx%). Concordance metrics were used to select the DILx% matching closest to GTVMRI (GTVPET). Prostate (36.25 Gy), pelvic nodes (25 Gy), GTVunion: GTVPET ∪ GTVMRI (40 Gy), and GTVoverlap: GTVPET ∩ GTVMRI (42.5 Gy) were planned for 5-fraction SBRT. All patients received androgen deprivation therapy (ADT) for 6 months. The primary endpoint for the present analysis was concordance (volumetric and spatial) between GTVMRI and GTVPET. Secondary endpoints included the percentage SUVmax threshold for GTVPET contouring (%SUVGTV-PET) and cumulative acute (≤90 days) urinary and gastrointestinal toxicity using Common Terminology Criteria for Adverse Event (CTCAE) v5.0.

RESULTS: Thirty patients (54% intermediate risk, 46% high risk) were enrolled. GTVMRI and GTVPET showed strong volumetric correlation (Spearman correlation coefficient ρ = 0.817, 95% CI, 0.64-0.91; P < .001). The median Dice similarity coefficient, Jaccard index, and the mean Hausdorff distance for PET and magnetic resonance imaging boost volumes were 0.56, 0.37, and 2.2, respectively. The median %SUVGTV-PET was 48% (IQR, 40%-58%). There was an inverse correlation between DIL SUVmax and %SUVGTV-PET (Spearman correlation coefficient ρ = -0.598, 95% CI lower -0.79, upper -0.29; P < 0.001). Cumulative grade 2 acute urinary and GI toxicity were 13.3% and 6.6%, respectively, with no grade ≥3 toxicities.

CONCLUSION: Boost volumes on 68Ga-PSMA-PET/CT and mpMRI were volumetrically similar, however, with poor spatial concordance. The %SUVmax threshold for GTVPET contouring correlated inversely with DIL SUVmax and was a median of 48%. Based on the favorable acute toxicity profile, PSMA-PET-guided intraprostatic boost is likely to be safe for dose escalation in prostate SBRT.