Optimal 68Ga-PSMA and 18F-PSMA PET window levelling for gross tumour volume delineation in primary prostate cancer

Purpose: This study proposes optimal tracer-specific threshold-based window levels for PSMA PET–based intraprostatic gross tumour volume (GTV) contouring to reduce interobserver delineation variability. Methods: Nine ⁶⁸Ga-PSMA-11 and nine ¹⁸F-PSMA-1007 PET scans including GTV delineations of four expert teams (GTV _manual) and a majority-voted GTV (GTV _majority) were assessed with respect to a registered histopathological GTV (GTV _histo) as the gold standard reference. The standard uptake values (SUVs) per voxel were converted to a percentage (SUV%) relative to the SUV _max. The statistically optimised SUV% threshold (SOST) was defined as those that maximises accuracy for threshold-based contouring. A leave-one-out cross-validation receiver operating characteristic (ROC) curve analysis was performed to determine the SOST for each tracer. The SOST analysis was performed twice, first using the GTV _histo contour as training structure (GTV _SOST-H) and second using the GTV _majority contour as training structure (GTV _SOST-MA) to correct for any limited misregistration. The accuracy of both GTV _SOST-H and GTV _SOST-MA was calculated relative to GTV _histo in the ‘leave-one-out’ patient of each fold and compared with the accuracy of GTV _manual. Results: ROC curve analysis for ⁶⁸Ga-PSMA-11 PET revealed a median threshold of 25 SUV% (range, 22–27 SUV%) and 41 SUV% (40–43 SUV%) for GTV _SOST-H and GTV _SOST-MA, respectively. For ¹⁸F-PSMA-1007 PET, a median threshold of 42 SUV% (39–45 SUV%) for GTV _SOST-H and 44 SUV% (42–45 SUV%) for GTV _SOST-MA was found. A significant pairwise difference was observed when comparing the accuracy of the GTV _SOST-H contours with the median accuracy of the GTV _manual contours (median, − 2.5%; IQR, − 26.5–0.2%; p = 0.020), whereas no significant pairwise difference was found for the GTV _SOST-MA contours (median, − 0.3%; IQR, − 4.4–0.6%; p = 0.199). Conclusions: Threshold-based contouring using GTV _majority-trained SOSTs achieves an accuracy comparable with manual contours in delineating GTV _histo. The median SOSTs of 41 SUV% for ⁶⁸Ga-PSMA-11 PET and 44 SUV% for ¹⁸F-PSMA-1007 PET form a base for tracer-specific window levelling. Trial registration: Clinicaltrials.gov;