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

Abstract

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;