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

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.

Nine 68Ga-PSMA-11 and nine 18F-PSMA-1007 PET scans including GTV delineations of four expert teams (GTVmanual) and a majority-voted GTV (GTVmajority) were assessed with respect to a registered histopathological GTV (GTVhisto) as the gold standard reference. The standard uptake values (SUVs) per voxel were converted to a percentage (SUV%) relative to the SUVmax. 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 GTVhisto contour as training structure (GTVSOST-H) and second using the GTVmajority contour as training structure (GTVSOST-MA) to correct for any limited misregistration. The accuracy of both GTVSOST-H and GTVSOST-MA was calculated relative to GTVhisto in the 'leave-one-out' patient of each fold and compared with the accuracy of GTVmanual.

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 GTVSOST-H and GTVSOST-MA, respectively. For 18F-PSMA-1007 PET, a median threshold of 42 SUV% (39-45 SUV%) for GTVSOST-H and 44 SUV% (42-45 SUV%) for GTVSOST-MA was found. A significant pairwise difference was observed when comparing the accuracy of the GTVSOST-H contours with the median accuracy of the GTVmanual contours (median, - 2.5%; IQR, - 26.5-0.2%; p = 0.020), whereas no significant pairwise difference was found for the GTVSOST-MA contours (median, - 0.3%; IQR, - 4.4-0.6%; p = 0.199).

Threshold-based contouring using GTVmajority-trained SOSTs achieves an accuracy comparable with manual contours in delineating GTVhisto. 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.

Clinicaltrials.gov ; NCT03327675; 31-10-2017.

European journal of nuclear medicine and molecular imaging. 2020 Oct 06 [Epub ahead of print]

Cédric Draulans, Robin De Roover, Uulke A van der Heide, Linda Kerkmeijer, Robert J Smeenk, Floris Pos, Wouter V Vogel, James Nagarajah, Marcel Janssen, Sofie Isebaert, Frederik Maes, Cindy Mai, Raymond Oyen, Steven Joniau, Martina Kunze-Busch, Karolien Goffin, Karin Haustermans

Department of Radiation Oncology, University Hospitals Leuven, Leuven, Belgium. ., Department of Radiation Oncology, University Hospitals Leuven, Leuven, Belgium., Department of Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, The Netherlands., Department of Radiation Oncology, Radboud University Medical Centre, Nijmegen, The Netherlands., Department of Nuclear Medicine, The Netherlands Cancer Institute, Amsterdam, The Netherlands., Department of Radiology & Nuclear Medicine, Radboud University Medical Centre, Nijmegen, The Netherlands., Department of Electrical Engineering, ESAT/PSI, KU Leuven, Leuven, Belgium., Department of Radiology, University Hospitals Leuven, Leuven, Belgium., Department of Urology, University Hospitals Leuven, Leuven, Belgium., Department of Nuclear Medicine, University Hospitals Leuven, Leuven, Belgium., Department of Radiation Oncology, University Hospitals Leuven, Leuven, Belgium. .

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