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

Abstract

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 68 Ga-PSMA-11 and nine 18 F-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 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 maximizes 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.Results: ROC curve analysis for 68 Ga-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 18 F-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).Conclusions: Threshold-based contouring using GTVmajority-trained SOSTs achieves an accuracy comparable to manual contours in delineating GTVhisto. The median SOSTs of 41 SUV% for 68 Ga-PSMA-11 PET and 44 SUV% for 18 F-PSMA-1007 PET form a base for tracer-specific window levelling.

Prostate cancer GTV delineation with biparametric MRI and 68Ga-PSMA-PET: comparison of expert contours and semi-automated methods

“…Recent studies have considered the impact of technical considerations such as image scaling on interobserver variability in contouring tasks: Zamboglou et al 25 described significantly greater agreement when uniform SUV windowing of 0-5 (DSC 0.80) was employed (as was used here) compared to nonstandardised scaling methods (DSC 0.56). 25 Draulans et al 26 found alternatively that windowing of 0-41% SUV max optimised the accuracy of contours with reference to histopathology. 26 Similar considerations may apply to bpMRI, where choice of b-values or the incorporation of simulated high b-value DWI could improve consistency between observers on bpMRI.…”

“…25 Draulans et al 26 found alternatively that windowing of 0-41% SUV max optimised the accuracy of contours with reference to histopathology. 26 Similar considerations may apply to bpMRI, where choice of b-values or the incorporation of simulated high b-value DWI could improve consistency between observers on bpMRI. 27 Ultimate optimisation of image scaling and display is likely dependent on specific scanner, image acquisition and patient characteristics, and should be further investigated.…”

“…We subsequently could not establish any benefit to the combined use with Boolean union/intersection of semi-automated volumes, despite previous findings of improved histological coverage with mpMRI/ PSMA-PET volume unions. 9 However, a wide range of cut-off values for this purpose is present in the literature: ADC values from approximately 700-1300 mm 2 /s x 10 −6 10-17 and %SUV max values from 20-50%, 18,19,25,26,29 have been variably suggested. Similarly to the issue of optimising image scaling for manual delineations, experimental factors such as acquisition parameters -b-values in the case of DWI, and other reconstruction factors in PET 30 -significantly impact the accuracy of derived thresholds.…”

Comparison of Manual and Semi-Automatic [18F]PSMA-1007 PET Based Contouring Techniques for Intraprostatic Tumor Delineation in Patients With Primary Prostate Cancer and Validation With Histopathology as Standard of Reference

“…Manual Contouring: According to recommendations for [ 68 Ga]-PSMA-PET imaging scaling of SUVmin-max 0-5 was firstly applied for intraprostatic tumor lesion contouring (11). Due to volume overestimation and differences in SUVuptake between both tracers (17,22), a second analysis applying individual scaling was performed to define an additional scaling range for manual contouring, which results in absolute volumes (GTV-Individual) more consistent with GTV-Histo. Therefore, PET images of each patient were scaled individually by modifying the scaling value of the upper SUV-limit, adjusting the volume to the available histological information.…”

“…Fluorine-18-labeled Tracers ([ 18 F]PSMA-1007) have been implemented in nuclear medicine practice, with suspected benefits due to lesser renal elimination and consequent less background signal in the bladder (15) and performed with good diagnostic accuracy (16). Since [ 68 Ga]-and [ 18 F]PSMA-1007 tracers show differences in SUV distribution scaling recommendations might not be used interchangeable (17). This prospectively designed study aims to validate [ 18 F]PSMA-1007 PET based contouring approaches for intraprostatic tumor contouring using whole mount histopathology as standard of reference, since a consensual method to accurately contour intraprostatic lesions for this tracer has not yet been established.…”