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

Objective: The optimal method for delineation of dominant intraprostatic lesions (DIL) for targeted radiotherapy dose escalation is unclear. This study evaluated interobserver and intermodality variability of delineations on biparametric MRI (bpMRI), consisting of T-2 weighted (T2W) and diffusion-weighted (DWI) sequences, and Ga-68-PSMA-PET/CT; and compared manually delineated GTV contours with semi-automated segmentations based on quantitative thresholding of intraprostatic apparent diffusion coefficient (ADC) and standardised uptake values (SUV). Methods: 16 patients who had bpMRI and PSMA-PET scanning performed prior to any treatment were eligible for inclusion. Four observers (two radiation oncologists, two radiologists) manually delineated the DIL on: (1) bpMRI (GTV(MRI)), (2) PSMA-PET (GTV(PSMA)) and (3) co-registered bpMRI/PSMA-PET (GTV(Fused)) in separate sittings. Interobserver, intermodality and semi-automated comparisons were evaluated against consensus Simultaneous Truth and Performance Level Estimation (STAPLE) volumes, created from the relevant manual delineations of all observers with equal weighting. Comparisons included the Dice Similarity Coefficient (DSC), mean distance to agreement (MDA) and other metrics. Results: Interobserver agreement was significantly higher (p < 0.05) for GTV(PSMA) (DSC: 0.822, MDA: 1.12 mm) and GTV(Fused) (DSC: 0.787, MDA: 1.34 mm) than for GTV(MRI) (DSC: 0.705, MDA 2.44 mm). Intermodality agreement between GTV(MRI) and GTV(PSMA) was low (DSC: 0.440, MDA: 4.64 mm). Agreement between semi-automated volumes and consensus GTV was low for MRI (DSC: 0.370, MDA: 8.16 mm) and significantly higher for PSMA-PET (0.571, MDA: 4.45 mm, p < 0.05). Conclusion: Ga-68-PSMA-PET appears to improve interobserver consistency of DIL localisation vs bpMRI and may be more viable for simple quantitative delineation approaches; however, more sophisticated approaches to semi-automatic delineation factoring for patient- and disease-related heterogeneity are likely required. Advances in knowledge: This is the first study to evaluate the interobserver variability of prostate GTV delineations with co-registered bpMRI and Ga-68-PSMA-PET.