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

Abstract

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 T2 weighted (T2W) and diffusion-weighted (DWI) sequences, and 68Ga-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 (GTVMRI), (2) PSMA-PET (GTVPSMA) and (3) co-registered bpMRI/PSMA-PET (GTVFused) 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 GTVPSMA (DSC: 0.822, MDA: 1.12 mm) and GTVFused (DSC: 0.787, MDA: 1.34 mm) than for GTVMRI (DSC: 0.705, MDA 2.44 mm). Intermodality agreement between GTVMRI and GTVPSMA 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: 68Ga-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 68Ga-PSMA-PET.

Evaluation of therapeutic radiographer contouring for magnetic resonance image guided online adaptive prostate radiotherapy

“…Comparisons with other studies are made difficult because of variation in metrics used, for example volume-only comparisons [23] , [24] and contouring variation [25] , [26] . Where published, our median MDA of < 1 mm is similar [27] , [15] . The trend of increasing volume may be due to prostate swelling during treatment [28] .…”

Reduction of inter-observer differences in the delineation of the target in spinal metastases SBRT using an automatic contouring dedicated system

“…To date, few experiences in the literature report on the use of softwares for automatic segmentation of the target in radiotherapy. Hearn et al have recently published the results of a comparison study between manual segmentation and semi-automated delineation of prostate GTV based on quantitative thresholding of intraprostatic apparent diffusion coefficient (ADC) and PET standardized uptake values (SUV), recording a significantly higher inter-observer agreement with the use of PET compared to MRI-based semiautomatic segmentation [ 19 ].…”

Feasibility of biology-guided radiotherapy using PSMA-PET to boost to dominant intraprostatic tumour

“…Recent studies have investigated determination of the DIL from PSMA uptake. DIL volumes were either first determined from mpMRI and then reproduced by %SUVmax threshold techniques [29] , [30] , [31] , manually delineated on both MRI and PSMA PET datasets [32] , determined first from histopathology specimens and then compared to manual and %SUVmax threshold delineation on PSMA PET images [33] , [34] , [35] , or determined from a %SUVmax threshold technique without any other imaging references [36] . DIL determination from PSMA uptake may be advantageous; for instance, studies reported higher sensitivity and comparable specificity as compared with mpMRI determination [37] , [38] , [39] .…”