Inter- and intra-rater variability of MRI-based lesion size measurements in active surveillance for prostate cancer: a multicentre study

OBJECTIVES: Prostate cancer (PCa) lesions can be measured on MRI using maximum or biaxial diameters, or as volumes derived by the ellipsoid formula or planimetry. We evaluated the inter- and intra-rater reliability (reliability between different radiologists and the same radiologist during different reading sessions) of lesion size measurements on baseline MRI scans for patients on active surveillance (AS). MATERIALS AND METHODS: Twenty patients with low- or intermediate-risk PCa (Gleason score 3 + 3 or 3 + 4) and MRI-visible lesions were selected from AS cohorts at two centres (United Kingdom and Italy). Five radiologists, blinded to clinical outcomes and reports, independently measured the index lesion on a baseline MRI scan in a single reading session using: (1) maximum diameter; (2) biaxial diameters; (3) ellipsoid volume, and (4) planimetry volume. Measurements were repeated after a 4-week washout period. Strip plots present lesion size measurements for all methods and readers. Bland-Altman plots were used to present intra-rater reliability. RESULTS: Graphical presentation of measurements across the twenty patients enabled examination of variability between methods, readers, and reads. There was considerable variation for all methods, and for a single lesion, size measurements spanned previously accepted definitions of clinically significant and insignificant disease. Inter-rater reliability decreased for larger lesions, with notable radiologist-specific differences, and intra-rater reliability appeared better overall. CONCLUSION: This study underscores the difficulty of reliably measuring PCa lesions during AS. Intra-rater reliability appeared greater than inter-rater reliability, emphasising that radiologists should remeasure lesions when tracking changes. More work is needed on measuring change in lesion size across serial MRI scans. KEY POINTS: Question Several methods exist for measuring prostate cancer lesion size on MRI for patients on active surveillance, but it is unclear how reliable these methods are. Findings Lesion size measurements varied widely across methods and readers, often spanning thresholds for clinically significant disease, and intra-rater reliability was generally better than inter-rater reliability. Clinical relevance Variability in lesion size measurements on MRI may lead to inconsistent clinical decisions during active surveillance. Our findings emphasise that regardless of the method used, lesions should be remeasured by the same radiologist when monitoring patients on active surveillance.