Abstract
PURPOSE: Cervical cancer metabolic tumour volume (MTV) derived from [18F]-FDG PET/CT has a role in prognostication and therapy planning. There is no standard method of outlining MTV on [18F]-FDG PET/CT. The aim of this study was to assess the optimal method to outline primary cervical tumours on [18F]-FDG PET/CT using MRI-derived tumour volumes as the reference standard. METHODS: 81 consecutive cervical cancer patients with pre-treatment staging MRI and [18F]-FDG PET/CT imaging were included. MRI volumes were compared with different PET segmentation methods. Method 1 measured MTVs at different SUV(max) thresholds ranging from 20 to 60% (MTV(20)-MTV(60)) with bladder masking and manual adjustment when required. Method 2 created an isocontour around the tumour prior to different SUV(max) thresholds being applied. Method 3 used an automated gradient method. Inter-observer agreement of MTV, following manual adjustment when required, was recorded. RESULTS: For method 1, the MTV(25) and MTV(30) were closest to the MRI volumes for both readers (mean percentage change from MRI volume of 2.9% and 13.4% for MTV(25) and - 13.1% and - 2.0% for MTV(30) for readers 1 and 2). 70% of lesions required manual adjustment at MTV(25) compared with 45% at MTV(30). There was excellent inter-observer agreement between MTV(30) to MTV(60) (ICC ranged from 0.898-0.976 with narrow 95% confidence intervals (CIs)) and moderate agreement at lower thresholds (ICC estimates of 0.534 and 0.617, respectively for the MTV(20) and MTV(25) with wide 95% CIs). Bladder masking was performed in 86% of cases overall. For method 2, excellent correlation was demonstrated at MTV(25) and MTV(30) (mean % change from MRI volume of -3.9% and - 8.6% for MTV(25) and - 16.9% and 19% for MTV(30) for readers 1 and 2, respectively). This method also demonstrated excellent ICC across all thresholds with no manual adjustment. Method 3 demonstrated excellent ICC of 0.96 (95% CI 0.94-0.97) but had a mean percentage difference from the MRI volume of - 19.1 and - 18.2% for readers 1 and 2, respectively. 21% required manual adjustment for both readers. CONCLUSION: MTV(30) provides the optimal correlation with MRI volume taking into consideration the excellent inter-reader agreement and less requirement for manual adjustment.