Inter‐Observer variability in MRI ‐based target volume delineation of uveal melanoma

Purpose: Several efforts are being undertaken towards MRI‐based treatment planning for ocular proton therapy for uveal melanoma (UM). The inter‐observer variability of the gross target volume (GTV) on MRI is one of the important parameters to design safety margins for a reliable treatment. Therefore, this study assessed the inter‐observer variation in tumour delineation of UM on MRI. Methods: Six observers delineated the tumour in 10 different UM patients using the Big Brother contouring software. Patients were scanned at 3 T MRI with a surface coil and tumours were delineated separately on contrast enhanced 3DT1 (T1gd) and 3DT2‐weighted scans with a resolution of 0.8 mm isotropic for the acquisition voxels (1). Volume difference and overall local variation were analysed for each patient. Additionally the local variation was analysed for four interfaces: sclera, vitreous, retinal detachment, and tumour‐choroid interface. Results: The average tumour contour was significantly larger on T1gd (0.57 cm 3 ) compared to T2 (0.51cm 3 , p = 0.01). A not significant higher interobserver variation was found on T1gd (0.4 1 mm) compared to T2 (0.35 mm). The largest variations were found at the tumour‐choroid interface, due to peritumoral enhancement (T1gd: 0.62 mm and T2: 0.5 2 mm). As a result, a larger part of this tumour‐choroid interface appeared to be included on T1gd based GTVs compared to T2, explaining the smaller volumes on T2. Conclusions: The interobserver variation of 0.4 mm on MRI are low with respect to the voxel size of 0.8 mm. We recommend to delineate based on the T1gd‐weighted scans, as choroidal tumour extensions might be missed. Reference 1. Ferreira, et al. MR imaging characteristics of uveal melanoma with histopathological validation. Neuroradiology. 2022; 64:171. doi: 10.1007/s00234‐021‐02825‐5 .