For proton beam therapy (PBRT) of Uveal Melanoma, tantalum clips are surgically sutured on the outside of the eye to mark the tumour edge. To aid in the 3D clip localisation and modelling of the tumor, a dedicated MRI protocol was developed and evaluated. In 51% of the clips, the MRI and per-operative measurements differed less than 1.0mm. In 28% of the clips the discrepancy between the MRI and peroperative measurement was attributed to the complex tumor geometry or anterior tumor localisation. For the majority of the patients MRI added valuable information to the peroperative measurements, improving PBRT planning.