Positron emission tomography/computed tomography (PET/CT) depicted vital tumour metabolism of OALs accurately. In cases scanned after orbital biopsy and under systemic therapy, no elevated tumour metabolic activity was expressed. This underlines the reasonable application of PET/CT for therapy monitoring besides whole-body staging. Higher-stage OALs show higher metabolic activity. Yet, for adequate therapy initiation, histology remains indispensable.