C. Positron emission tomography scan
The differential diagnosis of multifocal, amelanotic choroidal lesions is broad and includes choroidal metastasis, amelanotic melanoma, lymphoma, and other hematologic processes. Fine-needle aspiration biopsy (choice A) would be warranted if systemic workup was unrevealing, but it is preferred to first perform systemic workup and also biopsy a site with less risk. Rituximab and radiotherapy (choice B) would not be indicated prior to definitive diagnosis but could be used to treat choroidal lymphoma. Plaque brachytherapy (choice D) would not be recommended because the multifocal nature of the lesion made choroidal melanoma unlikely. A positron emission tomography scan (choice C) was recommended to look for systemic disease, which showed fluorodeoxyglucose-avid soft tissue abnormalities along the bilateral neuroforamina, upper ribs, and paraspinal muscles. A biopsy was performed of a lumbar paraspinal mass (Figure 2), which revealed a histiocytic lesion with a MAP2K2 p.E202_I203 in-frame deletion consistent with Erdheim-Chester disease (ECD).