GNAS and Langerhans cell histiocytosis: While many of these features can be confirmed on X-ray, computed tomography (CT) is the imaging modality of choice when diagnosing FD lesions.1,8, –10 Lesions that may have radiographic similarities to FD include ossifying fibromas, osteofibrous dysplasia, giant cell tumors, Langerhans cell histiocytosis, and aneurysmal bone cysts.8 In cases for which the diagnosis remains uncertain based on radiographic or clinical features, or when there is concern for underlying malignancy, a biopsy may be performed for histologic and genetic analysis testing for causative GNAS mutations.