Malignant transformation of polyostotic fibrous dysplasia with aberrant keratin expression

Malignant transformation of fibrous dysplasia (FD) is exceedingly rare, occurring in less than 1% of all FD cases, and has been described in both monostotic and polyostotic forms of this entity. We report a case of a large proximal femur mass arising in a 45-year-old man. The biopsy revealed a high-grade pleomorphic malignancy that focally expressed multiple keratins. Based on the presence of keratin immunoreactivity, the morphologic differential diagnosis included metastatic sarcomatoid carcinoma. However, review of the clinical information revealed a history of polyostotic FD, and imaging findings were compatible with malignant transformation of FD. The resected neoplasm was biphasic and composed of areas of conventional FD admixed with a high-grade pleomorphic malignancy. Activating GNAS mutations were identified in both components. To the best of our knowledge, this is the first description of keratin expression in malignant transformation of FD.