Conversion of FDG PET activity of fibrous dysplasia of the skull late in life mimicking metastatic disease.

Fibrous dysplasia (FD) accounts for 7% of benign bone tumors. It is a developmental disorder of unclear etiology. The lamellar cancellous bone of the medullary cavity is replaced with immature fibroosseous tissue. We describe a case of FD of the skull in a patient of advanced age (69 years) with recent diagnosis of colon cancer, which changed its FDG activity and CT appearance within 10 months of follow-up. Surgical biopsy confirmed FD. Several case reports describe FDG-avid FD, but conversion of metabolic activity late in life is probably unusual.