A case of giant cell tumor of sacrum with unusual pulmonary metastases: CT and FDG PET findings.

A 43-year-old woman was admitted for concern of postcurettage recurrence of left sacral giant cell tumor (GCT). Both MRI and FDG PET revealed a large lesion in the left sacrum with characteristics concerning for local recurrence. Preoperative staging CT showed no evidence of remote metastases. Subsequent restaging CT and FDG PET demonstrate the interval development of a 5 × 6-mm pulmonary nodule in the right lung base within 40 days after curettage with hypermetabolic features on PET, concerning for pulmonary metastasis of GCT, which was later confirmed by biopsy. As an exception, benign GCT can demonstrate metastases, rapid growth rate, and enhanced FDG avidity on PET for both primary and metastatic lesions. Our case illustrates the importance of recognizing these exceptional features when interpreting FDG PET of GCT to prevent the misinterpretation of metastases as perhaps other etiologies, such as infection.