Benign Bone Cyst Mimicking Bone Metastasis Demonstrated on 68Ga-FAPI.

A 59-year-old woman with lobular breast cancer was referred to nuclear medicine department for a 68Ga-FAPI PET/CT study after completion of treatment to rule out the possibility of metastatic disease. It revealed a lytic lesion showing increased FAPI accumulation in the right acetabulum, highly suspicious for bone metastasis. In comparison to an initial 18F-FDG PET/CT study done 2 years before, the lytic lesion showing no FDG uptake in the right acetabulum remained unchanged, and the findings were consistent with bone cyst. This case highlighted that benign bone cyst may demonstrate increased 68Ga-FAPI uptake mimicking bone metastasis on 68Ga-FAPI PET/CT.