False-positive bone metastases on PET/CT secondary to sarcoidosis in a patient with rectal cancer

Patient with newly diagnosed rectal cancer underwent staging PET/CT that showed an intensely [F-18]-fluoro-2-deoxy-D-glucose (FDG)-avid rectal tumor, but also multifocal, intensely FDG-avid mediastinal lymphadenopathy and skeletal foci. The bone foci had no corresponding lytic or blastic lesions on CT. There were mildly FDG-avid reticulonodular opacities in both lungs. This apparent metastatic distribution pattern was highly atypical for rectal cancer; therefore, biopsy was recommended. It showed granulomatous disease in a lymph node and the bone marrow, indicating sarcoidosis. Bone involvement in sarcoidosis is rare, but can lead to the false-positive appearance of metastatic disease on PET/CT.