Carcinoma of Unknown Original Identified as Renal Cell Carcinoma by 18F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography Scan: A Report of Two Cases

Imaging is useful in identifying the primary site of an unknown primary cancer, and (18)F-fluorodeoxyglucose positron emission tomography/computed tomography ((18)F-FDG PET/CT) is an excellent imaging modality for identifying the primary lesion. However, a potential limitation is that (18)F-FDG is physiologically excreted from the kidneys, thus masking renal lesions. In this report, we describe two cases of cancer of unknown origin that were detected as originating from renal cancer on (18)F-FDG PET/CT. Both cases showed abnormal nodular accumulation of (18)F-FDG in the kidney, which can be distinguished from the physiological excretion of (18)F-FDG in the urinary tract. It is clinically crucial to be able to confirm the possibility of renal cancer, and careful observation of the urinary tract with (18)F-FDG PET/CT can be useful.