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 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) is an excellent imaging modality for identifying the primary lesion. However, a potential limitation is that 18F-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 18F-FDG PET/CT. Both cases showed abnormal nodular accumulation of 18F-FDG in the kidney, which can be distinguished from the physiological excretion of 18F-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 18F-FDG PET/CT can be useful.