Extrarenal Retroperitoneal Angiomyolipoma Mimicking Metastatic Melanoma CT and FDG PET Correlation

A 38-year-old man with melanoma of the right flank underwent a staging 18F-fluoro-2-deoxyglucose (FDG) PET/CT scan, which demonstrated a hypermetabolic extrarenal mass in the left retroperitoneal space, concerning for metastatic melanoma. However, surgical pathology demonstrated an angiomyolipoma (AML). Although AMLs can rarely occur in an extrarenal location, extrarenal retroperitoneal AMLs are exceptional. AMLs have variable imaging appearances on multiple imaging modalities, including FDG PET, and can confound accurate diagnosis when in an extrarenal location. This case demonstrates the only known FDG PET description of an extrarenal retroperitoneal AML and highlights the challenge in accurate diagnosis based on FDG PET findings.