Fat-poor renal angiomyolipoma in a patient with an end-stage kidney

Angiomyolipoma in end-stage kidneys is very rare. The differentiation of an angiomyolipoma from a renal cell carcinoma is important because angiomyolipomas do not usually need to be surgically removed. The diagnosis of an angiomyolipoma is made by demonstrating bulky fat within a solid renal mass. When angiomyolipomas contain a large amount of internal fat, the diagnosis is not challenging. However, if the angiomyolipoma contains a small amount of fat, it can be challenging to detect on CT scanning. When a small amount of fat is suspected in a renal mass, an unenhanced CT examination with thin sections combined with a pixel analysis is the most sensitive test to confirm the presence of fat. A small number of angiomyolipomas do not contain macroscopic fat, and the imaging differentiation from a renal neoplasm is difficult. A 52-year-old woman with chronic renal failure was admitted to Oyokyo Kidney Research Institute Hirosaki Hospital for induction of hemodialysis. Screening computed tomography revealed an enhancing tumor without fat attenuation (size 3 × 2.5 cm) in the left kidney. Based on these findings, we diagnosed the patient with renal cell carcinoma and performed a radical left nephrectomy. However, histological examination of the tumor revealed the presence of angiomatous, myomatous, and lipomatous components. Furthermore, immunohistological examinations showed that the tumor cells were strongly positive for smooth muscle actin, desmin, and HMB-45. Based on these findings, we diagnosed fat-poor angiomyolipoma in a patient with end-stage kidneys. Diagnosing renal angiomyolipomas is challenging when the tumor contains a minimal amount of fat. It may be difficult to distinguish between fat-poor angiomyolipoma and renal cell carcinoma preoperatively based on cross-sectional imaging findings alone, especially in patients with end-stage kidneys.