[Myolipoma arising from the renal capsule: a case report].

A 53-year-old female visited us with an asymptomatic renal mass, which was incidentally found on abdominal ultrasonography (US). US showed a heterogeneous renal mass measuring 5 cm in diameter. Computed tomorraphy and magnetic resonance imaging showed that the tumor was mixed with muscle and fat tissue, faintly enhanced, and located at the lower portion of the left kidney. Although renal angiomyolipoma (AML) was suspected from these findings, we could not rule out a malignant tumor. Therefore, she underwent a laparoscopic partial nephrectomy. Histopathological examination revealed the tumor composed of smooth muscle and mature adipose tissue without cytological atypia arising from the renal capsule. The tumor contained no AML-like blood vessels, and was negative for HMB-45 staining. The final diagnosis was myolipoma arising from the renal capsule. Myolipoma is a rare benign neoplasm in the soft tissue of retroperitoneum and abdominal cavity. Since it is difficult to distinguish myolipoma from other benign and malignant tumors with fat tissue only by imaging studies, a surgical approach should be considered.