Intrarenal epidermal cyst.

An epidermal cyst is a very unusual cause of a renal mass, with only three cases reported in English literature. The authors report a case of this in a 51-year-old man with left flank pain and hematuria. A 5 x 4 cm-sized cystic mass, having an irregular margin with stippled calcification, was identified in the lower pole of the left kidney. A retroperitoneal laparoscopic simple nephrectomy was performed under the assumption that it was a renal tumor. Histologically, the tumor was identical in appearance to an epidermal cyst within the skin. Aberrant ectodermal implantation during embryogenesis has been postulated as a histogenesis of intrarenal epidermal cyst. However, the present case had a history of renal stone, which was treated with extracorporeal shock wave lithotripsy. Therefore, the authors suspected the pathogenetic mechanism of this lesion was a metaplasia of traumatic origin. The diagnosis of an epidermal cyst was not made preoperatively in the present case, or in the cases described in the literature. This diagnosis should be included in the differentiation of calcified intrarenal masses.