Unilocular retroperitoneal cyst of mesothelial origin presenting as a renal mass - A report of 2 cases

We report the first 2 cases, to our knowledge, of retroperitoneal cysts wit h features of mesothelial differentiation that clinically mimic renal masse s. The first lesion occurred in a 71-year-old man who presented with flank pain. Ultrasound and magnetic resonance imaging studies showed a unilocular cystic structure arising from the upper pole of the left kidney. The secon d lesion was in a 44-year-old woman who presented with left flank pain. Ima ging studies revealed an 8-cm hemorrhagic cyst at the lower pole of the lef t kidney. Histologic examination of the nephrectomy specimens in each case revealed a unilocular cyst with intracystic and pericystic hemorrhage. In e ach case, the cyst was lined by a single layer of cells with ample eosinoph ilic cytoplasm and benign nuclear features without mucinous or mullerian di fferentiation. Histochemical staining showed Alcian blue positivity on the cell surface, which was sensitive to hyaluronidase digestion. Intracytoplas mic mucin, however, was not detected. Immunostaining showed that the cyst l ining cells were positive for keratin, vimentin, HBME-1, WT1, and thrombomo dulin but negative for carcinoembryonic antigen, B72.3, Leu-M1, and BerEP4. The first case was positive for calretinin, whereas the second was negativ e. These findings support the mesothelial nature of the cysts.