Case 10 -

Mixed Epithelial and Stromal Tumor John N Eble, M.D. Holger Moch, M.D.

Case history: Radiologic evaluation for gallbladder disease disclosed a complex solid and cystic mass in the right kidney of a 55 year-old woman. A radical nephrectomy was done. Macroscopic findings: A 7 cm mass was present in the center of the kidney and protruded into the renal pelvic space and renal sinus. The cut surface showed solid areas and small and medium-sized cysts. Diagnosis: Mixed epithelial and stromal tumor In 1993, Pawade et al. described three cases of a novel cystic renal tumor which they termed "cystic hamartoma of the renal pelvis." Additional cases have been reported as "mesoblastic nephroma of adults" and as "mixed epithelial and stromal tumor of the kidney." The last terminology is preferred since these tumors clearly have no relationship to mesoblastic nephroma of infants and since they appear to be mixtures of neoplastic epithelial components and spindle cells, the term "hamartoma" is inappropriate. There is a strong predominance of women, often of perimenopausal age. Pain in the loin or flank was the most common presenting complaint. This tumor has not been reported in a child. No patient has had a recurrence.. Gross Pathology Most of these tumors appear to arise centrally in the kidney and to grow as expansile masses. Most have been large and the precise anatomic site of origin has been obscure. The frequency of herniation into the renal pelvic cavity lends support to the idea of origin from the renal medulla. The tumors lack a thick fibrous wall but compressed renal tissue usually forms a pseudocapsule in the larger tumors. The tumors are grossly composed of multiple cysts and solid areas. The solid areas may be extensive. The septa of the cysts are thicker than is typical of cystic nephroma, cystic partially differentiated nephroblastoma, and multilocular cystic clear cell renal cell carcinoma. Microscopic Pathology These are complex tumors composed of large cysts, microcysts, and tubules. The largest cysts are lined by columnar and cuboidal epithelium which sometimes forms small papillary tufts. Urothelium, which may be hyperplastic, may also line some of the cysts. The microcysts and tubules are lined by flattened, cuboidal, or columnar cells. Their cytoplasm ranges from clear, to pale, to eosinophilic, or is vacuolated. The architecture of the microcysts is varied and ranges from simple microcysts with abundant stroma between them to densely packed clusters of microcysts, to complex branching channels which may be dilated. These varied elements often are present intermingled in the same area of the tumor. The stroma consists of a variably cellular population of spindle cells with plump nuclei and abundant cytoplasm. Areas of myxoid stroma and fascicles of smooth muscle cells may be prominent. Blood vessels with thick walls may be present. Fat cells may be present. The stroma may be sparsely cellular and highly collagenous. Mitotic figures and atypical nuclei have not been reported. The spindle cells often decorate with antibodies to smooth muscle markers. HMB-45 is not present in these tumors. References Pawade J, Soosay GN, et al: Cystic hamartoma of the renal pelvis. Am J Surg Pathol 17:1169-1175, 1993 Michal M and Syrucek M: Benign mixed epithelial and stromal tumor of the kidney. Pathol Res Pract 194:445-448, 1998 Adsay NV, Eble JN, et al.: Mixed epithelial and stromal tumor of the kidney, Am J Surg Pathol 24:958-970, 2000