Sarcomatoid renal cell carcinoma. An immunohistochemical study of 18 cases.

Malignant spindle cell neoplasms are a diagnostic challenge regardless of their location. In the retroperitoneum a major consideration in the differential diagnosis is sarcomatoid renal cell carcinoma; if an epithelial component cannot be recognized histologically, special studies may be required to reach the correct diagnosis. In an attempt to better characterize this entity, 23 cases of sarcomatoid renal cell carcinoma (6.3%) were identified from a review of 363 renal cell carcinomas. Blocks were available for immunohistochemical analysis in 18 cases. The epithelial and sarcomatoid portions were studied with a panel of antibodies directed against cytokeratin (AE1/AE3, CAM 5.2, and 34 beta E12), epithelial membrane antigen, Leu-M1, muscle-specific actin, S100 protein, desmin, and vimentin. The epithelial nature of the spindle cell component was best demonstrated by positive reactivity with the anti-cytokeratin AE1/AE3 (in 17 [94%] of the 18 cases). The other epithelial markers stained the spindle cell component less frequently: cytokeratin CAM 5.2 in seven cases (39%); epithelial membrane antigen in nine cases (50%); and high-molecular-weight cytokeratin 34 beta E12 in no cases (0%). In 10 cases (56%) vimentin positivity and in six cases (33%) actin positivity was seen in the spindled areas. The spindle cell component stained for Leu-M1 in four cases (22%) and for S100 protein in one case (6%) and did not react for desmin in any case. From this study we conclude that in the majority of sarcomatoid renal cell carcinomas the epithelial nature of the spindle cells, as indicated by cytokeratin expression, can be documented using immunohistochemical methods.