Chromophobe renal cell carcinoma with osteosarcoma-like differentiation.

Sarcomatoid differentiation in renal cell carcinoma is thought to be the result of the dedifferentiation of the parent tumor, and it can be found in the chromophobe renal cell carcinoma just as other subtypes. We report a case of chromophobe renal cell carcinoma, which showed osteosarcoma-like differentiation. This is the first known case ever to be clearly identified as such. The patient was a 74-year-old man, and the CT scan revealed a huge retroperitoneal mass, which protruded from the lower half of the kidney and directly invaded the colon. Intraabdominal dissemination and metastases to the liver and lungs were also found. The resected tumor histologically showed sarcoma-like spindle cell proliferation and partly produced massive osteoid, which simulated the osteosarcoma. In addition, a typical histology of chromophobe renal cell carcinoma was found in part of the tumor. Immunohistochemically, spindle cells were reactive for epithelial membrane antigen, cytokeratin, and vimentin. The cell nests that were labeled by epithelial membrane antigen and cytokeratin were also found in the osteosarcoma-like area. We think that these phenomena were the result of "dedifferentiation" and metaplasia of the chromophobe renal cell carcinoma.