Osteosarcoma-like divergent differentiation in sarcomatoid renal cell tumor. A rare entity whose diagnosis may be difficult.

A sarcomatoid component can occur in all histologic subtypes of renal cell carcinoma (RCC) and indicates an aggressive tumor. It is reported in literature that the sarcomatoid component can resemble fibrosarcoma, malignant fibrous histiocytoma or rhabdomyosarcoma. We describe the case of a 59-years old patient diagnosed with a chromophobe renal cell carcinoma, presenting a very rare osteosarcoma-like differentiation of the sarcomatoid component. The differential diagnosis with retroperitoneal calcifying masses is also discussed.