Renal cell carcinoma with rhabdoid and sarcomatoid features presented as a metastatic thigh mass with an unusual immunohistochemical profile.

Renal cell carcinoma (RCC) may metastasize anywhere in the body and sometimes the primary tumor is missing and necessitates extensive investigations to detect. In this report, we describe a case of RCC metastasizing to the thigh in a 70 year old male with a highly pleomorphic morphology suggesting a high grade sarcoma that showed unequivocal positivity for desmin directing the diagnosis for pleomorphic rhabdomyosarcoma. After completion of 33 cycles of radiotherapy, the patient developed large intraabdominal mass that showed conventional areas of RCC with immunoreactivity for CD10, CK, EMA, carbonic anhydrase IX and vimentin. The tumor cells in other areas resembled that of thigh mass which raised suspicions whether the two masses represented the same tumor or not. Surprisingly, the tumor cells of thigh mass showed diffuse positivity for CD10 and focal expression for CK, EMA and carbonic anhydrase IX. Extensive investigations failed to detect any primary renal lesions. The present case demonstrated that RCC can metastasize to virtually any body site and can have significant morphologic overlap with other non-renal neoplasms. Absence of primary origin of RCC according to radiological and operative data should not hinder the diagnosis of metastatic RCC. RCC with sarcomatoid and rhabdoid features carries aggressive behavior manifested by great metastatic potential and short survival time.