Unclassifiable Renal Cell Carcinoma With an Estimated 100% Sarcomatoid Component: A Case Report

Sarcomatoid renal cell carcinoma (RCC) is an aggressive tumour with a poor prognosis. It is not a distinct histological entity, as it can be found in any subtype of renal cell carcinoma. The majority of cases will present with advanced or metastatic disease requiring systemic treatment. However, pathologists are sometimes confronted with renal cell tumours that do not fit any definition, so these tumours must be considered unclassified renal cell carcinomas. The criteria for this classification are varied and include tumours with more than one histological subtype, tumours with unrecognized epithelial histological subtypes, mucin-producing tumours, and tumours with an exclusively sarcomatoid or rhabdoid morphology. The latter subgroup of unclassified renal cell carcinomas is rare in renal pathology. Consequently, the presence and quantity of the sarcomatoid component must be reflected in pathology reports. We report the case of a 76-year-old male patient presenting with intermittent left lumbar pain for two months, in a deteriorating general condition. Histological and immunohistochemical examinations revealed an unclassifiable renal cell carcinoma with an estimated 100% sarcomatoid component.