Solitary fibrous tumour of the kidney with sarcomatous overgrowth. Case report and review of the literature.

Solitary fibrous tumour (SFT) rarely occurs in the kidney, with only one case exhibiting malignant behaviour. We report the case of a typical SFT of the kidney with sarcomatous overgrowth in a 34-year-old woman. This malignant component, grossly apparent as a nodular area arising in the context of the main tumour mass, consisted of CD34+ mitotically active atypical plump spindle- to epithelioid-shaped cells, including pleomorphic multinucleated giant cells. A novel immunohistochemical finding was diffuse and strong S-100 protein expression by sarcomatous cells. This should be kept in mind by pathologists to avoid confusion with other S-100 protein-positive malignant neoplasms.