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.