Mixed epithelial and stromal tumours of the kidney with malignant transformation: a clinicopathological study of four cases.

Mixed epithelial and stromal tumour of the kidney is a complex benign neoplasm in which malignancy rarely arises. In this study, we report four mixed epithelial and stromal tumours in which sarcoma or carcinoma developed. In the first, a multifocal adenocarcinoma arose and areas of transition from benign to malignant epithelium were observed. Oestrogen and progesterone receptors were diffusely present in the nuclei of the spindle cell stroma of the benign component. The second was a sarcoma in which benign epithelial elements were intermixed. Outside the renal parenchyma, clusters of small benign glands surrounded by oestrogen receptor-positive benign stroma were present, supporting the diagnosis of mixed epithelial and stromal tumour. Fluorescence in situ hybridisation for SYT-SSX translocation and immunohistochemical results, specifically TLE1 -ativity, argued against primary renal synovial sarcoma. The patient died 24 months after surgery. The third tumour consisted of small blue round cells, positive for epithelial membrane antigen, BCL2, CD99, and FLI1. Throughout the tumour, the presence of benign appearing branching tubules in fibromuscular stroma, reactive for smooth muscle actin, desmin and progesterone receptor, supported the diagnosis of mixed epithelial and stromal tumour in which a small round blue cell sarcoma with EWSR1 rearrangement arose. In the fourth tumour, adenocarcinoma with papillary architecture arose in a typical mixed epithelial and stromal tumour. In summary, we present four cases of mixed epithelial and stromal tumour with malignant transformation, two showing carcinomatous and the other two with sarcomatous transformation. Identification of typical benign looking elements and the absence of SYT-SSX translocation are helpful in recognition of this entity.