Malignant rhabdoid tumor of the orbit.

Malignant rhabdoid tumor (MRT), originally described as a sarcomatous variant of Wilms' tumor, is now recognized as a distinct, highly malignant entity. The authors describe, for the first time, a primary tumor of the orbit with histologic, immunohistochemical, and ultrastructural features of MRT. Their findings suggest both epithelial and mesenchymal differentiation of this unique tumor.