POLYPHENOTYPIC TUMORS IN THE CENTRAL-NERVOUS-SYSTEM - PROBLEMS IN NOSOLOGY AND CLASSIFICATION

In recent years, there is increasing recognition of polyphenotypic hig h-grade malignancies in the non-central nervous system (CNS) tumor lit erature. Some of these tumors have been regarded as variants of primit ive neuroectodermal tumor (PNET) or as extrarenal malignant rhabdoid t umors (MRTs). This report concerns two posterior fossa neoplasms, both of which displayed a ''polyphenotypic'' expression of neural, epithel ial, myogenic, and glial markers, including synaptophysin, neurofilame nt, vimentin, glial fibrillary acidic protein, S-100, neuron-specific enolase, desmin, S antigen, MIC2, cytokeratin, epithelial membrane ant igen, and carcinoembryonic antigen. One tumor showed complex intercell ular junctions, cytoplasmic intermediate filaments, well-developed rou gh and smooth endoplasmic reticulum and Golgi apparatus, cilia, and ne urosecretory granules. The other neoplasm showed pools of glycogen, de smosomes, and tonofilaments. The histological and ultrastructural appe arances were inconsistent with glioma, PNET, meningioma, ependymoma, c horoid plexus carcinoma, sarcoma, germ cell tumor, and other tumors in the World Health Organization classification. Although the polyphenot ype raises the issue that these may represent variants of MRT or the a typical teratoid-rhaboid tumor, the morphologic findings in the two ca ses were very dissimilar. Our two cases underscore the problems in nos ology and classification of polyphenotypic tumors of the CNS. This is particularly significant, as therapeutic protocols for PNET, MRT, and non-CNS polyphenotypic tumors are different. We review the literature on polyphenotypic tumors and reiterate the difficulties in precise cla ssification of these complex tumors.