Sections stained by conventional methods showed a moderately cellular tumor with an organoid pattern. The tumor cells were divided into groups by a network of incomplete septae resulting in a lobular appearance. They contained reticulin, collagen fibers and vessels, and were regionally hyalinized. The tumor cells were small to medium sized with round nuclei and eosinophilic cytoplasm without clear boundaries. Interspersed were larger cells with slightly pleomorphic, hyperchromatic nuclei containing an eccentric nucleolus and often abundant cytoplasm, suggestive of atypical ganglioid cells. In addition, there were scattered giant cells with single or multiple bizarre nuclei. Occasionally, the cytoplasm was vacuolated. There were small amounts of an eosinophilic matrix between the tumor cells in a central location, resembling neuropil. (Image 02). A network of neurites could be visualized here and along vessels and septae by Bodian silver impregnation (Image 03). Typical pineocytomatous rosettes were not detectable. Only a few mitoses were observed and there was no necrosis. The tumor showed few microcalcifications and was bounded by a fibrous capsule.
Immunohistochemically, a large number of cells showed a positive cytoplasmic reaction with antibodies to neuron specific enolase (BioGenex). Synaptophysin (38 kD, Dako) was conspicuously positive in the neuropil islands in a fine granular distribution (Image 04). Neurofilament protein (70 kD and 200 kD, Dako) was demonstrable in perikarya and cytoplasmic processes in many cells. Antibodies to glial fibrillary acidic protein, cytokeratin, placenta-specific alkaline phosphatase, and vimentin gave no positive reaction. MIB-I was positive in approximately 1.5% of tumor cell nuclei.