Oncocytic variant of malignant gastrointestinal neuroectodermal tumor: a potential diagnostic pitfall.

Malignant gastrointestinal neuroectodermal tumor (MGNET) is a very rare, aggressive malignant neoplasm that may occur in any location in the gastrointestinal tract. Malignant gastrointestinal neuroectodermal tumors typically consist of sheet-like to pseudopapillary proliferation of primitive-appearing epithelioid cells with a moderate amount of lightly eosinophilic cytoplasm, round nuclei and small nucleoli, often in association with osteoclast-like giant cells. By immunohistochemistry, these tumors show expression of S100 protein and SOX10, in the absence of expression of more specific melanocytic markers (eg, HMB45, Melan A). Genetically, malignant gastrointestinal neuroectodermal tumors are characterized by rearrangements of the EWSR1 or FUS genes with CREB1 or ATF1. We report a case of gastric malignant gastrointestinal neuroectodermal tumor occurring in a 46-year-old woman and showing striking oncocytic cytoplasmic change, a previously undescribed potential diagnostic pitfall. An initial needle biopsy showed large, eosinophilic cells with S100 protein and SOX10 expression and lacking expression of KIT, DOG1, Melan A, keratin, chromogranin, or smooth muscle actin, and was interpreted as representing a granular cell tumor. The subsequent excision specimen showed similar-appearing areas, but also contained small more primitive-appearing areas, lacking oncocytic change and having high nuclear grade and brisk mitotic activity. This resection specimen was initially diagnosed as a malignant granular cell tumor. However subsequent gene expression profiling studies showed an EWSR1-ATF1 fusion, confirmed with fluorescence in situ hybridization for EWSR1, and a final diagnosis of MGNET with oncocytic change was made. This case highlights a previously undescribed pitfall in the diagnosis of MGNET, oncocytic change, and suggests that MGNET should be included in the differential diagnosis for unusual oncocytic neoplasms of the gastrointestinal tract.