Adult variant of atypical teratoid/rhabdoid tumor: immunohistochemical and ultrastructural confirmation of a rare tumor in the sella tursica.

Atypical teratoid/rhabdoid tumor (AT/RT) is a distinctive neoplasm of young children characterized by diverse histology and fatal course. Adult presentation is rare. We describe the diagnostic problems associated with an AT/RT arising in the sellar region in a 46-year-old female. Vimentin, keratin, synaptophysin, CD34, SMA, PLAP, GFAP, S-100, NSE, desmin, MYF-4, LCA, and CD99 were performed on tissue obtained from the paraffin block. INI1 protein expression was immunohistochemically determined on tumor tissue. Electron microscopy was performed from the tissue block. The tumor was composed of large atypical "rhabdoid" cells having macronucleoli and abundant eosinophilic cytoplasm. Immunohistochemistry showed that the tumor cells were positive for vimentin, CD34, CD99, and reacted variably for keratin, synaptophysin, NSE, and SMA. All were negative for GFAP, S-100, desmin, MYF-4, and LCA. The tumor cells lacked nuclear expression of INI1. Electron microscopy revealed cells with large paranuclear intracytoplasmic collections of intermediate filaments. AT/RT should be considered when dealing with a malignant neoplasm with rhabdoid features, regardless of age. Immunohistochemistry is of importance in differentiating this entity from primitive neuroectodermal tumors (PNET) and carcinosarcomas. Lack of nuclear INI1 protein expression by immunohistochemical methods is required for a reliable diagnosis.