Spindle cell oncocytoma of the adenohypophysis: report of a case with marked cellular atypia and recurrence despite adjuvant treatment.

Spindle cell oncocytoma (SCO) of the adenohypophysis is a recently defined pituitary tumor mimicking a non-functioning macroadenoma and composed of mitochondrion rich tumor cells, positive for S-100, vimentin, epithelial membrane antigen and galectin-3 but lacking cytokeratins, pituitary hormones, and neuroendocrine markers. Derivation from pituitary folliculostellate cells (FSCs) has been suggested based upon immunohistochemical and ultrastructural characteristics shared by SCO and FSCs. 10 cases of SCO have been reported to date; of these, 8 underwent a benign clinical course and 2 recurred. We report a case of SCO with typical histologic and immunohistochemical features in addition to marked cellular pleomorphism and nuclear atypia. It showed slow regrowth over a 30-month period of follow-up despite combined surgical and radiotherapy. Despite the benign course of most reported cases, additional experience with longer follow-up are needed to assess clinical, histopathologic, and proliferative indices and their relevance to optimal therapy for this rare pituitary tumor.