TTF-1-positive Oncocytic Sellar Tumor with Follicle Formation/ependymal Differentiation: Non-adenomatous Tumor Capable of Two Different Interpretations As a Pituicytoma or a Spindle Cell Oncocytoma

We describe herein the unique case of a 70-year-old male with a TTF-1-positive non-adenomatous sellar tumor that has unusual morphological and immunohistochemical features. MRI examination detected a 2-cm sellar mass that was enhanced heterogeneously. By histology, the tumor was composed of epithelioid and oncocytic cells arranged in a trabecular pattern with occasional luminal structures. The lesion was diffusely immunopositive for thyroid transcription factor-1 (TTF-1) and vimentin but negative for S100 protein and GFAP. Immunoreactivity for epithelial membrane antigen, low molecular weight cytokeratin (CAM 5.2), and neuronal markers was also observed in the tumor cells. By electron microscopy, the tumor cells were filled with abundant mitochondria and extended microvillous projections into small extracellular and intracellular lumens. TTF-1 is considered to be an excellent marker of pituicytes, specialized glia of the neurohypophysis. This case can be regarded as a variant of pituicytoma, showing both ependymal differentiation and oncocytic changes. However, the immunoprofile was not completely consistent with a pituicyte lineage; the epithelial features suggested a possibility of folliculostellate cell origin. TTF-1-positive sellar neoplasms might therefore have variable morphological and immunohistochemical profiles. For suitable classification of TTF-1 positive sellar neoplasms, their histological features should be carefully re-evaluated.