Pituitary adenoma presenting as sinonasal tumor: pitfalls in diagnosis.

Pituitary adenomas may cause significant difficulties in histological diagnosis when presenting in unusual sites either as extension from an intrasellar lesion or as ectopic tumor. Three such cases, involving the sinonasal tract are described herein, and the differential diagnoses are discussed. Two of them were invasive intrasellar macroadenomas that presented as unilateral nasal polyp, and one was an ectopic pituitary adenoma involving the sphenoid sinus. There was notable cellular atypia in two cases, with nuclear pleomorphism, giant cells, chromatin clumping, and distinct nucleoli, leading to serious consideration of the possibility of poorly differentiated carcinoma. In none of the cases was the diagnosis of pituitary adenoma suspected clinically. The clues to diagnosis were an endocrine growth pattern comprising tumor cells arranged in packets, ribbons, or rosettes, with prominent delicate vascularized stroma; a high index of suspicion; and immunohistochemical showing of neuroendocrine markers and pituitary hormones in the tumor cells. A correct diagnosis is important because in contrast to neuroendocrine carcinoma as a whole or to poorly differentiated carcinoma, pituitary neoplasms have a much more favorable prognosis and a low metastatic potential.