New aspects of tumor pathology of the pituitary

Abstract

Pituitary adenomas have to be studied in detail for structural characteristics, especially regarding the degree of granulation and immunohistochemical hormone expression, such as growth hormone (GH), prolactin, adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH), luteinizing hormone (LH) and proliferation markers (e.g. Ki-67 and p53) for correlation to clinical data and assessment of the prognosis. If histological and immunostaining data do not correlate to the patient data, explanations for the discrepancies must be found. All active adenoma types can also be present as inactive, so-called silent adenomas showing the same features. An increased Ki-67 index (> 3%), significant nuclear expression of protein p53 and mitoses are characteristic of atypical adenomas. Up to now the biological relevance of these atypical adenomas, especially their role as preneoplasms for pituitary carcinomas has not been fully elucidated. The only proof of a pituitary carcinoma is the existence of metastases. Extensive local invasion and a greatly increased Ki-67 index are not sufficient for this diagnosis. Craniopharyngiomas have to be classified into adamantinomatous types (intrasellar and suprasellar) and papillary types (only suprasellar). Regressive changes are found in adamantinomatous types only. Strong regression may lead to difficulties in the differential diagnosis of Rathke's cleft cysts with squamous metaplasia. Demonstration of nuclear expression of beta-catenin in these cases enables the diagnosis of craniopharyngioma. Papillary craniopharyngiomas are characterized by BRAF mutations that may be helpful in the differential diagnosis. All pituicytomas of the neurohypophysis, all spindle cell oncocytomas of the anterior pituitary and all granular cell tumors of the posterior pituitary express thyroid transcription factor 1 (TTF-1) and are thought to be variants of a common uniform spindle cell tumor of the pituitary.