Multiple Endocrine Neoplasia Type 1 (MEN1) is a rare autosomal dominantly inherited endocrine tumor predisposition syndrome, caused by mutations in the MEN1 gene. Cardinal manifestations are primary hyperparathyroidism (pHPT), pituitary adenomas (PA), and neuroendocrine tumors (NETs) of the pancreas (PanNET) and duodenum. Other manifestations are NETs of thymus, lung, and stomach, adrenal tumors, and an increased breast cancer risk in women. Malignant NETs are the most important cause of disease-related mortality, mainly NF-PanNETs, gastrinomas and thymus NETs. MEN1 can be diagnosed genetically and a clinical diagnosis in patients with negative comprehensive testing has been debated. Timely recognition of MEN1, referral for genetic testing and swift cascade screening is essential. MEN1-related pHPT (penetrance >95%) is a multiglandular disease and recurrence after initial operation is to be expected. Subtotal parathyroidectomy is the preferred initial operation. Prolactinomas are the most prevalent PA in MEN1, followed by non-functioning (NF) PAs. Treatment and treatment results do not differ from sporadic PAs. Life-time penetrance of duodenopancreatic NETs is >80%. NF-PanNETs are most frequent, followed by gastrinomas and insulinomas. Surgical resection is the mainstay of treatment, and is indicated in non-gastrinoma functional PanNETs and NF-PanNETs >2cm or with progression during follow-up. No consensus exists on the surgical treatment of MEN1-related gastrinoma. MEN1-related dpNETs are currently detected at earlier stages and more indolent small dpNETs are seen. The main challenge is to identify patients at risk for an aggressive disease course. Thymic NETs (2-8%) occur predominantly in males and have a poor prognosis. Bronchopulmonary NETs are more frequent than previously thought, occur in both sexes, and are usually indolent although cases with a deviant progressive course occur. Adrenal tumors are mostly indolent non-functioning adenomas, but adrenocortical carcinomas and pheochromocytomas do occur. Women with MEN1 have an increased (RR 2.8) risk of breast cancer, at a younger age than the general population. Given the complexity of the disease, it is strongly advised that patients, whenever possible, be followed and treated in centers of expertise. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text,
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