[Epithelial tumors with thymus differentiation of the thyroid gland and the neck]

Epithelial tumors with thymus-like differentiation occurring in the thyroid gland and the neck, delineate four entities with distinct histological features and different behaviour. The spindle epithelial tumors with thymus-like differentiation are potentially malignant tumors occurring in the thyroid gland of young patients. They are highly cellular with pattern of spindle cells and tubulopapillary formations and mucinous glands. Carcinomas showing thymus-like differentiation of the thyroid gland are histologically identical to some thymic carcinomas. The ectopic hamartomatous thymoma is strictly benign and occurs in the soft tissues of the lower neck. It is characterized by spindle and solid or cystic islands of epithelial cells associated with adipose cells. The ectopic cervical thymoma is usually benign and histologically similar to mediastinal thymomas. All these tumors are particularly rare and arise from ectopic thymus or remnants of branchial pouch. The differential diagnoses of these tumors can be difficult and include some other proliferations occurring in the neck.