DISTINCTIVE FEATURES

The tumors that arise in the thymus consist of those originating from the thymic epithelial cells (thymoma), the lymphoid tissue (Hodgkin's and non-Hodgkin's lymphomas) as well as germ cell tumors, and carcinoid.
Thymomas are most commonly found in adults over the age of 40.
Thymomas are found both in males and females at an equal frequency.
Most commonly occur in the anterior or superior mediastainum. However, may occur in the neck, thyroid or pulmonary hilum.
Grossly appears as a lobulated, firm, gray-white mass measuring from few to 20 cm.
These tumors may be benign (70%) or malignant (30%). The first group of tumors are both histologically and biologically benign. The second set of tumors may be histologically benign (type I; 25%) or malignant (type II; 5%) and have an aggressive biologic behavior. Microscopically, these tumors may consist of spindle cells resembling the thymic medullary cells (medullary thymoma) and may in addition contain cells resembling the cortical-type cells (mixed thymoma). The diagnosis of the malignant type I thymoma is based on demonstration of invasion of the tumor capsule or adjacent structures. The malignant type II thymoma or thymic carcinoma most commonly appears as squamous cell carcinoma with various degrees of differentiation. Other types include clear cell type, lymphoepitheliomatous type, sarcomatoid type and basaliod type.