Thymic cysts can also occur uncommonly in the absence of a thymoma and are thought to be developmental in origin. Thymus glands can also undergo hyperplasia which may or may not produce a measurable enlargement and which can be due to lymphoid follicle formation only or to an increase in the entire gland structure. Thymomas can be subtyped based on histologic appearance but staging is the most important determinant of management and prognosis. Most cases can be cured by excision and can be considered benign. Locally invasive tumors may also need radiation.
Some cases spread to pleura and pericardium but distant metastases are rare.
In adults the thymus involutes and consists of islands of mainly lymphoid tissue scatterred through fat 2. Hassal's corpuscles are rare or absent. Goitres, adenomas and carcinomas can all arise in ectopic thyroid although this is fairly rare. More common is a cervical thyroid goitre which extends substernally. Distinguishing between these scenarios is important prior to surgical resection as the blood supply differs.