Thymomas are rare tumors that grow from the epithelial cells of the thymus, a gland essential to the immune and endocrine systems. Less than one person in 1.5 million develops the disease -- about 400 cases per year. Thymomas are malignant, even though they are slow-growing, because of their location and potential to invade vital spaces and cause complications. There are various types of thymomas.
The thymus is a gland behind the sternum, in front of the pericardium. It gets blood from the internal thoracic and pericardiophrenic arteries, and the inferior thyroid. The thymus weighs between one and 1.5 ounces and is responsible for producing the hormone thymosin, which stimulates T cells that keep the immune system from turning on itself. Past puberty, the gland becomes inactive and shrinks, leaving behind bundles of cytoplasmic fibers called Hassall's corpuscles.
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