Why is thymic carcinoma rare?

Small organ: The thymus is a small organ in the anterior mediastinum. There are several different tumors, but a thymoma is the most common. Thymic carcinoma is just not a common tumor. It may be related to the volume of the gland. Same as eye: very rare to have eye cancers, and it is also a small organ. This is not always true though -- testes are small, but they have many cancers per year.

Less active organ: The most common cancers arise from glandular tissues in the body and the more frequently that tissues turn over or replicate themselves the higher is the incidence of cancer. The thymic gland is active early in life but then becomes less active after adolescence and regresses in size which probably plays a role in its lower risk of generating a cancer.