thymoma
Meaning of thymoma: A slow-growing growth in the front of the chest, often discovered because of unrelated muscle weakness.
Definition of thymoma
Thymoma is a tumor that grows from the surface cells of the thymus, a small gland in the front of the chest that trains immune cells during childhood. Most thymomas grow slowly and stay within their capsule, although some push into nearby tissue. Many are found only because of an associated autoimmune illness rather than because of chest symptoms.
Where is the thymus and what does it do?
The thymus sits behind the breastbone and in front of the heart. It is part of the lymphatic system, and its work is mostly done in childhood: it schools T cells, the immune cells that learn to tell the body apart from an invader. After puberty the gland shrinks and is gradually replaced by fat, so an adult thymus is small.
How is thymoma different from thymic carcinoma?
Both start in the same gland, but they behave differently enough to be treated as separate diseases.
| Thymoma | Thymic carcinoma | |
|---|---|---|
| Growth | Slow, often contained | Faster, invasive |
| Cells under the microscope | Resemble normal thymus | Clearly abnormal |
| Spread beyond the chest | Uncommon | More likely |
| Autoimmune links | Frequent | Rare |
Why is it linked to muscle weakness?
A large share of people with thymoma also have myasthenia gravis, a disorder in which the immune system attacks the junction between nerve and muscle. The result is weakness that worsens with use and improves with rest, often first noticed as drooping eyelids or double vision. A thymoma that has lost its normal control over developing immune cells is thought to allow these self-directed cells to escape. Weakness of this kind is a common reason the tumor is looked for at all. Other autoimmune conditions turn up as well, including pure red cell aplasia, in which the marrow stops making red cells, and low antibody levels that leave a person prone to infection.
How is it staged?
Staging here does not depend on size, as it does for many tumors. What matters is how far the growth has pushed outward: whether it remains inside its fibrous capsule, has entered the fat around the gland, has reached neighboring structures such as the lung covering or the sac around the heart, or has seeded deposits further afield. A thymoma confined to its capsule and removed whole carries a very different outlook from one that has invaded. Because recurrence can be late, follow-up imaging continues for years after surgery.
Used in a sentence
A chest radiograph taken for another reason revealed an anterior mediastinal mass, later confirmed as a thymoma.
Good to know
Thymoma and thymic carcinoma both arise in the thymus but are distinct diseases; thymoma is the slower of the two and is the one associated with myasthenia gravis.
Word origin
Where “thymoma” comes from
From the Greek thymos, the name of the gland, plus the suffix -oma, meaning a swelling or tumor.