Thymus cancer is a rare type of cancer that starts in the thymus, a small organ behind your breastbone that plays a key role in training your immune system. There are two main forms: thymoma, which grows slowly and tends to stay contained, and thymic carcinoma, which is more aggressive and often spreads before it’s found. Together, they account for a small fraction of all cancers, but they can cause significant problems because of where the thymus sits and how it interacts with your immune system.
What the Thymus Does
The thymus sits in the upper chest, just behind the top of the breastbone, in the space between your lungs. Its main job is producing and training a type of white blood cell called T-cells, which are central to your immune defense. Inside the thymus, developing T-cells go through a rigorous screening process. About 95% of them are eliminated because they either can’t recognize threats properly or might mistakenly attack your own body. Only the cells that pass both rounds of screening are released into your bloodstream.
The thymus is most active during childhood and gradually shrinks as you age, which is one reason thymus cancers tend to appear in middle-aged and older adults. Even though the gland becomes less active over time, the cells that line it can still become cancerous.
Thymoma vs. Thymic Carcinoma
These two cancers start in the same type of cell (the epithelial cells of the thymus), but they behave very differently.
Thymoma cells still resemble normal thymus tissue under a microscope. Thymomas grow slowly, rarely spread beyond the thymus, and are generally more treatable. They’re also more likely to trigger immune-related complications, which can sometimes be the first clue that something is wrong.
Thymic carcinoma cells look abnormal and no longer resemble thymus tissue. They grow faster, are more likely to invade surrounding structures or spread to distant parts of the body, and have a much higher chance of coming back after treatment. Thymic carcinomas have usually already spread by the time they’re diagnosed. The five-year survival rate for thymic carcinoma ranges from roughly 30% to 50%, compared to significantly better outcomes for most thymomas.
Pathologists further classify thymomas into subtypes (labeled A, AB, B1, B2, and B3) based on how the cells look and behave. These subtypes help predict how aggressively a particular tumor might act, though the subtype alone doesn’t always tell the full story.
Symptoms to Recognize
Many thymus cancers are found incidentally on a chest scan done for another reason, because early tumors often cause no symptoms at all. When symptoms do appear, they’re usually caused by the tumor pressing on structures in the chest:
- Persistent cough, sometimes with bloody mucus
- Chest pain or a feeling of pressure
- Shortness of breath
- Trouble swallowing
- Unexplained weight loss or loss of appetite
If the tumor presses on the large vein that carries blood from the head and arms back to the heart, it can cause a condition called superior vena cava syndrome. This shows up as swelling in the face, neck, and upper chest, sometimes with a bluish-red skin color. Visible veins may bulge in these areas, and you might feel dizzy or lightheaded. This is a serious situation that needs prompt medical attention.
The Myasthenia Gravis Connection
One of the most distinctive features of thymus cancer, particularly thymoma, is its link to autoimmune conditions. Somewhere between 30% and 50% of people with thymoma also develop myasthenia gravis, a condition where the immune system attacks the connections between nerves and muscles. This causes muscle weakness that worsens with activity, especially in the eyes, face, and limbs. Drooping eyelids, double vision, and difficulty chewing or speaking are common early signs.
In some cases, myasthenia gravis is diagnosed first, and the thymoma is discovered during follow-up imaging. About 10% to 20% of people who have myasthenia gravis turn out to have a thymoma. A smaller percentage of thymoma patients, around 5%, develop a blood disorder called pure red cell aplasia, where the bone marrow stops producing enough red blood cells, leading to severe anemia. These immune-related conditions are much more common with thymomas than with thymic carcinomas.
How Thymus Cancer Is Diagnosed
A CT scan of the chest is typically the first step, often revealing a mass in the front of the upper chest. The scan helps doctors assess the size of the tumor and whether it has grown into nearby tissues.
Getting a tissue sample for confirmation can be tricky. Image-guided needle biopsies, where a needle is inserted through the chest wall using CT or ultrasound for guidance, are the most common nonsurgical approach. However, fine-needle aspiration alone correctly identifies the tumor type in only about 50% to 60% of cases, partly because the small tissue sample makes it hard to distinguish a thymoma from other conditions like lymphoma or thymic hyperplasia. Core needle biopsies, which collect a larger piece of tissue, improve accuracy. In some situations, a surgical biopsy is needed for a definitive diagnosis.
Staging is based on how far the tumor has grown beyond the thymus, whether it’s invaded surrounding structures like the lining of the lungs or heart, and whether it has spread to lymph nodes or distant organs. A tumor that’s fully contained within its capsule has a much better outlook than one that has broken through into neighboring tissue.
Treatment Options
Surgery is the cornerstone of treatment for thymus cancers that haven’t spread widely. The operation, called a thymectomy, removes the thymus along with any involved surrounding tissue. Traditionally this has been done through a vertical incision in the breastbone, but minimally invasive approaches using robotic-assisted or camera-guided techniques are increasingly used for smaller, earlier-stage tumors. Surgeons may convert to an open procedure if the tumor turns out to be more invasive than expected, such as when it’s growing into major blood vessels.
For tumors that can’t be fully removed or that have spread, radiation and chemotherapy play a larger role. Radiation may also be used after surgery to reduce the chance of recurrence, particularly for tumors that were found growing into nearby structures.
For advanced thymic carcinoma that has progressed after initial chemotherapy, newer immunotherapy drugs that target a protein called PD-1 or PD-L1 on cancer cells offer an additional option. In clinical trials, these drugs have produced tumor shrinkage in roughly 19% to 23% of patients with previously treated thymic carcinoma, while stabilizing the disease in a larger proportion. Response tends to be stronger in tumors that test positive for PD-L1. Several clinical trials are also testing combinations of immunotherapy with chemotherapy or targeted drugs as earlier-line treatments.
Because thymus cancers are rare, current guidelines strongly recommend that treatment decisions involve a multidisciplinary team with specific experience in these tumors. The rarity means that expertise matters more than usual, and being treated at a center that sees a higher volume of cases can make a real difference in outcomes.
Long-Term Outlook and Recurrence
For thymomas caught at an early stage and completely removed surgically, the prognosis is generally favorable, with many patients living well beyond ten years. Even some advanced thymomas carry a reasonable outlook because of their slow growth. The challenge is that thymomas can recur years or even decades later, so long-term follow-up with regular imaging is standard.
Thymic carcinoma carries a tougher prognosis. With five-year survival rates between 30% and 50%, and a tendency to come back after treatment, ongoing surveillance is especially important. Recurrences often happen, and second or third rounds of treatment may be needed over time. For people who also have myasthenia gravis, removing the thymus often improves the autoimmune symptoms, though the improvement may take months and isn’t guaranteed.