Thymoma is a rare cancer that begins in the thymus, a small gland tucked behind the breastbone that plays a vital role in building the body’s immune system during childhood. While these tumors grow slowly and often don’t cause noticeable symptoms in their early stages, understanding them and the treatment options available can help patients navigate their diagnosis with greater confidence and hope.

How Common Is Thymoma?

Thymoma and its more aggressive relative, thymic carcinoma, are remarkably uncommon cancers. Together, they represent the most frequently occurring tumors of the thymus gland, yet only about 400 people receive this diagnosis each year in the United States.^[1] Despite their rarity overall, these cancers account for roughly 20 percent of all tumors found in the mediastinum, which is the central area of the chest between the lungs.^[4]

The annual worldwide incidence of thymic malignancies ranges from 1.3 to 3.2 cases per million people, making them truly uncommon in the broader landscape of cancer diagnoses.^[14] When thymomas do occur, they typically affect adults, with most diagnoses happening between the ages of 40 and 75.^[1] Some sources note that the mean age at diagnosis is between 50 and 60 years, though these cancers can appear in elderly patients and, rarely, even in children.^[14]

In the United States, most people diagnosed with thymoma or thymic carcinoma have Asian or Pacific Islander heritage.^[1] However, there appears to be no clear difference in how frequently thymoma affects men versus women, and no specific racial groups appear to be at significantly higher risk globally.^[4]

What Causes Thymoma?

The exact reasons why thymoma develops remain unknown to medical researchers. Unlike some cancers where clear risk factors such as smoking or sun exposure have been identified, no specific environmental exposures, lifestyle choices, or inherited genetic mutations have been definitively linked to the development of thymic tumors.^[4]

The thymus itself is a remarkable organ. Located behind the breastbone and in front of the heart and major blood vessels, it consists of two main types of cells: epithelial cells, which form the structural framework of the organ, and lymphocytes, which are white blood cells that help fight infections.^[1] During childhood and adolescence, the thymus is highly active, training specialized immune cells called T-lymphocytes or T-cells to recognize and attack foreign invaders while leaving the body’s own tissues alone.^[2]

After puberty, the thymus naturally becomes less active and gradually shrinks, transforming into fatty tissue as part of the normal aging process.^[4] Thymoma arises when the epithelial cells in this gland begin growing abnormally. If lymphocytes instead grow abnormally within the thymus, the result is typically a lymphoma rather than a thymoma.^[1]

While researchers cannot point to specific causes, there is a notable association between thymoma and various autoimmune conditions. Between 30 and 40 percent of patients with thymoma experience symptoms of an autoimmune disorder, with myasthenia gravis being the most common.^[4] This connection works both ways: about 10 to 15 percent of patients with myasthenia gravis have a thymoma, and conversely, 30 to 45 percent of patients with thymoma develop myasthenia gravis.^[3] The relationship between thymoma and these immune system disorders was discovered somewhat accidentally in 1939 when a surgeon removed a thymic cyst from a young woman with myasthenia gravis and found that her muscle weakness improved dramatically.^[5]

Risk Factors for Thymoma

One of the most puzzling aspects of thymoma is that medical science has not identified any clear risk factors that increase a person’s likelihood of developing this cancer. There are no known behaviors to avoid, no occupational exposures to prevent, and no screening tests recommended for early detection in the general population.^[4]

Age does appear to play a role, as thymomas occur most commonly in middle-aged and older adults, particularly those in their fourth through sixth decades of life.^[4] However, this is an observation about when thymomas tend to appear rather than a true risk factor that someone can modify or control.

The strong association with autoimmune conditions, particularly myasthenia gravis, suggests that abnormalities in immune system function may be involved in thymoma development, but whether these immune disorders increase the risk of developing thymoma or result from its presence remains unclear.^[4] An additional 5 percent of patients with thymoma have other autoimmune or paraneoplastic syndromes, which are conditions that occur when the immune system attacks not only cancer cells but also normal, healthy tissues throughout the body.^[5]

Symptoms of Thymoma

Many people with thymoma experience no symptoms at all in the early stages of the disease. Between one-third and one-half of all thymomas are discovered incidentally when a chest X-ray or CT scan is performed for an entirely unrelated medical reason.^[3] This silent nature of early thymoma means that many patients feel completely healthy when they receive their diagnosis, which can make the news particularly shocking and difficult to process.

When symptoms do appear, they typically fall into two categories: those caused by the tumor physically pressing on or invading nearby structures in the chest, and those resulting from autoimmune paraneoplastic syndromes that accompany the cancer.

Symptoms from the Tumor Pressing on Chest Structures

As a thymoma grows, it can push against or compress the organs, blood vessels, and airways in the chest cavity. About one-third of patients experience symptoms related to this physical expansion of the tumor.^[3] The most common of these symptoms include chest pain or a feeling of pressure in the chest, a persistent cough that doesn’t resolve, shortness of breath, and difficulty swallowing.^[1]

In some cases, the growing tumor can block or squeeze the vena cava, which are the large veins that carry blood from the upper body back to the heart. When this happens, a serious condition called superior vena cava syndrome can develop.^[1] This syndrome causes blood to back up in the veins of the head, neck, and arms, leading to noticeable swelling in the face, neck, and upper body. Patients may also experience visible veins in the upper chest and neck, along with cough, shortness of breath, fatigue, dizziness, and headaches.^[1]

Some patients may notice their voice becoming hoarse, which can occur if the tumor affects the nerves that control the voice box.^[2] These symptoms develop gradually and may be mistaken for other, less serious conditions, which is why persistent or worsening chest symptoms should always be evaluated by a healthcare provider.

Symptoms from Autoimmune Paraneoplastic Syndromes

Another one-third of thymoma patients have their cancer discovered because they develop symptoms of an associated autoimmune disorder.^[3] In these conditions, the immune system malfunctions and begins attacking the body’s own healthy tissues alongside the cancer cells. This happens through mechanisms that scientists are still working to fully understand.

The most common autoimmune condition associated with thymoma is myasthenia gravis, which causes progressive muscle weakness, particularly in the muscles that control eye movement, facial expression, swallowing, and breathing.^[1] Patients with myasthenia gravis often first notice drooping eyelids, double vision, difficulty chewing or swallowing, or weakness in their arms and legs that worsens with activity and improves with rest.

Another paraneoplastic syndrome is pure red cell aplasia, a blood disorder where the bone marrow stops producing enough red blood cells, leading to anemia. People with this condition feel persistently tired and weak, may appear pale, and can experience shortness of breath even with mild exertion.^[1]

Hypogammaglobulinemia, also known as Good syndrome, is an immune system disorder where the body fails to produce enough antibodies, making patients highly susceptible to repeated infections.^[1] People with this condition may find themselves battling one respiratory or other infection after another without the normal periods of good health in between.

Less frequently, thymoma has been associated with a wide range of other autoimmune conditions including polymyositis (muscle inflammation and weakness), lupus erythematosus (a systemic inflammatory disease), rheumatoid arthritis (joint inflammation), thyroiditis (thyroid gland inflammation), and Sjögren syndrome (dry eyes and mouth).^[1]

Prevention of Thymoma

Because no risk factors for thymoma have been identified, there are unfortunately no specific prevention strategies that can reduce the likelihood of developing this cancer. Unlike lung cancer, where avoiding tobacco can dramatically reduce risk, or skin cancer, where sun protection makes a meaningful difference, thymoma appears to develop without clear connections to lifestyle, environmental exposures, or preventable causes.^[4]

There are no recommended screening programs for thymoma in the general population, as the disease is too rare to justify widespread screening tests. Even in people with myasthenia gravis, who have a somewhat higher likelihood of having a thymoma, routine screening of asymptomatic individuals is not standard practice.

The best approach to thymoma, given the current state of medical knowledge, is to remain attentive to persistent or unusual symptoms, particularly those affecting the chest, breathing, or muscle strength, and to seek prompt medical evaluation when such symptoms arise. Early detection, even when it happens incidentally during imaging for other purposes, can lead to treatment at a stage when outcomes are most favorable.

How Thymoma Affects the Body

Understanding what happens in the body when thymoma develops requires looking at both the physical changes caused by the tumor itself and the complex immune system disruptions that often accompany it.

Physical Changes from Tumor Growth

Thymomas originate from the epithelial cells that form the structural framework of the thymus gland. Under a microscope, the cancer cells in a thymoma often look quite similar to normal thymus cells, which partly explains why these tumors typically grow slowly.^[2] The tumor usually remains well-contained within a capsule of tissue and rarely spreads beyond the thymus in its early stages.

As the tumor enlarges, it occupies more space within the anterior mediastinum, the front portion of the chest cavity between the lungs. This area is crowded with vital structures including the heart, major blood vessels, the windpipe, and the esophagus. The growing tumor can press against any of these structures, interfering with their normal function. When it pushes on the trachea or bronchi, breathing becomes difficult. When it compresses the esophagus, swallowing becomes uncomfortable. When it blocks the vena cava, blood flow back to the heart is impaired.^[1]

In more aggressive cases, particularly with thymic carcinoma rather than thymoma, the cancer cells may invade through the tumor capsule and into surrounding tissues. The cancer cells might spread to the lining of the lungs or heart, a membrane called the pleura or pericardium. In advanced stages, thymic malignancies can metastasize to distant organs, though this is relatively uncommon with thymoma itself.^[1]

Immune System Disruption

The most puzzling aspect of thymoma is its profound effect on the immune system, which extends far beyond the physical presence of the tumor. The thymus is responsible for training T-cells, the specialized white blood cells that coordinate immune responses and distinguish between the body’s own tissues and foreign invaders. When a thymoma develops, this careful training process can go awry.^[2]

In autoimmune paraneoplastic syndromes, the immune system becomes confused and begins producing antibodies or T-cells that attack the body’s own tissues. In myasthenia gravis, for example, antibodies block or destroy the receptors at the junction between nerves and muscles, preventing the nerves from properly signaling the muscles to contract. This results in progressive weakness that characteristically worsens with repeated use of the affected muscles.^[1]

In pure red cell aplasia, the immune system attacks the precursor cells in the bone marrow that develop into red blood cells, effectively shutting down red blood cell production. In Good syndrome, the immune system fails to produce adequate amounts of protective antibodies, leaving patients vulnerable to infections.^[1]

These immune-related complications can sometimes be more troublesome than the tumor itself and may persist even after the thymoma has been successfully removed, requiring ongoing management with medications that modulate immune system activity.