Thymoma

Thymomas are rare cancers that develop in a small gland behind your breastbone called the thymus. While they often grow slowly and rarely spread, these tumors can cause serious symptoms and may be linked to conditions where your immune system attacks your own body.

Table of contents

• What is thymoma?
• The thymus gland and its location
• Thymoma versus thymic carcinoma
• Who is affected
• How common is thymoma
• Connection with autoimmune conditions
• Signs and symptoms
• What causes thymoma
• How thymoma is diagnosed
• Stages of thymoma
• Treatment options
• Outlook and survival
• Life after treatment and follow-up care

What is thymoma?

Thymoma is a type of cancer that begins in the thymus gland, a small organ in your chest. This cancer forms when cells in the outer covering of the thymus, called epithelial cells, begin to grow abnormally^[1]. Along with a related cancer called thymic carcinoma, thymomas are known as thymic epithelial tumors or TETs^[2].

Your thymus contains two main types of cells: epithelial cells and lymphocytes. When epithelial cells grow out of control, they can form thymoma or thymic carcinoma. When lymphocytes become cancerous instead, they cause different diseases called Hodgkin or non-Hodgkin lymphoma^[1].

The thymus gland and its location

Your thymus is positioned in the upper chest, tucked behind your breastbone and in front of your heart and the major blood vessels that carry blood to and from your heart^[2]. This small gland extends sideways to structures called the phrenic nerves, which control your diaphragm muscle for breathing^[4].

• Thymus gland
• Anterior mediastinum (front part of chest cavity)
• Area behind breastbone
• Upper chest

The thymus plays a vital role in your body’s defense system, particularly during childhood. It helps develop special white blood cells called T-lymphocytes or T-cells, which protect you from infections. These cells learn in the thymus to tell the difference between your body’s own cells and foreign invaders^[2]. As you grow older, your thymus naturally becomes smaller and less active, gradually turning into fatty tissue. This is a normal part of aging^[4].

Thymoma versus thymic carcinoma

Although thymoma and thymic carcinoma both develop in the thymus gland and originate from epithelial cells, they behave very differently. Thymoma typically grows slowly and rarely spreads beyond the thymus itself. The cancer cells in thymoma look quite similar to normal thymus cells when examined under a microscope^[2].

Thymic carcinoma, on the other hand, is much more aggressive. It develops faster and is more likely to metastasize, meaning spread to other parts of your body. Under the microscope, thymic carcinoma cells look very different from healthy thymus cells. About one in every five thymic epithelial tumors is a thymic carcinoma, and these are considerably harder to treat than thymomas^[2].

The treatment your healthcare provider recommends and your chances of recovery depend largely on whether you have thymoma (less aggressive) or thymic carcinoma (more aggressive)^[1].

Who is affected

Thymoma and thymic carcinoma most commonly affect adults between the ages of 40 and 75, though they can occur in older people and even children. These cancers affect men and women equally. In the United States, most people diagnosed with these tumors have Asian or Pacific Islander heritage^[1].

The average age when people are diagnosed is between 50 and 60 years^[3]. No specific lifestyle factors or exposures have been identified that increase your risk of developing thymoma^[4].

How common is thymoma

Thymoma and thymic carcinoma are rare cancers. Only about 400 people in the United States are diagnosed each year^[1]. Despite their rarity overall, these tumors are actually the most common type of growth found in the front part of the chest cavity, called the anterior mediastinum. They account for approximately 20% of all masses found in this area^[4].

Connection with autoimmune conditions

Thymomas have a strong connection with certain medical conditions where your immune system mistakenly attacks your own body. These are called autoimmune paraneoplastic syndromes. These conditions may appear in people with cancer, but they are not caused directly by the cancer itself. Instead, they happen when your immune system, while trying to fight cancer cells, also attacks normal, healthy cells^[2].

Between 30 and 40% of people with thymoma experience symptoms of one or more of these autoimmune conditions^[5]. The three most common autoimmune syndromes associated with thymoma are:

• Myasthenia gravis: This is the most frequently occurring autoimmune condition linked to thymoma. It causes weakness in your muscles because your immune system attacks the connections between your nerves and muscles. About 10 to 15% of people with myasthenia gravis have a thymoma, and conversely, 30 to 45% of people with thymoma develop myasthenia gravis^[3].
• Pure red cell aplasia (PRCA): In this blood disorder, your bone marrow stops making enough red blood cells, which leads to anemia. Anemia means you don’t have enough healthy red blood cells to carry oxygen throughout your body^[1].
• Hypogammaglobulinemia (also called Good syndrome): This immune system disorder prevents your body from making enough antibodies, the proteins that help fight off diseases and infections^[1].

Other autoimmune conditions that occur less frequently with thymoma include polymyositis, lupus erythematosus, rheumatoid arthritis, thyroiditis, and Sjögren syndrome^[1].

Signs and symptoms

Many people with thymoma don’t have any symptoms at first. In fact, between one-third and one-half of all thymomas are discovered by accident when a chest X-ray or CT scan is performed for an unrelated health issue^[3]. Most patients do not have signs or symptoms when they are first diagnosed^[2].

When symptoms do appear, they usually occur because the tumor has grown large enough to press on or squeeze nearby organs in your chest. About one-third of people with thymoma have symptoms caused by this compression^[3].

Common symptoms that may indicate thymoma include:

• Chest pain or a feeling of pressure on your chest
• A cough that won’t go away
• Shortness of breath or difficulty breathing
• Trouble swallowing
• A hoarse voice^[2]

In some cases, a thymoma tumor can block or squeeze the large veins that carry blood to your heart, called the vena cava. This compression can cause a serious condition known as superior vena cava syndrome. Warning signs of this condition include cough, shortness of breath, fatigue, dizziness, headaches, swelling in your neck and face, swelling in your arms and hands, and visible veins bulging in the upper part of your body^[1].

Another third of people with thymoma are diagnosed because they develop symptoms from one of the associated autoimmune conditions, particularly myasthenia gravis^[3].

What causes thymoma

The exact cause of thymoma remains unknown. Researchers have not identified any specific risk factors, lifestyle choices, or environmental exposures that lead to the development of these tumors^[4]. What scientists do know is that thymomas have a strong association with myasthenia gravis and other autoimmune conditions^[4].

How thymoma is diagnosed

If your doctor suspects you might have thymoma, the diagnostic process typically begins with a physical examination, a review of your medical history, and discussion of any symptoms you’re experiencing^[2].

Several tests may be used to diagnose thymoma:

Imaging tests are usually the first step. A chest X-ray can often detect most thymomas, but your doctor may order additional scans to get more detailed information about the tumor’s size, location, and characteristics^[2].

A computed tomography (CT) scan of the chest uses thin X-ray beams that rotate around your chest. A computer processes the information to create detailed three-dimensional images that can reveal a tumor. CT scans can also show if the tumor has spread and help estimate its extent^[2].

A positron emission tomography (PET) scan uses a harmless radioactive substance called a tracer to collect detailed images of body tissues. This highly sensitive technique can help identify cancer cells^[2].

An MRI scan (magnetic resonance imaging) uses a powerful magnet linked to a computer to create detailed pictures of the tissues and organs in your chest^[2].

If imaging tests reveal a tumor, your doctor may perform a biopsy. During this procedure, a small sample of tissue is removed and examined under a microscope. Sometimes this is done using a needle guided by CT images. Limited biopsies carry very small risks, including pneumomediastinum (air in the chest) or mediastinitis (inflammation), and an even lower risk of damaging the heart or large blood vessels^[3].

Stages of thymoma

Doctors use staging systems to describe how far a cancer has spread. The most widely used system for thymoma was developed by a doctor named Masaoka and later revised in 1994^[12]. This system stages thymoma based on how much the tumor has spread through its outer covering and into surrounding structures:

• Stage I: The tumor is completely enclosed within a capsule with no invasion visible under the microscope
• Stage II: The tumor has invaded through the capsule or into nearby fat tissue or the covering of the lungs (pleura), either visible to the eye or under the microscope
• Stage III: The tumor has invaded into surrounding structures such as the pericardium (heart sac), great vessels, or lung
• Stage IV: The tumor has spread to the lining of the lungs and heart (pleura and pericardium) or to distant parts of the body^[12]

The degree of tumor encapsulation and the extent of invasion into surrounding tissue relate directly to prognosis—how well you’re likely to do^[12].

Doctors also classify thymomas by how they look under the microscope. The World Health Organization system, introduced in 1999, divides thymomas into types A, AB, B1, B2, and B3, depending on the appearance of the cells^[3].

Treatment options

The treatment for thymoma depends on several factors, including the stage of the cancer, whether it can be completely removed with surgery, and your overall health. Surgery forms the cornerstone of treatment for early-stage disease^[4].

Surgery is the main treatment when thymoma can be completely removed. For patients requiring surgery, many centers now use minimally invasive techniques, including robot-assisted surgery and video-assisted thoracic surgery. These approaches use smaller incisions than traditional surgery, which can decrease complications and improve results^[1].

The surgical procedure to remove the thymus is called a thymectomy. Doctors typically perform this through either a cut down the center of the breastbone (median sternotomy) or through the neck (cervical approach)^[5]. The goal is to remove the entire tumor along with the thymus gland.

Radiation therapy uses high-energy beams to kill cancer cells. Evidence indicates that all thymomas, except those that are completely encapsulated Stage I tumors, benefit from radiation therapy given after surgery (adjuvant radiation therapy)^[5].

Chemotherapy uses drugs to kill cancer cells. For advanced or unresectable disease (tumors that cannot be removed with surgery), platinum-based chemotherapy is considered the standard treatment^[14]. For the best outcomes with advanced disease, doctors often recommend a multimodality approach that combines chemotherapy given before surgery (induction chemotherapy), surgical removal of the tumor, radiation therapy after surgery, and additional chemotherapy (consolidation chemotherapy)^[5].

If a tumor cannot be removed surgically, there remains some debate about whether biopsy or partial removal is better^[5]. For recurrent disease—cancer that comes back after treatment—surgical removal represents the most important factor for long-term survival^[12].

Clinical trials test new treatments to see if they work better than current options. Because thymomas are rare, patients should consider participating in clinical trials when available^[1].

Outlook and survival

The outlook for people with thymoma varies depending on whether the tumor is invasive (spreads beyond its capsule) and how far it has spread. Early-stage thymomas that are completely encapsulated have excellent cure rates, with over 95% of patients surviving long-term when properly treated^[1].

The 15-year survival rate is 47% for people with noninvasive thymoma, compared to 12.5% for those with invasive thymoma^[4]. Thymic carcinoma is more difficult to treat than thymoma^[2].

When thymoma leads to death, it usually occurs from complications involving the heart and breathing, such as cardiac tamponade (fluid around the heart that prevents it from beating properly) or other cardiorespiratory problems^[4].

Even when successfully treated, thymoma can come back many years later. Most recurrences appear as growths on the lining of the lungs (pleural recurrences)^[12].

Life after treatment and follow-up care

Recovering from thymoma and adjusting to life after treatment is different for each person. It depends on the stage of cancer you had, the treatments you received, and many other factors. Even after treatment ends, there may be other issues to address, such as coping with long-term side effects^[15].

One of the biggest concerns for people treated for thymoma is the worry that the cancer will come back. Thymomas can recur up to 20 years after treatment, which is why lifetime follow-up with your doctor is recommended^[15]. Living for such a long time with the possibility of recurrence can be very stressful, and regular doctor’s appointments may serve as constant reminders.

However, regular follow-up care increases the chance that any recurrence will be found earlier, when it’s easier to treat. You should report any signs or symptoms that might suggest the cancer has returned as soon as possible^[15].

People who have been diagnosed with thymoma are at greater risk of developing a second cancer. This risk appears similar whether you were treated with surgery or radiation therapy, or if you have a history of myasthenia gravis. The second cancers that can develop after thymoma include non-Hodgkin lymphoma and soft tissue sarcoma^[15].

Many centers offer support services to help with recovery and overall wellness, including support groups, nutrition education, and mental health care^[1]. If fear of cancer returning is preventing you from moving forward, you may want to talk with a counselor or close friend. You can also join a cancer survivor support group or online community, where talking with others who share the same fears may be helpful^[15].

Finding ways to cope with stress and improve your health can make a significant difference. Many people find comfort through meditation, prayer, imagery and relaxation techniques, physical activity, and eating a healthy diet^[15].