What Is Thymoma?

A type of neoplasm that develops in the outer layer of the thymus

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Thymoma is a tumor (growth) of the thymus gland. It may be benign or malignant (cancer). The thymus gland is part of the lymphatic system—the tissues that help produce, store, and transport white blood cells. The thymus helps white blood cells called T lymphocytes grow and multiply. It’s found in the chest behind the breastbone.

Thymomas are rare. There are only about 400 cases of thymomas per year in the United States. Thymic carcinoma accounts for 20% of all tumors in the thymus.

This article will review the types of thymoma and the symptoms of this cancer, its causes, risk factors, diagnosis, treatment, and prognosis. It will also cover how to cope with a diagnosis of thymoma. 

Healthcare provider pointing at an X-ray image on a tablet

krisanapong detraphiphat / Getty Images

Types of Thymoma

Thymoma is often lumped together with thymic carcinomas. They are both thymic epithelial tumors (TETs) that start from cells in the outer layer of the thymus. The two types of TET are different, though. Thymoma cells look normal, grow slowly, and usually stay in the thymus.

In thymic carcinoma, the cancer cells look abnormal and grow quickly. Thymic carcinoma is more likely to spread to other body parts and is more challenging to treat. They make up about 20% of TETs.

Thymomas are classified using a system developed by the World Health Organization (WHO). It assigns letters to the different types of thymomas.

Type A thymomas look the closest to normal thymus cells. It’s the rarest type but has the best prognosis. Overall survival is reported to be 100% at five and 10 years.

Type AB or mixed thymomas look like Type A thymomas but have areas of more abnormal cells. The overall survival is reported to be 80% to 100% at five and 10 years.

Type B is divided into three types.

  • Type B1 has mostly normal-looking thymus cells but contains a lot of white blood cells. Most are stage 1.
  • Type B2 has many white blood cells and big thymus cells with abnormal structures.
  • Type B3 has few white blood cells and is mostly normal thymus cells.

Type C is the more dangerous type of thymoma. It usually spreads with a high risk of coming back and a poor survival outlook. It is another term used for thymic carcinoma. Its thymus cells look very abnormal and have often grown into nearby tissues. These tumors may have spread to distant tissues and organs (metastasized) when found.

The most common types of thymoma are type AB and type B2. The least common is type A.

Thymoma Symptoms

Symptoms of thymoma are typically mild. Often, people with a thymoma don’t have symptoms. Many thymomas are found when someone is getting a scan for another reason.

Symptoms that do show up can include:

  • A lingering cough
  • Trouble breathing, including shortness of breath
  • Pain in the chest
  • Problems swallowing 
  • Loss of appetite
  • Weight loss 
  • Changes to the voice, including hoarseness
  • Swelling in the face, neck, upper body, or arms

If the tumor presses on a large blood vessel, it may produce these symptoms:

  • Enlarged veins in the face, neck, upper body, or arms
  • Headaches
  • A dizzy or light-headed feeling

Thymomas are associated with the development of autoimmune paraneoplastic syndromes. These diseases develop in people with cancer, where the immune system attacks the body’s normal cells. They include:

  • Myasthenia gravis: A muscle weakness that typically shows up first in the eyes. It may cause blurred or double vision and drooping eyelids. It is seen in 30% to 65% of people with thymomas but has many other underlying causes.
  • Thymoma-associated hypogammaglobulinemia (Good syndrome): About 5% to 10% of people with thymoma have this syndrome. It results in low levels of immune system proteins called antibodies. It may increase the risk of infection.
  • Thymoma-associated autoimmune pure red cell aplasia: In about 5% of people with thymomas, this syndrome leads to anemia (a low number of healthy red blood cells), which causes weakness, dizziness, shortness of breath, and tiring quickly.


Researchers don’t know much about what causes thymomas or other thymus cancers. Researchers haven’t found any specific inherited, environmental, or lifestyle risk factors that increase your risk of thymoma.

Thymomas are seen more often in middle-aged adults than in younger people and are most common in those in their 70s. In the United States, thymomas are diagnosed more often in Asians and Pacific Islanders than those of other ethnicities.


If a healthcare provider thinks you may have a thymoma, they’ll start with a thorough checkup, including a physical exam and a discussion of your medical and family history.

They’ll likely order a variety of imaging tests of the thymus. Thymomas are generally diagnosed based on how the thymus gland looks on these imaging tests.

Blood tests don’t necessarily help diagnose thymoma but can help screen for related autoimmune syndromes.

They may want to take a thymus sample to review in the lab (called a biopsy), but this isn’t always needed. They’re more likely to wait until surgery is performed for treatment.


Treatment for thymoma most often starts with surgery to remove the tumor. Typically, surgery removes the entire thymus. Parts of nearby organs or blood vessels may also be removed.

Many early-stage thymomas won’t need additional treatment. Larger thymomas may have spread to nearby tissues and may need other treatments to kill any cancer cells that were left, including:

If the tumor is too large or close to vital structures to remove, chemotherapy may be used before surgery to shrink it. If the tumor shrinks, chemo may be followed by surgery and more treatments.


Thymoma is slow growing and not likely to spread, so it typically has a good prognosis. The five-year overall survival rate is approximately 90% for thymoma.

For thymus cancers, the survival rates are a little worse. Overall the five-year survival rate for thymus cancers is 71%. The prognosis is better for thymus cancers caught early before they spread.

  • Cancer just in the thymus has a 95% five-year survival rate.
  • Thymus cancer that has spread to other areas of the chest or lymph nodes has a 79% five-year survival rate. 
  • Thymus cancers that have spread to far-flung areas of the body have a five-year survival rate of 40%.

Factors that can impact a thymoma prognosis include:

  • If cancer cells spread
  • If the surgeon can take the tumor out with surgery
  • If it is new cancer or has come back after being treated

While thymomas can often be successfully treated, they may come back much later. Cancer can reappear in the thymus or other parts of the body.

Thymomas are also linked to a higher risk of different types of cancer. Your healthcare provider will likely ask you to come back for regular follow-ups throughout your life.


A diagnosis of thymoma can be very upsetting. Symptoms from an associated autoimmune paraneoplastic syndrome it can affect your quality of life. Cancer that has already spread or can’t be removed with surgery may make you scared for your life, and more intensive treatment can cause stress and physical symptoms.

Bring any questions to your treatment team so you understand your choices in treatment, what side effects may occur, and your prognosis. Knowing your challenges can help prepare you and allow you to plan for additional support.

Your family and friends can be resources for emotional and physical support. It can be challenging to ask for support, but you can benefit from this help during diagnosis and treatment.

Even if you are symptom-free and your treatment is successful, you may have strong feelings, including joy, concern, relief, guilt, and fear.

Cancer can make you very anxious about your health and how to deal with everyday life. Returning for required follow-up visits may cause stress.

 While many thymomas can be treated with surgery alone, a thymoma can come back and require additional treatments. It’s normal to worry about this.

Ask your treatment team about online or in-person support groups for people with thymoma or similar conditions. This may also be a time to get the help of a mental health professional or counselor.


Thymoma is a rare neoplasm of the thymus gland. Thymomas are classified and lettered from best prognosis (A) to worst (C) based on how abnormal their cells look and how likely they are to spread. Some of them behave literally like benign tumors. Others have malignant behavior and tend to invade locally and spread at a distance.

Type A is the best prognosis but the least common. Type AB and type B2 are the most common. More common. Type B1, B2, and B3 thymomas are more likely to have a worse prognosis than Type A. Type C is also called thymic carcinoma and is the most dangerous thymoma. 

Thymoma symptoms include cough and hoarseness, breathing problems, chest pain, and swelling in the upper body. There are no known causes of thymomas. Instead, they are diagnosed with imaging scans. 

Surgery is the most common and sometimes only treatment needed for thymomas. Other treatment options include radiation, chemotherapy, targeted therapy, and clinical trials. 

Thymoma is slow growing and not likely to spread, so it typically has a good prognosis. The five-year overall survival rate is approximately 90% for thymoma. 

A Word From Verywell

A thymoma diagnosis may be frightening as you may not know how dangerous or malignant your tumor is until surgery. Talk to your care team about your prognosis and treatment for your symptoms. While the prognosis and survival rates are suitable for many types of thymoma, seek support during this challenging time.

19 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Jennifer Welsh
Jennifer Welsh is a Connecticut-based science writer and editor with over ten years of experience under her belt. She’s previously worked and written for WIRED Science, The Scientist, Discover Magazine, LiveScience, and Business Insider.