Thymoma is a type of thymus cancer. The thymus is an organ within the chest. Thymoma can cause symptoms such as chest pain, coughing, and shortness of breath. Treatment may involve surgery, radiation therapy, or chemotherapy.

This article provides a detailed overview of thymoma. It looks at causes, symptoms, diagnosis, and treatment and discusses the outlook for this condition.

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The thymus gland is a small organ inside the chest. It is part of the lymphatic system. According to the National Cancer Institute (NCI), growths of cancer cells in the outermost layer of the thymus are called thymic epithelial tumors (TETs).

The two main forms of TETs are:

  • Thymoma: Thymoma cancer cells look similar to regular thymus cells. They grow slowly and rarely spread outside the thymus.
  • Thymic carcinoma: These cancer cells look very dissimilar to regular thymus cells. They grow more quickly than thymoma cells and have more potential to spread beyond the thymus.

Around 1 in 5 TETs are thymic carcinomas.

Learn more about thymic carcinoma.

Medical experts do not know what causes TETs.

All cancers occur when cells develop abnormally. They might grow too quickly, multiply too easily, or live too long. Abnormal cell development stems from genetic changes.

Some people are born with cancer-causing genetic changes. Others acquire them during their lives. Doctors know what prompts cancer-causing genetic changes for some cancers but not for TETs.

Scientists are uncertain about the risk factors for TETs.

According to the American Cancer Society (ACS), research has not revealed any lifestyle or environmental risk factors. Many demographic factors, such as gender, also seem irrelevant to the risk of TETs. There are only two known risk factors:

  • Age: TETs become more common as people get older. These cancers are most common in people in their seventies.
  • Ethnicity: In the United States, TETs are most common in people of Asian and Pacific Islander descent and least common in white and Latino people.

TETs are often asymptomatic. Any symptoms a person experiences often reflect the TET’s location. Symptoms of thymoma and thymic carcinoma include:

People with thymoma may also experience paraneoplastic syndromes such as muscle weakening. These syndromes occur when the immune system reacts to cancer and kills both cancer cells and healthy cells.

Myasthenia gravis (MG) is the most common paraneoplastic syndrome linked with thymoma. An estimated 30–40% of people with thymoma experience symptoms of MG, including:

An individual with these symptoms of thymoma should seek medical advice.

Learn more about MG.

Doctors use various tests to diagnose thymoma or thymic carcinoma. The two main types of diagnostic tests for TETs are imaging tests and biopsies.

Imaging tests produce images of a person’s thymus and surrounding areas. If these tests show any abnormal growth, healthcare professionals will recommend further tests. Imaging tests include:

Biopsies involve surgically removing samples of suspicious-looking thymus tissue for testing in a laboratory. Thymus biopsies are necessary for doctors to diagnose a TET confidently.

However, a biopsy may be unnecessary if imaging tests clearly show a thymoma. Instead, doctors may recommend removing all the suspicious-looking tissue for analysis in a laboratory.

Learn more about biopsy.

According to the NCI, thymoma has four main stages:

  • Stage 1: All cancer cells are within the capsule, or sac, surrounding the thymus.
  • Stage 2: Some cancer cells have spread beyond the capsule. They might be in the fat near the thymus or the lining of the chest cavity.
  • Stage 3: The cancer cells have spread to nearby structures such as the lungs, the large blood vessels, or the sac surrounding the heart.
  • Stage 4: This stage has two substages. In stage 4A, the cancer cells have dispersed widely throughout the heart or lungs. In stage 4B, the cancer cells have spread into the blood or the lymphatic system.

Determining whether a TET is resectable is an important factor in treatment decisions. If a TET is resectable, doctors can surgically remove it.

Almost all stage 1 and 2 TETs and most stage 3 TETs are resectable. Most stage 4 TETs are unresectable. Other TETs may be unresectable because they are too close to vital organs or because the individual cannot undergo surgery.

Doctors may recommend the following treatments for TETs:

  • Resectable TETs: Doctors will recommend surgical removal of the thymus and possibly some surrounding tissues. Especially for later resectable TETs, doctors may recommend other treatments after surgery, such as radiation therapy and chemotherapy.
  • Unresectable TETs: Doctors may recommend removing some cancer tissues surgically. This is “debulking” surgery. However, with or without debulking surgery, radiation therapy and chemotherapy are the primary forms of treatment.

Scientists continue to research treatments for TETs. An individual with thymoma or thymic carcinoma may want to consider taking part in a clinical trial.

Learn about chemotherapy and radiation therapy.

TETs are slow-growing cancers, so doctors have more time to treat them. In theory, this should mean a more positive outlook.

According to the ACS, the overall 5-year relative survival rate for thymus cancer is 72%. This means a person with this cancer is 72% as likely to live for 5 years after diagnosis as someone without cancer.

However, this figure varies with cancer staging. For instance, for TETs localized within the thymus, the 5-year relative survival rate is 94%. However, if a TET has spread to distant organs, the rate drops to 38%.

Thymic carcinoma tends to have a worse outlook than thymoma. This is because it is harder to treat and because doctors tend to diagnose it later.

TETs are thymus cancers originating in the outer layer of thymus cells. Thymoma and thymic carcinoma are the two main types of TETs. Thymoma occurs when thymus cells begin to grow abnormally quickly.

Thymoma can cause various symptoms, usually affecting the organs in the chest. It may also cause paraneoplastic symptoms, including muscle weakening.

Doctors may recommend imaging tests or a biopsy to help diagnose thymoma. Treatment often involves surgery to remove tumors from the thymus and surrounding areas. Radiation therapy and chemotherapy may also help.