Most of us learn early on where our hearts and stomachs are in our bodies. But it’s understandable if you only heard about the thymus after you or a loved one was diagnosed with myasthenia gravis (MG).

Thymoma and thymic carcinoma are rare cancers that begin in the thymus, a small gland in the upper chest that helps develop the body’s immune system. Issues with the thymus may be closely connected to MG.

In this article, we review the main facts you need to know about thymus tumors in MG.

1. Thymus Tumors Are Very Rare

In general, tumors of the thymus gland are incredibly uncommon. Thymomas in particular only account for about 0.5 percent of all cancer cases, most often affecting adults aged 40 to 75. They’re equally common in all people regardless of gender.

However, MG is present in 30 percent to 50 percent of those with thymoma, and thymomas are found in 10 percent to 20 percent of all people with MG, suggesting that the connection between these conditions is still not fully understood.

Nevertheless, tumors of the thymus gland can still be considered rare: only about 400 people in the United States are diagnosed with thymoma and thymic carcinoma each year.

Researchers do not know why some people develop thymus tumors. According to Cleveland Clinic, being of Asian or Pacific Islander descent may increase your likelihood of having the condition, though there is conflicting evidence on this point.

2. There Are Different Types of Thymus Tumors

There are multiple types of tumors that can develop in the thymus. The two main types, in which cancer cells form in the outer layer of the thymus, are thymomas and thymic carcinomas. These are sometimes referred to as thymic epithelial tumors, or TETs. Other types of tumors, such as germ cell tumors or lymphomas, may also form in the thymus. However, these are not specifically associated with MG.

Thymomas

Thymomas are a type of cancer. They start in the thymus and often look like normal thymus cells. They usually grow slowly and don’t spread much, but some types can be more aggressive.

Although researchers used to divide them into benign and malignant (noncancerous and cancerous) types, doctors now consider all thymomas to be a type of cancer. Today, doctors divide thymomas into subtypes depending on what their cells look like under the microscope.

Thymic Carcinomas

In cases of thymic carcinoma, the abnormal cancer cells grow more rapidly and are more likely to spread, making this type of thymus tumor more difficult to treat and more likely to come back.

Unfortunately, even though both of these tumors originate from the same cells, the outlook for people with thymic carcinomas tends to be worse than the outlook for people with thymomas. The good news is that thymic carcinomas are found much less frequently than thymomas: they occur in only 1 in 5 cases of all TETs.

3. The Connection Between Thymus Tumors and MG Goes Both Ways

For many years, researchers have understood that the thymus is important to the development of MG. In healthy people without MG, the thymus gland becomes smaller and smaller during the teen years, and it is almost undetectable in adulthood.

However, in people with MG, the thymus remains the same size for the rest of their lives. This is thought to potentially lead to the development of thymomas, which makes them more common in people with MG.

Autoimmune paraneoplastic diseases occur in people with certain types of cancer, but they are not caused by the cancer itself. Instead, these diseases develop when the immune system attacks healthy cells along with the cancer cells. MG is the most common autoimmune paraneoplastic disease linked with thymomas.

Sometimes, developing MG symptoms can be the only sign that someone has a thymoma. These symptoms may include:

• Muscle weakness
• Drooping eyelids (ptosis)
• Blurry or double vision
• Altered facial expressions
• Difficulty breathing and talking
• Difficulty swallowing and chewing

4. At First, Thymus Tumors May Not Cause Any Symptoms

In the early stages of thymoma or thymic carcinoma, people usually do not notice any symptoms. Sometimes, a thymoma can be an incidental (unexpected) finding on an imaging study, such as a chest X-ray that’s been taken for other reasons.

Thymoma symptoms often arise only when the tumor grows larger in size and starts affecting the surrounding organs. Then, one may notice:

• Chest pain
• Shortness of breath
• Voice changes (e.g., hoarseness)
• A cough that doesn’t go away
• Swelling in the upper neck and face

5. Diagnosing Thymus Tumors Requires Additional Testing

Everyone who has recently been diagnosed with MG should be evaluated for thymomas using a CT scan or an MRI of the chest. A positron emission tomography (PET) scan can tell if the cancer has spread to other areas of the body.

Healthcare providers might be able to see the difference between thymic cysts and thymic carcinomas by looking at an MRI of the chest. However, they need a tissue-sample biopsy to confirm what type of tumor it is with certainty. They may take a sample with a needle or during surgery, if the tumor can be removed.

6. All Thymus Tumors Require Treatment

If a person has MG and develops a thymus tumor, they are going to require different kinds of treatment than those used for managing MG symptoms.

Treatment options for thymus tumors are affected by many factors, including:

• Its type (thymoma or thymic carcinoma)
• Its localization (whether the cancer is local or it has spread)
• Its characteristics (which will help doctors determine if it can be removed by surgery)
• Its status (whether the cancer has just been diagnosed or has come back)

After evaluating all of these factors, your healthcare provider may suggest surgery, chemotherapy, radiation therapy, hormone therapy, or targeted therapy. Some of these treatments may be combined.

7. Thymectomy May Alleviate MG Symptoms

In the early stages of thymoma and thymic carcinoma, doctors often suggest a thymectomy (surgical removal of the thymus).

Interestingly, this can lead to a significant relief from MG symptoms both in people who have been previously diagnosed with MG and in those who develop MG as an autoimmune paraneoplastic disease.

Even if a person with MG doesn’t have a thymus tumor, doctors may recommend a thymectomy. It’s been shown that the surgical removal of the thymus in those with MG may lead to significant improvements in MG symptoms, and, in some cases, even remission (complete disappearance of the symptoms).

Talk With Others Who Understand

MGteam is a social network for people with myasthenia gravis and their loved ones. On MGteam, members come together to ask questions, give advice, and share their stories with others who understand life with MG.

Have you been diagnosed with a thymus tumor? Did you notice any symptoms before the diagnosis? Perhaps you’ve undergone a thymectomy? Share your experience in the comments below, or start a conversation by posting on your Activities page.