Most of us learn early on where our hearts and stomachs are. The thymus, however, may be less familiar.

This small gland in your upper chest plays an important role in developing your immune system. It can be affected in people with the autoimmune condition myasthenia gravis.

While not everyone with myasthenia gravis has a problem with their thymus, some people may develop a rare tumor in the gland called a thymoma. Doctors recommend people with myasthenia gravis get their thymus gland checked.

What Is a Thymoma?

A thymoma is a rare tumor that develops in the thymus. It grows from thymic epithelial cells on the thymus’s outer surface.

Doctors used to describe some thymomas as noncancerous, but today any thymoma is considered a form of cancer. Thymomas make up about 20 percent of tumors found in the middle of the chest, though they’re still rare overall.

In general, tumors of the thymus gland are incredibly uncommon. Thymomas in particular account for less than 0.5 percent of all cancer cases, most often affecting adults aged 40 to 75.

While these tumors are still rare, myasthenia gravis is present in 30 percent to 50 percent of those with a thymoma. Thymomas are also found in 10 percent to 20 percent of all people with myasthenia gravis.

Despite this, the connection between thymomas and myasthenia gravis is still not fully understood.

Multiple types of tumors can develop in the thymus. The two main types, which form from thymic epithelial cells, 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 myasthenia gravis.

Thymomas

Thymoma cells often look like normal thymus cells. They usually grow slowly and don’t spread much, but some types can be more aggressive.

Doctors divide thymomas into subtypes depending on what their cells look like under a microscope.

These subtypes can help predict how a thymoma is likely to grow and behave, though the tumor’s stage — how far a tumor has grown or spread — still matters more than its subtype alone.

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.

The outlook for thymic carcinoma is less favorable than thymoma because it tends to grow and spread more rapidly.

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.

For years, researchers have understood that the thymus is important to the development of myasthenia gravis.

The thymus normally shrinks with age, and much of its tissue is gradually replaced by fat, but some thymic tissue remains throughout adulthood. For people with myasthenia gravis, this normal shrinking process doesn’t always happen as expected.

Some people with myasthenia gravis develop thymic hyperplasia, an enlargement of the thymus (which is not considered an early stage of thymoma). Others develop a thymoma, and many have neither.

How a Thymoma Can Trigger an Autoimmune Response

The connection between a thymoma and myasthenia gravis goes beyond the two conditions sometimes occurring together.

An immune reaction known as autoimmune paraneoplastic disease can also occur. Myasthenia gravis is actually the most common autoimmune paraneoplastic disease linked with thymomas.

This reaction happens because a thymoma can disrupt the thymus’s normal job of filtering out immune cells that react against the body’s own tissues. As a result, immune cells and antibodies can escape into the body and mistakenly target healthy tissue. In myasthenia gravis, this disrupts the signals between nerves and muscles, which can lead to muscle weakness.

Everyone who has recently been diagnosed with myasthenia gravis should be evaluated for thymomas. The main imaging test doctors use to look for a thymic tumor is a contrast-enhanced CT scan of the chest.

They may also order:

These scans can provide helpful information in some cases, but they aren’t part of every evaluation.

Another diagnostic tool is examining a tissue sample, sometimes through a biopsy.

A needle biopsy or surgical biopsy before treatment is more likely if the tumor can’t be safely removed, if other treatments are being considered first, or if the diagnosis is uncertain.

However, if imaging strongly suggests a thymoma that can be completely removed, doctors may proceed directly to surgery and examine the tumor tissue after it’s removed.

If a person has myasthenia gravis and develops a thymus tumor, they’ll need different kinds of treatment than those used for managing myasthenia gravis symptoms.

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

After evaluating these factors, your healthcare provider may suggest surgery, radiation therapy, chemotherapy, or, in certain cases, targeted therapy or immunotherapy. Some of these treatments may be combined.

In rare cases, some small thymomas may be observed rather than immediately treated, particularly when the risks of treatment are significant.

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

Thymectomy may improve myasthenia gravis symptoms in people who have been previously diagnosed with myasthenia gravis, as well as those who develop myasthenia gravis as an autoimmune paraneoplastic disease.

That said, myasthenia gravis doesn’t always improve or go away completely after surgery, and some people continue to need myasthenia gravis medications afterward.

Even without a thymus tumor, doctors may recommend a thymectomy for some people with myasthenia gravis.

The strongest evidence of benefit is in people with generalized myasthenia gravis who test positive for acetylcholine receptor (AChR) antibodies. In this group, thymectomy may reduce symptoms, the need for medication, and hospitalizations over time.

With that said, the benefits of thymectomy haven’t been established equally for all forms of myasthenia gravis.

Thymoma and Myasthenia Gravis: Staying on Top of Your Care

Thymoma is rare, but its close connection with myasthenia gravis means healthcare providers often evaluate the two conditions together. With appropriate imaging, treatment, and follow-up care, many people can manage these conditions effectively.

A few small steps can help you stay on top of both conditions. If you have myasthenia gravis, ask your doctor whether you’ve already had chest imaging to check for a thymoma.

If you’ve had a thymectomy, it also helps to keep track of any changes in your myasthenia gravis symptoms and to keep taking your myasthenia gravis medications as prescribed unless your doctor tells you otherwise.

Your neurologist and surgical team can help you understand how these conditions affect you specifically. Bring any questions or concerns about thymoma or myasthenia gravis to your next appointment.