Thymoma staging can help doctors determine your best treatment options. It may also provide insight into your outlook.

Thymomas are cancerous tumors that grow in your thymus — a gland in your chest that helps make immune system cells.

Thymomas and thymic carcinomas are types of thymus cancer. But unlike thymic carcinomas, thymomas grow slowly and are unlikely to spread beyond your thymus.

When diagnosing thymomas, doctors use a thymoma staging system to help guide treatment. The most common staging system is the TNM system, but some doctors may use an alternative staging system.

Read on to learn more about these staging systems and what the stages mean for your symptoms, treatment, and outlook.

Doctors group thymoma into four main stages, 1 through 4, with the help of the TNM system:

  • Tumor size: How far has the thymoma grown into the tissues of the thymus and any body structures near the thymus?
  • Nodes: Has the thymoma spread to any lymph nodes? If so, has it grown into shallow or deep lymph nodes?
  • Metastasis: Has the thymoma spread to organs far beyond the thymus, like the lungs or heart?

Numbers and letters after T, N, and M indicate more advanced disease. For example, a T2 tumor is more invasive than a T1 tumor. N1 indicates that cancer has spread to lymph nodes in the front of your chest, but N2 means they have spread to nodes deep in your chest.

Your thymoma stage will depend on your TNM findings. Because thymomas don’t usually spread beyond the thymus, any N or M findings typically suggest an advanced (stage 4) thymoma.

In stage 1, the thymoma is still contained to the sac around the thymus. It hasn’t grown very deeply into the thymus or nearby tissues.

At this stage, doctors can perform a surgery called a thymectomy to remove the thymus gland and any affected tissue. This helps make sure the thymoma doesn’t grow into any nearby tissues or organs.

According to 2021 research, the median survival time for people with stage 1 thymoma is 166 months (about 14 years). This means that half of the people with stage 1 lived longer, and half lived less than 166 months.

The Canadian Cancer Society notes that about 3 in 4 people with stage 1 thymoma live at least 5 years after diagnosis.

At stage 2, the thymoma has grown beyond the tissues of the thymus and thymus capsule.

It may grow into the mediastinal pleura — a layer of tissue between the lungs — or into the pericardium that surrounds your heart and keeps it in place.

You may notice chest pain or pressure as well as symptoms like:

Doctors may perform a thymectomy at this stage, too. They may also recommend chemotherapy or radiation therapy to kill any remaining cancer cells.

The 5-year survival rates for stage 2 thymoma are similar to rates for stage 1. However, the median survival time is lower — 107 months (about 9 years).

Stage 3 thymomas are divided into two categories:

  • Stage 3A: The thymoma has grown beyond the thymus into the lungs, blood vessels around the lungs and heart, and nerves in your diaphragm. But it still hasn’t spread to any lymph nodes or organs that aren’t in the area around the thymus.
  • Stage 3B: The thymoma has grown into your windpipe, esophagus, or blood vessels around the heart. But it hasn’t spread to lymph nodes or organs beyond this area.

You may start to notice new symptoms at this stage, including:

  • having trouble seeing
  • face or arm swelling
  • drooping eyelids
  • feeling fatigued
  • feeling dizzy

In stage 3, a thymectomy can still help stop the spread of cancerous cells. But radiation and chemotherapy are more critical in helping kill cancer cells that have spread into lung or heart tissue.

Around 64% of people with stage 3 thymomas live at least 5 years after diagnosis. The median survival time is 108 months (9 years) for people with stage 3A thymoma but only 22 months for people with stage 3B.

Stage 4 thymoma is also divided into two categories:

  • Stage 4A: The thymoma has grown into nearby lymph nodes and the linings of the heart and lungs.
  • Stage 4B: The thymoma has grown into the heart, lungs, or other organs. It may have also spread into neck tissue and lymph nodes far beyond the thymus.

Symptoms may be very noticeable at stage 4. Treatment may include a thymectomy in stage 4A but not stage 4B. You’ll need radiation and chemotherapy to remove cancer cells that have spread beyond the thymus.

The 5-year survival rate for people with stage 4 thymoma is 45%.

Before the introduction of the TNM system, doctors traditionally used the Masaoka-Koga system. It uses slightly different stage definitions:

1Contained in the thymus capsule
2AGrown into or outside the thymus capsule
2BGrowing into thymus and mediastinal pleura or pericardium
3Growing into the pericardium, blood vessels, or lung tissue
4AMetastasized into tissues around the lungs and heart
4BMetastasized into nearby lymph nodes or distant organs

The World Health Organization (WHO) uses a complex system of codes that allows doctors to identify the origin, location, and impact of a thymoma.

Depending on the features of the tumor cells, doctors may classify thymoma as:

  • type A
  • type AB
  • type B1, B2, or B3
  • micronodular thymoma with lymphoid stroma
  • metaplastic thymoma

Some tests doctors use to stage thymoma include:

Lower stages typically mean a more positive outlook. Some research suggests that people with stage 4 thymomas tend to live longer than people with stage 3B thymomas. This may be due to where the tumor has spread but also a low number of study participants.

Your outlook also depends on whether doctors can remove the thymoma. When doctors remove the entire thymus, 64–80% of people survive for at least 5 years.

The following table summarizes some of the research regarding survival rates and life expectancy:

Stage5-year survival rateTNM stage median survival time (months)MK stage median survival time (months)
273%107A: 166
B: 58
365%A: 108
B: 22

A thymoma may return after treatment. Research from 2022 suggests that recurrence rates are higher for thymomas in later stages, particularly stage 3B.

Your overall outlook is not as good if the thymoma returns within 40 months of treatment.

Thymus cancer has a high survival rate if caught and treated early. You may catch thymomas earlier by getting regular physicals and letting your doctor know about symptoms like chest pain or a cough that doesn’t go away on its own.

Thymus cancer is rare. You may want to join a clinical trial to help researchers better understand thymus cancer and develop more effective treatments for it.