Thymus Cancer

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Thymus Cancer Stages

After someone is diagnosed with thymus cancer, doctors will try to figure out if it has spread. If it has spread, they look to see how far. This process is called staging. The stage of a cancer describes the extent of the cancer in the body. It helps determine how serious the cancer is and how best to treat it. Doctors also use a cancer's stage when talking about survival statistics.

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How is the cancer stage determined?

Thymus cancer ranges from stages I (1) through IV (4). The lower the number, the less the cancer has spread. And within a stage, an earlier letter means a lower stage.

Although each person’s cancer experience is unique, cancers with similar stages tend to have a similar outlook and are often treated in much the same way.

The TNM staging system

The staging system used for thymus cancer is the American Joint Committee on Cancer (AJCC) TNM system, which is based on 3 key pieces of information:

  • The extent (size) of the main (primary) tumor (T): Has the cancer grown into nearby structures. And if so, how far has it grown?
  • The spread to nearby lymph nodes (N): Has the cancer spread to nearby lymph nodes?
  • The spread (metastasis) to distant parts of the body (M): Has the cancer spread to the outer linings of the lungs or heart or to distant organs?

The staging system described below is the most recent AJCC system. This system is used to stage thymomas, thymic carcinomas, and thymic neuroendocrine tumors.

You may also see your cancer described using an older system called the Masaoka staging system (sometimes called Masaoka-Koga). This system is based mainly on whether the tumor has spread through its outer covering (capsule) and into nearby tissue, and if so, how far. Many doctors still use Masaoka staging alongside the AJCC system.

How your overall stage is determined

Numbers or letters after T, N, and M provide more details about each of these factors. Higher numbers mean the cancer is more advanced. Once a person’s T, N, and M categories have been determined, this information is combined in a process called stage grouping to assign an overall stage. For more information see Cancer Staging.

The staging system in the table below uses the pathologic stage. This is also called the surgical stage. It is determined by examining tissue removed during surgery. Sometimes, if surgery is not possible right away or at all, the cancer will be given a clinical stage instead. This is based on the results of a physical exam, biopsy, and imaging tests (CT or MRI scan, x-rays, or PET scan) done before surgery.

Cancer staging can be complex, so ask your doctor to explain it to you in a way you understand.

Stage grouping: T1a, N0, M0

  • The tumor has not spread outside the thymus OR it has grown into the nearby fatty tissues or into the mediastinal pleura (the thin layer covering the space between the lungs).
  • The tumor is no larger than 5 centimeters (cm) across (T1a).
  • The cancer has not spread to nearby lymph nodes (N0) or to distant parts of the body (M0).

Stage grouping: T1b, N0, M0

  • The tumor has not spread outside the thymus OR it has grown into the nearby fatty tissues or into the mediastinal pleura (the thin layer covering the space between the lungs).
  • The tumor is larger than 5 centimeters (cm) across (T1b).
  • The cancer has not spread to nearby lymph nodes (N0) or to distant parts of the body (M0).

Stage grouping: T2, N0, M0

  • The tumor has grown into the pericardium (the tissue sac containing the heart), into the lung, or into the phrenic nerve (the nerve that controls the diaphragm and breathing) (T2).
  • The cancer has not spread to nearby lymph nodes (N0) or to distant parts of the body (M0).

IIIA

Stage grouping: T3, N0, M0

  • The tumor is growing into other nearby tissues or organs, including the chest wall, the vessels carrying blood into or out of the lungs, the large vein that carries blood from the head and arms back to the heart (the brachiocephalic vein), or the main blood vessel taking blood back to the heart (the superior vena cava) (T3).
  • The cancer has not spread to nearby lymph nodes (N0) or to distant parts of the body (M0).

IIIB

Stage grouping: T4, N0, M0

  • The tumor is growing into nearby tissues or organs, including the trachea (windpipe), esophagus (swallowing tube), or the main blood vessels pumping blood away from the heart (T4).
  • The cancer has not spread to nearby lymph nodes (N0) or to distant parts of the body (M0).

Stage grouping: Any T, N1, M0

  • The tumor might or might not have grown into nearby tissues or organs (any T) AND the cancer has spread to nearby lymph nodes in the front chest cavity (N1).
  • It has not spread to distant parts of the body (M0).

IVA

Stage grouping: Any T, N0 or N1, M1a

  • The tumor might or might not have grown into nearby tissues or organs (any T) and the cancer might or might not have spread to nearby lymph nodes (N0 or N1).
  • The cancer has spread to the pleura (lining of the lung) or the pericardium (lining of the heart) (M1a).

IVB

Stage grouping: Any T, N2, M0 or M1a

  • The tumor might or might not have grown into nearby tissues or organs (any T) AND the cancer has spread to the lymph nodes deep in the chest or the neck (N2).
  • It might or might not have spread to the pleura (lining of the lung) or the pericardium (lining of the heart) (M0 or M1a).

Stage grouping: Any T, any N, M1b

  • The tumor might or might not have grown into nearby tissues or organs (any T).
  • The cancer might or might not have spread to nearby lymph nodes in the chest cavity or neck (any N), but it has spread to the inside of the lungs or to other distant organs (M1b).

* The following additional categories are not listed in the table above:

  • TX: Main tumor cannot be assessed due to lack of information.
  • T0: No evidence of a primary tumor.
  • NX: Regional lymph nodes cannot be assessed due to lack of information.

Other outlook factors

A person’s outlook (prognosis) after treatment of a thymus cancer depends largely on its stage. But other factors are also important, such as the type of thymus cancer and if surgery can remove the entire tumor.

Learn more about types of thymus cancer in What Is Thymus Cancer?

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Developed by the American Cancer Society medical and editorial content team with medical review and contribution by the American Society of Clinical Oncology (ASCO).

American Joint Committee on Cancer. Thymus. In: AJCC Cancer Staging System. Version 9. American College of Surgeons; 2024.

Fang W, Girard N, Cilento V, et al; Members of the Staging and Prognostic Factors Committee. The International Association for the Study of Lung Cancer Thymic Epithelial Tumors Staging Project: Proposals for the N and the M Components for the Forthcoming (Ninth) Edition of the TNM Classification of Malignant Tumors. J Thorac Oncol. 2024;19(1):52-70.

Klug M, Kirshenboim Z, Truong MT, et al. The Ninth Edition TNM Staging System for Thymic Epithelial Tumors: A Comprehensive Review. Radiographics. 2025;45(10):e250144.

Ruffini E, Fang W, Hsu CP, et al; Members of the Staging and Prognostic Factors Committee. The International Association for the Study of Lung Cancer Thymic Epithelial Tumors Staging Project: Proposal for a Stage Classification for the Forthcoming (Ninth) Edition of the TNM Classification of Malignant Tumors. J Thorac Oncol. 2024;19(2):198-211.

Last Revised: August 12, 2026

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