How Is Thymus Cancer Staged?

Staging is the process of finding out if and how far a cancer has spread. Your treatment and prognosis (the outlook for chances of survival) depend, to a large extent, on the cancer’s stage.

Masaoka staging system

There is no single staging system for thymomas that all doctors agree on, perhaps because these tumors are so uncommon. The system most often used to stage thymomas is the Masaoka system, although other systems exist. Staging in the Masaoka system is based on:

  • The extent of disease as seen on imaging tests such as CT or MRI scans
  • Whether the surgeon finds the tumor hard to separate from nearby tissues (indicating the tumor is invasive)
  • Whether the doctor sees tumor cells beyond the thymus when looking at the tumor sample under the microscope

The Masaoka system has 4 main stages.

Stage I

The thymoma is non-invasive. That is, it has not spread into the capsule (outer layer) of the thymus.

Stage II, which is divided into IIA and IIB

  • Stage IIA: The thymoma is growing into the capsule (the outer layer of tissue of the thymus).
  • Stage IIB: The tumor has grown through the capsule into the nearby fatty tissue, and may be stuck to the mediastinal pleura (the thin layer covering the space between the 2 lungs) or the pericardium (the tissue sac containing the heart).

Stage III

The thymoma is growing into nearby tissues or organs of the lower neck or upper chest area, including the pericardium (the tissue sac containing the heart), the lungs, or the main blood vessels going into or exiting from the heart (the superior vena cava and aorta).

Stage IV, which is divided into IVA and IVB

  • Stage IVA: The thymoma has spread widely throughout the pleura (lining of the lungs and chest wall) and/or pericardium.
  • Stage IVB: The thymoma has spread to distant organs. The most common sites of spread are bone, the liver, and the lungs.

Resectable versus unresectable cancer

The Masaoka staging system divides thymomas into different groups that help give doctors an idea about a person’s prognosis (outlook). But for treatment purposes, doctors often use a simpler system based on whether these cancers are likely to be resectable (where all visible tumor can be removed by surgery) or unresectable.

In general terms, almost all stage I and II thymomas, most stage III thymomas, and even some stage IV thymomas are potentially resectable, but there are exceptions. Resectability is based on whether the tumor appears to have grown into nearby tissues or spread to distant sites, as well as on whether or not a person is healthy enough to have surgery.

Surgery is typically part of the treatment plan whenever possible. In some cases, other forms of treatment such as radiation therapy or chemotherapy may be recommended as well.

Other prognostic factors

The prognosis (the outlook for chances of survival) after treatment of a thymoma depends to a large extent on its stage. But other features are important as well, such as its cellular classification (described in What Is Thymus Cancer?) and whether the surgeon is able to remove the entire tumor.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master’s-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: February 7, 2014 Last Revised: March 17, 2015

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