Staging is a way of describing where the cancer is located, if or where it has spread, and whether it is affecting the functions of other organs in the body. Doctors use diagnostic tests to determine the cancer's stage, so staging may not be complete until all the tests are finished. Knowing the stage helps the doctor to decide what kind of treatment is best and can help predict a patient's prognosis (chance of recovery). There are different stage descriptions for different types of cancers.
The most commonly used staging system for thymoma is called the Masaoka system, and it classifies thymoma into the following stages:
Stage I: The cancer is limited to the thymus and the capsule that surrounds it.
Stage II: The cancer has spread into fat surrounding the thymus or into the lining of the lung next to the tumor, called the mediastinal pleura.
Stage III: The cancer has spread to other organs near the thymus, such as the lung, blood vessels, and the sac around the heart, called the pericardium.
Stage IVA: The cancer has spread more extensively into the lining of the lung or the sac around the heart.
Stage IVB: The cancer has spread to organs further away from the thymus, or has spread through the vessels carrying blood or lymph.
In addition to its stage, thymoma can be classified into different categories, developed by the World Health Organization (WHO), based on what the tumor cells look like under a microscope:
Type A thymoma. This is also called spindle cell thymoma or medullary thymoma. The chance of recovery for people with type A thymoma is good, with a 15-year relative survival rate (percentage of people who survive at least 15 years after the cancer is detected, excluding those who die from other diseases) near 100%.
Type AB thymoma. Also called mixed thymoma, type AB thymoma is similar to type A thymoma; however, type AB thymoma has lymphocytes in the tumor. The chance of recovery for people with type AB thymoma is also good, with a 15-year relative survival rate of approximately 90%.
Type B1 thymoma. This is also known as lymphocyte-rich thymoma, lymphocytic thymoma, predominantly cortical thymoma, and organoid thymoma. This type of thymoma has very many lymphocytes in the tumor, but the cells of the thymus appear normal. The chance of recovery for people with type B1 thymoma is also good, with a 20-year relative survival rate (percentage of people who survive at least 20 years after the cancer is detected, excluding those who die from other diseases) of approximately 90%.
Type B2 thymoma. Type B2 thymoma also has many lymphocytes, like type B1 thymoma; however, the thymus cells do not appear normal. Type B2 thymoma is also known as cortical thymoma and polygonal cell thymoma. The 20-year relative survival rate for people with type B2 thymoma is approximately 60%.
Type B3 thymoma. Type B3 thymoma is also known as epithelial thymoma, atypical thymoma, squamoid thymoma, and well-differentiated thymic carcinoma. This type of thymoma has few lymphocytes, and the thymus cells look abnormal. The 20-year relative survival rate is approximately 40%.
Thymic carcinoma. Thymic carcinoma, previously called type C thymoma, is more aggressive. Thymic carcinoma cells do not look like normal thymus cells, but like cancers in other organs of the body. This type of tumor is often advanced when diagnosed. It is sometimes classified into two categories: low grade, which has a better chance of recovery, and high grade, which is more likely to grow and spread quickly. Low-grade thymic carcinoma includes basaloid, mucoepidermoid, and well-differentiated squamous cell types. High-grade thymic carcinoma includes anaplastic/undifferentiated, clear cell, poorly differentiated squamous cell, sarcomatoid, and small cell/neuroendocrine types. The five-year relative survival rate of people with thymic carcinoma is 35%. The 10-year relative survival rate of people with thymic carcinoma is 28%.
Cancer survival statistics should be interpreted with caution. These estimates are based on data from thousands of cases of this type of cancer in the United States, but the actual risk for a particular individual may differ. It is not possible to tell a person how long he or she will live with thymoma.
Source: National Cancer Institute.