Staging is a way of describing a cancer, such as where it is located, where it has spread, and if 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.

There is no standard system for staging thymoma; however, the most commonly used system is called the Masaoka system. This system was developed in 1981, and divides thymoma into the following stages:

Stage I: The cancer is limited to the thymus and the sac that surrounds it.

Stage II: The cancer has spread into fat surrounding the thymus or into the mediastinal pleura (the lining of the lung adjacent to the tumor).

Stage III: The cancer has spread to other organs that are near the thymus, such as the lung, the sac around the heart, and the blood vessels.

Stage IVA: The cancer has spread more extensively into the sac surrounding the heart or the lining of the lung.

Stage IVB: The cancer has spread to distant organs, or has spread through the vessels carrying blood or lymph.

Noninvasive thymoma: Stage I thymoma is often referred to as noninvasive thymoma, when the tumor is limited to the thymus and has not invaded other tissues.

Invasive thymoma: Stage II through stage IVB thymoma is also called invasive thymoma, when the tumor has invaded other tissues.

Classification

In addition to its stage, thymoma can be classified into different categories based on what the cells look like under a microscope. The World Health Organization (WHO) has developed the following system for classifying thymoma:

Type A thymoma. This type accounts for approximately 4% to 7% of all cases of thymoma, and is also referred to as spindle cell thymoma or medullary thymoma. The prognosis (chance of recovery) for people with type A thymoma is good, with a 15-year relative survival rate (percentage of patients who survive at least 15 years after the cancer is detected, excluding those who die from other diseases) near 100%.

Type AB thymoma. Type AB thymoma, or mixed thymoma, accounts for approximately 28% to 34% of thymoma cases. Type AB thymoma is similar to type A; however, there are lymphocytes that are mixed in the tumor, and approximately 16% of cases are thought to be associated with myasthenia gravis. The prognosis for people with type AB thymoma is also good, with a 15-year relative survival rate of approximately 90%.

Type B1 thymoma. Type B1 thymoma accounts for approximately 9% to 20% of thymoma cases, and is also known as lymphocyte-rich thymoma, lymphocytic thymoma, predominantly cortical thymoma, and organoid thymoma. This type of thymoma has a high concentration of lymphocytes in the tumor, but the cells of the thymus appear normal. Approximately 57% of type B1 thymoma cases are thought to be associated with myasthenia gravis. The prognosis for people with type B1 thymoma is also good, with a 20-year relative survival rate (percentage of patients 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 has a high concentration of 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, and accounts for approximately 20% to 36% of all thymoma cases. About 71% of cases of type B2 thymoma are thought to be associated with myasthenia gravis. 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 thymoma. It accounts for approximately 10% to 14% of thymoma cases. This type of thymoma has few lymphocytes, and the thymus cells look close to normal. About 46% of type B3 thymoma cases are thought to be associated with myasthenia gravis, and the 20-year relative survival rate is approximately 40%.

Type C thymoma (or Thymic carcinoma). Type C thymoma is also referred to as thymic carcinoma and is very aggressive. The cells in thymic carcinoma do not look like normal thymus cells, but like cancers in other organs of the body. Thymic carcinoma may start from a pre-existing thymoma that has transformed into a more aggressive tumor. This type of thymoma is often advanced when diagnosed, and can be divided into two categories: low grade (better prognosis) and high grade (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. Most people with thymic carcinoma do not have associated myasthenia gravis. The five-year relative survival rate of people with thymic carcinoma is 38%. The 10-year relative survival rate of people with thymic carcinoma is 28%.