Thymus Cancer
The thymus is a small organ located in the upper/front portion of your chest, extending from the base of the throat to the front of the heart. The thymus is composed of an inner medulla surrounded by an outer layer called the cortex, which in turn is surrounded by a thin covering called the capsule. The thymus reaches its maximum weight of about 1 ounce during puberty, then slowly decreases in size during adulthood as it is gradually replaced by fat tissue.
During fetal development and childhood, the thymus is involved in the production and maturation of T-lymphocytes, a type of white blood cell important in the immune system. T-lymphocytes develop in the thymus and then travel to lymph nodes (bean-sized collections of immune system cells) throughout the body. There they help the immune system protect the body from viruses, fungus, and other types of infections.
The thymus contains 2 main types of cells, thymic epithelial cells and lymphocytes. Thymic epithelial cells can give rise to thymomas and thymic carcinomas. Lymphocytes, whether in the thymus or in the lymph nodes, can become malignant and develop into cancers called Hodgkin disease and non-Hodgkin lymphomas. More Information about these cancers is available in other documents from the American Cancer Society.
The thymus also contains another much less common type of cells called Kulchitsky cells, or neuroendocrine cells, which normally release certain hormones. These cells can give rise to cancers, called carcinoids or carcinoid tumors that often release the same type of hormones and are similar to other tumors arising from neuroendocrine cells elsewhere in the body. There are American Cancer Society documents on carcinoid tumors of the lungs ("Lung Carcinoid Tumor") and the digestive system ("Gastrointestinal Carcinoid Tumors") , the two most common locations for carcinoid tumors. Much of the information in these documents also applies to carcinoids of the thymus.
Doctors can tell all these different thymic cancers apart by how they look under the microscope and by the results of other lab tests done on tissue samples.
Doctors disagree about the best way to describe and classify thymomas. In the past, thymomas were sometimes divided into benign thymomas (not cancerous) and malignant thymomas (considered cancerous because of evidence they had invaded or penetrated beyond the thymus into other tissues or organs). The view now held by most doctors is that all thymomas are potentially cancerous, and the best way to predict whether they can be cured or how likely they are to come back and spread after treatment is to describe whether they have invaded beyond the thymus and, if so, how far. The extent of invasion is identified by the surgeon who notes whether or not the thymus gland tumor appears attached to nearby organs and by the pathologist who looks at samples from the margins (edges) of the tumor under the microscope to see whether thymoma cells have grown into the capsule and spread to other tissues or organs.
Thymomas are classified by a system recently developed by the World Health Organization (WHO). This system assigns a letter grade to the thymomas based on their appearance under the microscope. These are:
Type A: The cells are spindle-shaped or oval epithelial cells and do not appear very malignant. Around 5% of thymomas are this type. The outlook for people with this kind of thymoma is quite good and most are cured.
Type AB: This type looks like type A except in addition there are lymphocytes mixed in the tumor. Nearly one third of thymomas are this type. The outlook for people with this type is also quite good. Most people are cured.
Type B1: This type is recognized because it has a lot of lymphocytes, the cells responsible for immunity along with normal-appearing thymus cells. It has a very good outlook. About 90% of people with this type are cured. About 10% to 20% are this type.
Type B2: This type also has a lot of lymphocytes, but the thymus epithelial cells do not appear normal. Instead, they are larger with abnormal nuclei (the DNA-containing part of the cell). This has a less favorable outlook, with about 60 to 70% of patients cured. About 20% to 35% are this type.
Type B3: This type has few lymphocytes and mostly consists of thymus epithelial cells that look pretty close to normal. The outlook for this type is somewhat less favorable than for B2 thymomas, with only about 40 to 60% of patients cured. About 10% to 15% of thymomas are this type.
Type C: This is the most dangerous form and is also known as thymic carcinoma. It is the most likely kind of thymus tumor to invade into surrounding tissues and spread. It has a very different appearance from normal thymus tissue. Thymic carcinomas contain cells that have a very abnormal appearance that can be recognized under the microscope. These cells usually invade nearby tissues and can often metastasize (spread to distant tissues and organs).
Thymic carcinomas are further divided into low-grade (better prognosis) and high-grade (worse prognosis, that is, more likely to grow and spread quickly) categories. Around 25% of people with thymic carcinoma are cured.
Low-grade thymic carcinomas include well-differentiated squamous cell, mucoepidermoid, and basaloid types. High-grade thymic carcinomas include poorly differentiated squamous cell, small cell/neuroendocrine, clear cell, anaplastic/undifferentiated, and sarcomatoid types.
Although we have listed survival statistics listed for these different thymus cancer types, most doctors feel that the stage of the thymus cancer is the best predictor of a person’s outcome.
Information Obtained From National Institute Of Health