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| Detailed Guide: Thymus Cancer |
What Is Thymus Cancer? |
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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 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.
Revised: 11/10/2006
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