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Thymus cancers are uncommon cancers that start in the thymus.
This small organ is located just behind the breast bone (sternum) in
the front part of the mediastinum, the space in the chest between the
lungs. The thymus sits just in front of and above the heart.
The thymus is divided into 2 halves, called lobes. It has an
irregular shape and a surface that is made up of many small bumps
called lobules. The thymus has 3 main layers:
- The medulla
is the innermost part of the thymus.
- The cortex is
the layer surrounding the medulla.
- The capsule
is the thin covering over the outside of the thymus.
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.
The thymus is an important part of the body's immune system.
During fetal development and childhood, the thymus is involved in the
production and maturation of T lymphocytes (also known as T cells), a
type of white blood cell. 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 different types of cells, each of which
can develop into different types of cancer:
Epithelial cells
give the thymus its structure and shape. They can give rise to thymomas
and thymic carcinomas, which are the main focus of the rest of this
document.
Lymphocytes
make up most of the rest of the thymus. Whether in the thymus or in
other parts of the body, these immunes system cells can develop into
cancers called Hodgkin disease and non-Hodgkin lymphomas, which are
described in other documents from the American Cancer Society.
Kulchitsky cells,
or neuroendocrine cells, are much less common cells that normally
release certain hormones. These cells can give rise to cancers called
carcinoid tumors. Much of the information in the American Cancer
Society documents Lung Carcinoid Tumor
and Gastrointestinal Carcinoid Tumors
also applies to carcinoids of the thymus.
Doctors can tell the different thymic cancers apart by how
they look under the microscope and by the results of other lab tests
done on tissue samples.
Thymomas and thymic carcinomas
Thymomas and thymic carcinomas are tumors that start from
thymic epithelial cells. Not all doctors agree about the best way to
describe and classify these tumors. In the past, thymomas were
sometimes divided into benign (non-cancerous) thymomas and malignant
(cancerous) thymomas, based on whether they had invaded 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 how likely they are to come back after treatment is to describe
whether they have invaded beyond the thymus (and if so, how far). This
is done 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. The system
used to describe the stage (extent) of thymomas is discussed in the
section "How
is thymus cancer staged?"
WHO classification system for thymomas
Most doctors also classify thymomas based on how they look
under a microscope. The system used for this classification, which was
developed by the World Health Organization (WHO), assigns letters to
the different types of thymomas.
Type A:
The cells in these tumors are spindle-shaped or oval epithelial cells
that appear to be fairly normal looking. Around 5% of thymomas are of
this type. The outlook for people with this kind of thymoma is quite
good and almost all are cured.
Type AB: This
type, also known as a mixed thymoma, looks like type A except that
there are also areas of lymphocytes mixed in the tumor. About 1 out of
3 thymomas are of this type. The outlook for people with this type is
also quite good, with at least 9 out of 10 people being cured.
Type B1:
This type looks a lot like the normal structure of the thymus. It has a
lot of lymphocytes along with normal-appearing thymus cells. About 10%
to 20% are of this type. It has a very good outlook, with about 9 out
of 10 people being cured.
Type B2: This
type also has a lot of lymphocytes, but the thymus epithelial cells are
larger with abnormal nuclei (the DNA-containing part of the cell).
About 20% to 35% are of this type. About 6 out of 10 patients with this
type are cured.
Type B3: This
type has few lymphocytes and mostly consists of thymus epithelial cells
that look pretty close to normal. About 10% to 15% of thymomas are this
type. The outlook for this type is somewhat less favorable than for B2
thymomas, with about 4 out of 10 patients cured.
Type C: This
is the most dangerous form and is also known as thymic carcinoma.
It contains cells that have a very abnormal appearance under the
microscope. The cells may no longer even look like thymus cells. These
tumors have often invaded nearby tissues and/or metastasized (spread to
distant tissues and organs) at the time they are found. Around 1 out of
4 people with thymic carcinoma survive at least 10 years after being
diagnosed.
Although we have listed survival statistics for these
different thymus cancer types, most doctors feel that the stage (extent
of growth and spread) of the thymus cancer is the best predictor of a
person's outcome. Survival statistics based on the stage of the cancer
are discussed in the section "How
is thymus cancer staged?"
Last Medical Review: 05/18/2009 Last Revised: 05/18/2009
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