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Normal Anatomy
The anus is body's opening that leads into the lower end of
the intestines. The anal canal is the tube that connects the lower part
of the large intestine (rectum) to the outside of the body, allowing
feces (stool) to pass out through the anus during a bowel movement.
When digested food enters the large intestine (colon) from the small
bowel, feces are formed as the colon absorbs water and liquid from the
digested food. The feces is then stored in the rectum until it is
passed through the anus as a bowel movement.
The anal canal is about an inch and a half long. Its inner
lining (called the mucosa) is made up of several different kinds of
cells. Learning a little about these cells is helpful in understanding
the kinds of cancer that develop in various parts of the anal canal.
Glands and ducts (tubes leading from the glands) are found under the
mucosa. These "anal glands" make mucus, which acts as a lubricating
fluid.
The anal canal
is the tube that goes from the rectum to the anal margin (where
the canal meets the outside skin at the anus). About midway down the
anal canal is the dentate
line, which is where most of the glands empty into the
anus. Cells above the anal canal (in the rectum) are mainly shaped like
tiny columns, while most of those in the upper anal canal just above
the dentate line are shaped like cubes and are called transitional
cells. This area is called the transitional zone. Below the dentate
line are flat (squamous) cells. The anal margin (also called the anal
verge) is lined by squamous cells that merge with the skin just outside
the anus (called perianal
skin). The anal canal is surrounded by a sphincter, which
is a circular muscle that keeps feces from coming out until it is
relaxed during a bowel movement.
Anal Tumors
Many types of tumors can develop in the anus. Some of these tumors are
benign (non-cancerous). Others are cancerous, which means they can grow
into and damage nearby tissues and can spread to other parts of the
body. There are also some growths that start off as benign but over
time can develop into cancer. These are called pre-cancerous
conditions. This section discusses all of these types of abnormal
growths.
Benign (Non-cancerous) Anal
Tumors
Polyps: Polyps are small, bumpy, or
mushroom-like growths that develop in the mucosa or just under it.
There are several kinds, depending on their cause and location.
-
inflammatory polyps arise because of inflammation from
injury or infection.
- lymphoid
polyps develop in lymph tissue (part of the
immune system). Small nodules of lymph tissue are normally present
under the anal inner lining.
- skin tags are
benign growths of connective tissue that are
covered by a layer of squamous cells. Skin tags are also called
fibroepithelial polyps.
Condylomas: Condylomas (also called warts)
are growths that occur just outside the anus and in the lower anal
canal below the dentate line. Occasionally condylomas can be found just
above the dentate line. They are caused by infection with a human papilloma virus
(HPV).
People who have had condylomas are more likely to develop anal
cancer (see next section, "Potentially Pre-cancerous Anal
Conditions.").
Other tumors: In rare cases, benign tumors
can grow in other tissues of
the anus. These include:
- adnexal
tumors -- usually benign growths that start in hair
follicles
or sweat glands of the skin just outside of the anus. These tumors stay
in the perianal skin area and do not grow into the anal
region.
- leiomyomas --
develop from smooth muscle cells
- granular
cell tumors -- develop from nerve cells and are composed
of
cells that contain lots of tiny spots (granules)
- hemangiomas
-- start
in the lining cells of blood vessels
- lipomas
-- start from fat cells
- schwannomas --
develop from cells that cover nerves
Potentially Pre-cancerous Anal
Conditions
Sometimes changes in
the anal
mucosa are harmless in their early stages but may later develop into a
cancer. A common term for these potentially pre-cancerous conditions is
dysplasia.
Some warts, for example, contain areas of dysplasia that can
develop into cancer.
Dysplasia occurring in the anus is also known as
anal intraepithelial
neoplasia or AIN.
Anal intraepithelial neoplasia
is divided into low-grade and high-grade. Low-grade AIN is not likely
to be harmful. But many doctors feel that high-grade AIN may eventually
become cancer, and therefore advise that high-grade AIN be treated.
Cancerous Anal Tumors
Squamous cell carcinomas:
Most anal cancers in
the United States are squamous cell carcinomas. These tumors come from
the squamous cells that line the anal margin and most of the anal
canal. In its earliest stage, squamous cell carcinoma is known as
carcinoma in situ,
(pronounced "in SY-too"), or CIS. Another name for
this condition is Bowen
disease (also called Bowen's disease). The
cancerous cells of CIS are limited to the inner lining; they have not
spread into any other tissues. Cells of invasive squamous cell
carcinomas have already spread beyond the inner lining (mucosa).
Cloacogenic carcinomas
(also called basaloid or transitional cell
carcinomas) are sometimes listed as a subclass of squamous cell
cancers. They develop in the transitional zone, also called the cloaca.
While they look slightly different under the microscope, these cancers
behave and are treated like other squamous cell carcinomas.
Adenocarcinomas: A
small number of anal cancers are known as
adenocarcinomas, which develop in glands located under the anal mucosa
that release their secretions into the anal canal. They can also start
in apocrine glands (a type of sweat gland of the perianal skin). Most
often they arise from the rectum and are treated as rectal carcinomas.
For more information, see the American Cancer Society document,
Colorectal Cancer.
Paget disease (also called Paget's disease) is a type of apocrine gland
carcinoma that spreads through the surface layer of the skin. Paget
disease can affect skin anywhere in the body but most often affects
skin of the perianal area, vulva, or breast. This condition should not
be confused with Paget disease of the bone, which is a different
disease.
Basal cell carcinomas: Basal cell carcinomas are a type of skin cancer
that can develop in the perianal skin. These tumors are much more
common in areas of skin that are exposed to sun, such as the face and
hands, and account for only a small number of anal cancers. For more
information, see the American Cancer Society document, Nonmelanoma
Skin Cancer.
Malignant melanoma: This cancer develops from cells in the skin or anal
lining that make the brown pigment called melanin. Only about 1% to 2%
of anal cancers are melanomas. Melanomas are far more common on parts
of the body that are exposed to sun. If melanomas are found at an early
stage before they have invaded deeply into the skin or spread to lymph
nodes, the outlook for long-term survival is very good. But most anal
melanomas are found at a later stage because they are hard to see. For
more information, see the American Cancer Society document, Melanoma
Skin Cancer.
Gastrointestinal stromal tumors: These are rare anal
cancers that are much more commonly found in the stomach or small
intestine. For more information, see the American Cancer Society
document, Gastrointestinal Stromal Tumor.
Last Medical Review: 04/25/2007
Last Revised: 05/13/2009
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