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Overview
Cancer is predicted to become
the leading cause of death in the United States in this decade (surpassing heart
disease). Colon and rectal cancer is second only to lung cancer as the most
common cause of cancer deaths in the United States. Over 55,000 people die each
year from colon and rectal cancer. This is almost as many people as the total
number of Americans who died during the entire Vietnam conflict. Sadly
colorectal cancer is a preventable disease. It usually begins as a benign polyp
that can be removed before it becomes cancerous.
What is a polyp?
A polyp is a benign
growth on the inside of the colon or rectum. It typically looks like a little
mushroom sprouting from the wall of the bowel. Most polyps start off as small
bumps, about the size of the end of a pen. With time about 10% grow larger and
become cancerous. It is estimated to take about 5 years for a polyp to become a
cancer.
How does cancer develop?
Polyps and
cancer develop when there are mutations or errors in the genetic code that
controls the growth and repair of the cells lining the large intestine (another
name for the colon and rectum). It is an accumulation of these serial genetic
changes in the lining tissue, which allows normal tissue to become a polyp,
which can progress to a precancerous polyp (called a dysplastic polyp), and
finally a cancer. It is extremely rare for a polyp to develop or become
cancerous in the small intestine. Over the last 30 years, our understanding of
how the genetic alterations culminate in cancer has progressed rapidly, though
the complete process is not fully understood.
There are two common types of polyps in the large intestine.
- Hyperplastic polyps
These are small, benign polyps that do not carry a risk of developing
into cancer.
- Adenomas
These are also benign polyps, but some enlarge and become cancerous over
time.
Why do polyps develop?
It is thought
that colon and rectal cancer develop due to a variety of reasons. Some people
inherit a defect in their core DNA, which comes from their parent's egg or the
sperm at fertilization (this defect is also referred as a germline mutation). It
is estimated that up to 13% of colorectal cancers are from this type an
inherited abnormality. Two syndromes of inherited colorectal cancers are
familial adenomatous polyposis (where hundreds of polyps cover the colon) and
hereditary nonpolyposis colorectal cancer syndrome (also called HNPCC). In
HNPCC, the cancers tend to occur at a young age, across generations (such as
colorectal cancer in a mother's mother, the mother, and then the son), and may
be associated with other cancers (such as uterine, ovarian, stomach, esophagus,
and others).
The majority of colorectal cancers are classified as sporadic meaning that
the germline mutation is not present and environmental factors probably play a
more important role. These people may still inherit a mild abnormality, which
gives them a genetic predisposition to develop colorectal cancer, but a polyp
only forms after stimulation from an environmental factor.
Diet has been blamed as one of the most important environmental factors
leading to colorectal cancer. Diets, which are high in fat and low in fiber
(such as diets of Western cultures), seem to promote a high rate of polyps to
form. The exact reason why is not clear. Interestingly, areas of the world whose
diet is low in fat and meat and high in fiber have very low rates of colorectal
cancer.
Another group of people who are at an increased risk for developing
colorectal cancer, are those with a long history of inflammatory bowel disease (Crohn's
disease or ulcerative colitis) of the colon. This group probably accounts for 1%
of people who get colorectal cancer. It is believed that the chronic irritation
and inflammation over stimulate the colorectal cells until a mistake occurs in
the reproduction of these cells. This mistake then predisposes to becoming a
cancerous cell.
Conclusion
Colon and rectal cancer is
a preventable disease, which almost always originates from a polyp. The dilemma
is in discovering polyps when they are small and benign so they can be removed.
© Copyright 1995-2005 The Cleveland Clinic Foundation. All
rights reserved.
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