Colon Cancer
The colon is the part of the digestive system where
the waste material is stored. The rectum is the end of the colon
adjacent to the anus. Together, they form a long, muscular tube
called the large intestine (also known as the large bowel).
Tumors
of the colon and rectum are growths arising from the inner wall
of the large intestine. Benign tumors of the large intestine are
called polyps. Malignant tumors of the large intestine are called
cancers. Benign polyps do not invade nearby tissue or spread to
other parts of the body. Benign polyps can be easily removed during
colonoscopy, and are not life threatening. If benign polyps are
not removed from the large intestine, they can become malignant
(cancerous) over time. Most of the cancers of the large intestine
are believed to have developed from polyps. Cancer of the colon
and rectum (also referred to as colorectal cancer) can invade
and damage adjacent tissues and organs. Cancer cells can also
break away and spread to other parts of the body (such as liver
and lung) where new tumors form. The spread of colon cancer to
distant organs is called metastasis of the colon cancer. Once
metastasis has occurred in colorectal cancer, a complete cure
of the cancer is unlikely.
The first part of the Colon is involved with the digestion of
food and is known as the 'small Colon' because the tube is narrower
here.
The 'large Colon' follows the small Colon and in a
healthy person, the main part of the large Colon is responsible mostly
for absorbing water from the faeces. The last part of the large Colon is known
as the rectum, which leads to the anus.
Colon (colorectal) cancer is cancer of the colon or rectum, and
it arises from the cells that line the Colon. The small Colon is strikingly
free from cancer risk, and almost all Colon cancers arise in the large Colon.
About 6 per cent of the population in Western countries develop
Colon cancer at some time during their lives, making this the second commonest
cause of cancer-related death. However, it is curable in 40 to 50 per cent of
cases, usually by surgery.
The cancer develops when one of the cells in the colon develops
a series of changes (mutations) in some of the genes that control how the cell
divides and survives. As a result, the cell divides uncontrollably to form a
clump of malignant (cancerous) cells. Initially, these cell changes commonly
produce a polyp (a clump of abnormal cells the size of a pea on the end of a
stalk of normal cells) called an adenoma.
At this stage, an adenoma is still pre-cancerous (a stage at
which it may or may not become cancer), and probably only about 5 per cent of
the polyps progress further to become life-threatening cancers.
The polyp enlarges very slowly, probably over about 10 years, up
to between 1cm and about 5cm in diameter. The abnormal cells first invade the
stalk of the polyp, then the underlying tissue of the colon to which the stalk
is attached. This invasion indicates that cancer has developed. The patient
will then usually have symptoms, which can include bleeding from the ulcerated
tip of the cancer and diarrhoea caused by disturbance in the muscle activity of
the colon or to obstruction. The risk of invasive cancer becomes appreciable
once the polyp diameter has exceeded 1cm.
About 30 per cent of Colon cancers arise from flat lesions and
do not pass through a polyp stage. This particularly occurs with cancers of the
proximal (right-sided) colon and caecum.
If the cancer is not removed quickly, cancerous cells can break
off from the tumour and move through veins or lymph vessels to form tumour
growths (called metastases or secondaries) elsewhere, particularly in lymph
glands or in the liver. The cure rate falls sharply once this has happened.
The average age when Colon cancer is first discovered is 65, and
it becomes increasingly common with advancing age. Very occasionally, it may
affect much younger adults from the age of 20. The rates do not differ
strikingly between the sexes, although men are slightly more prone to
developing rectal cancer and women to developing cancer of the caecum. This is
the point where the appendix is attached.
The appendix itself is rarely the site of cancer, although it
can be the site of a much rarer tumour called a 'carcinoid'. Previous
appendicectomy (removal of the appendix) seems to have no effect on the
subsequent risk of Colon cancer.
Globally, cancer of the colon and rectum is the third
leading cause of cancer in males and the fourth leading cause
of cancer in females. The frequency of colorectal cancer varies
around the world. It is common in the Western world, and is rare
in Asia and Africa. In countries where the people have adopted
western diets, the incidence of colorectal cancer is increasing.
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