Colon cancer is a malignant tumor of the large intestine and is sometimes referred to as colorectal cancer. The latter is a broader term that refers to the cancers arising from the gastrointestinal tract extending from the cecum to the distal extension of colon and the rectum. Almost 95% of the malignant tumors in the colon are adenocarcinomas and many are believed to progress from adenomatous polyps with dysplasia to colon cancer. Colon polyps are most common benign tumors of large intestine.
How does colon cancer develop?
A number of causative and risk factors have been implicated in the development of colon cancer. This tends to lead to one of two types of genetic instability which may trigger the uncontrolled growth of cells. Chromosomal instability is where the tumor suppressing genes are inactivated to genetic mutations or deletions. With microsatellite instability, the repair of errors in DNA replication is hampered by mutation of specific genes which allow for this repair process.
Malignant tumors that form in the proximal colon, around the cecum and ascending colon, tend to occur as polypoid masses. In contrast, malignancies in the distal colon form annular lesions (ring-like) that constrict the colon and narrow the lumen. Cancer in the proximal colon are therefore less likely to cause an obstruction while those in the distal colon are more likely to do so.
Incidence of Colon Cancer
Colon cancer is one of the most common cancers in developed countries like the United States, Canada, Australia, New Zealand, and certain European nations with an intermediate incidence in nations like the United Kingdom. In contrast, developing countries have a fairly low incidence but this has been changing as a westernized diet and lifestyle is adopted in certain nations. The annual incidence of colon cancer is about 800,000 globally of which about 150,000 of the new cases are reported from the US.
The number of deaths related to colon cancer globally is about 450,000 annually and that in the United States is about 50,000. The deaths from colon cancer accounts for about 15% of the cancer related deaths in the US. Colon cancer as a cause of cancer-related death is second only to lung cancer in the United States. The overall death rate related to colon cancer in the US has however reduced in last few decades.
Colon cancer primarily affects individuals past the age of 45 to 50 years and both sexes have similar incidence of colon cancer. Only 10% colon cancers develop before the age of 50 years.