Two types of growth occur in the colon:

• Noncancerous growths, such as polyps
• Malignant or cancerous growths, which usually begin as polyps

Polyps
Colorectal cancer can occur without polyps, but this is rare and accounts for less than 1% of all colorectal cancers diagnosed in the US each year. It usually happens in individuals with long-standing inflammatory bowel diseases, such as chronic ulcerative colitis and Crohn's colitis.

Intestinal polyps are masses of tissue that arise from the intestinal wall and protrude into the lumen (the interior space of the intestine). It's not clear what produces polyps, but they appear to be caused by both environmental and genetic factors. Cigarette smoking, diets that are high in fat and low in fiber, lack of exercise, and obesity may all contribute to polyp growth.

Most cancers in the colon and rectum are adenocarcinomas. "Adeno" means "gland," and colorectal adenocarcinomas arise in abnormal gland cells that form in the inner lining of the colon or rectum. About three-quarters of cancers occur in the colon and one-quarter in the rectum.

There are other forms of colorectal cancer, but they occur much less frequently than adenocarcinomas:

• Sarcomas (tumors that begin in connective tissue)
• Lymphomas (tumors that begin in the lymphatic system)
• Rare cancers such as carcinoids and gastrointestinal stromal tumors

Polyps and Cancer Spread
Polyps grow slowly and may at first be benign. However, because they may become cancerous over time, they should be removed. Once a polyp becomes cancerous, it starts to invade the intestinal wall and may spread into nearby lymph nodes. From there it can enter into the lymphatic system and spread to areas distant from the original site, or "metastasize." Cancerous colorectal cells can also penetrate into capillaries and migrate via the bloodstream, most often to the liver. There the cells may form new tumors.

Types of Polyps
There are three types of polyps:

• Adenomatous polyps (sometimes referred to as adenomas)
• Hyperplastic polyps
• Inflammatory polyps Adenomatous polyps account for only about 10% of all colonic polyps, but they are of the greatest concern because they are the most likely to become cancerous. Fortunately, they grow slowly: an adenomatous polyp just under half an inch (10 mm) in diameter takes 5-10 years to become dangerous. In general, the larger the polyp, the more likely it is to become cancerous. Adenomatous polyps are divided into three different subtypes: villous (hairy), tubular, and tubulovillous. Villous adenomas are larger than the other types and are the most likely to turn malignant.

Polyps come in different shapes, as well, termed pedunculated or sessile. Pedunculated polyps have fleshy stalks, like mushrooms, and can be easily removed during a colonoscopy. Sessile polyps have broad bases and grow flat against the wall of the intestine, making their removal difficult.

The Lymphatic System
The lymphatic system works in concert with the immune system and has several functions in the body.The lymphatic system:

• Combats invasive foreign bodies like bacteria, viruses, and fungi
• Drains interstitial fluid back into the bloodstream from the tissues
• Filters lymph
• Filters the blood

The lymphatic system includes the lymphatic vessels, lymph, lymph nodes, spleen, thymus, bone marrow, and a few other organs.

• Lymphatic vessels form a sort of parallel circulatory system, branching throughout the tissues of the body.
• Lymph is a colorless liquid found within the lymphatic vessels. It's collected from tissues in all parts of the body as interstitial fluid, and then travels to at least one lymph node for filtering before returning to the bloodstream via the veins. Lymph carries large numbers of lymphocytes (white blood cells).
• Lymph nodes are found in the armpits, on either side of the groin, on either side of the neck, and in the abdomen, pelvis, and chest. Lymph nodes filter the lymph fluid as it passes through them, and macrophages inside the lymph nodes kill intruders, such as bacteria.
• The spleen produces lymphocytes and filters the blood.
• The thymus helps to produce white blood cells.
• Bone marrow produces white (and red) blood cells. When cancer cells break off of a tumor, they often become trapped in the nearest lymph node. That's why doctors check the lymph nodes first when they are determining how far a cancer has grown or spread.

More on this topic

Colorectal Cancer Screening (VIDEO)
Colorectal Cancer
Colon Anatomy & Function
What Is Colorectal Cancer
Tumor Growth
Cancer Staging
Signs & Symptoms
Screening & Diagnosis
A Less Invasive Look
Treating Colorectal Cancer
Detecting Cancer Recurrence
Preventing Cancer

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Breast Cancer, Cancer Introduction, Colorectal Cancer, Prostate Cancer