Colorectal Cancer: An Overview

It seems nothing can prepare us for a diagnosis of cancer. Being told you have cancer can bring on a flood of emotions — shock, fear, and confusion. At first, it may be difficult to comprehend the fact that you have cancer. And, in addition to coming to grips with the diagnosis, you're faced with undergoing medical tests and a series of treatments, all of which you probably know little about. It can be a stressful time for you, your family, and friends.

The term "colorectal cancer" actually refers to two diseases. Colon cancer is cancer found in the tissues of the colon, and rectal cancer forms in the tissues of the rectum. Both cancers have the same characteristics and the same risk factors. In some cases, they are treated the same way, while at other times the treatments are different.

Colorectal Cancer Statistics

Not counting skin cancer, colon cancer is the third most commonly diagnosed cancer in the United States. It affects approximately 102,000 Americans each year, according to the American Cancer Society. Rectal cancer is less common than colon cancer, affecting nearly 40,000 people annually. Both cancers are curable if detected early. And, thanks to improved treatments and greater public awareness about preventive screening, death rates from colorectal cancer have been dropping over the last twenty years.

Anatomy and Function of the Colon and Rectum

Before discussing colorectal cancer and how it develops, let's first take a brief look at the anatomy of the colon and rectum. The colon and rectum are important parts of your gastrointestinal (GI) tract. This tract includes your mouth, esophagus, stomach, duodenum (first part of your small intestine), small intestine, colon (large intestine), rectum, and anus. This system takes in food, digests it, absorbs nutrients, and excretes waste.

The primary job of the colon is to manage and remove solid waste. When you eat, food spends several hours in the stomach being digested. Once the nutrients are absorbed in the small intestine, any remaining liquid enters the colon, which is also called the large bowel.

By the time the liquid enters the first part of the colon, it contains no nutrients and is pure waste product. This waste liquid is slowly propelled toward the rectum. Over a four- to six-hour period, the cells in the colon absorb remaining water from the waste material. The colon can absorb nearly two gallons of water a day. The end product is the solid waste, or feces.

The rectum's primary function is to store processed fecal material before it is excreted from the body. Once there is enough fecal material in the rectum, sensory nerves tell the brain it is time to have a bowel movement.

The Colon

The colon is a hollow, tubelike organ that is five to six feet long and up to five inches in diameter. It sits in the abdominal cavity and is divided into segments. The first segment is the cecum, which is a pouch that receives waste material from the small intestine. The next segment is the right (ascending) colon, located on the right side of the abdomen. The transverse colon crosses over the midsection of the abdomen. The left (descending) colon is on the left side of the abdominal cavity. The left colon leads into the sigmoid colon, an S-shaped section that connects to the rectum. The rectum is a muscular tube, the last six to ten inches of the colon, and it exits into the anus.

The colon itself has four layers of tissue. The innermost layer, the mucosa, is in direct contact with fecal material moving through the colon and is responsible for much of the colon's function. The thin mucosa is composed of specialized cells that are in a constant state of flux, continually dying, sloughing off, and being replaced with new cells. The layer under the mucosa is the submucosa. It's a specialized layer of cells that helps support the mucosa.

The next layer of tissue is the muscularis propria. The muscle cells in this layer give strength to the colon wall and cause the contractions that push fecal material along the colon. The outermost layer of tissue is the serosa. The serosal cells add more support to the colon wall and act as a barrier, protecting the colon from any outside invading disease.

The Rectum

The rectum, the last six to ten inches of the colon, connects the colon to the anus. Unlike the rest of the colon, the walls of the rectum are primarily composed of muscle cells that propel the fecal material out of the body.

The last part of the digestive system is the anus. It is a canal, about two inches long, that contains sphincter muscles; it's the job of these muscles to hold in waste material until you're ready to have a bowel movement.

How Colorectal Cancer Develops

Colorectal cancer usually develops from polyps, which are small, abnormal growths that can occur anywhere along the gastrointestinal tract. Polyps are common in the colon and rectum; in fact, as many as 50 percent of the U.S. population has polyps in the colon and/or rectum. Polyps occur more frequently in people over age fifty; they are caused by genetic changes in the cells lining the inside of the colon and grow slowly over a period of ten to fifteen years. Cancer is an overgrowth of these abnormal cells and, sometimes, polyps start growing uncontrollably. Most people have no idea a tumor is growing inside their colon or rectum unless they go for routine tests or start to experience symptoms such as pain or bleeding.

Types of Polyps

Colorectal polyps may be benign or malignant (cancerous). Fortunately, most polyps are benign. Only about 1 percent of polyps that are removed prove to be cancerous. Still, it's important to know that benign polyps can turn into cancer if not removed.

Benign Polyps

Polyps referred to as benign are noncancerous. Several types of benign polyps may occur in the colon. The polyps are defined by their size and the appearance of their cells under the microscope.

• Hyperplastic. About 90 percent of all polyps are hyperplastic polyps. They are small, about the size of a pea, and consist of a dense cluster of normal cells with no microscopic abnormalities. They are totally benign, with no potential to become cancerous.

• Inflammatory. This type of benign polyp, also small, is made up of inflamed cells commonly associated with inflammatory diseases of the colon, including ulcerative colitis and Crohn's disease.

• Hamartoma. This type of small, benign polyp is found in patients with genetically inherited polyp syndromes. (See discussion of these syndromes later in this chapter.) These polyps are frequently numerous and scattered throughout the colon.

Precancerous Polyps

A polyp can also be...