Colorectal Cancer Overview

Colorectal cancer originates in the colon or rectum, parts of the large intestine. Depending on the site of origin, it is referred to as colon cancer or rectal cancer. These cancers are often collectively termed colorectal cancer due to their shared characteristics and similarities in treatment approaches.

The Colon and Rectum

To understand colorectal cancer, it’s essential to know the normal structure and function of the colon and rectum.

The colon and rectum together form the large intestine (also known as the large bowel), which is a vital part of the digestive system, or gastrointestinal (GI) system.

Structure of the Colon

The colon is the largest part of the large intestine, measuring approximately 5 feet (1.5 meters) in length. It is divided into four main sections:

Ascending Colon: This section begins at the cecum, where undigested food enters from the small intestine. It travels upward on the right side of the abdomen.
Transverse Colon: Extending across the abdomen from right to left, this section connects the ascending colon to the descending colon.
Descending Colon: Moving downward on the left side, this section leads to the sigmoid colon.
Sigmoid Colon: Named for its “S” shape, this section connects to the rectum.

Function of the Colon

The primary functions of the colon include:

Absorbing Water and Nutrients: The colon absorbs water and electrolytes from undigested food received from the small intestine.
Forming and Storing Stool: It compacts waste into stool and stores it until elimination.
Housing Beneficial Bacteria: The colon contains a vast number of bacteria that aid in digestion and contribute to overall health.

Structure of the Rectum

The rectum is the final section of the large intestine, measuring about 12 centimeters in length. It serves as a temporary storage site for stool before it is expelled through the anus.

Summary

Understanding the anatomy and function of the colon and rectum is crucial for comprehending how colorectal cancer develops and affects the body.

Symptoms & Presentation

Early-stage colorectal cancer often causes no symptoms, which is why screening is vital.
When symptoms occur, they may include:
- Changes in bowel habits (diarrhea, constipation, narrowing of stool),
- Blood in stool (bright red or dark tar-like),
- Abdominal pain or bloating, unexplained weight loss, fatigue or iron deficiency anemia
Because such symptoms can be mistaken for hemorrhoids or IBS, prompt evaluation is essential.

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Risk Factors

Non-modifiable risks:
- Age (risk rises sharply after 50), family history of colorectal cancer or polyps, personal history of inflammatory bowel disease, and inherited syndromes such as Lynch syndrome (HNPCC) and familial adenomatous polyposis (FAP)
Modifiable risks:
- Obesity, sedentary lifestyle, diets high in red or processed meat with low fruit, vegetables, and fiber intake, alcohol consumption, and smoking. ACS estimates ~55% of cases and deaths are attributable to these factors.

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American Cancer Society

Screening & Early Detection

American Cancer Society recommends that average-risk adults begin screening at age 45, continuing through at least age 75. Between 76–85, decisions should be individualized based on health status Screening options include:
- Colonoscopy (gold standard, every 10 years if no polyps are found),
- Stool-based tests such as FIT, gFOBT, or multi-target stool DNA (e.g., Cologuard),
- Sigmoidoscopy or CT colonography (virtual colonoscopy)
Screening not only detects early cancers but also prevents them by enabling removal of precancerous polyps

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Epidemiology & Incidence

In 2025, ACS estimates approximately 154,270 new cases of colorectal cancer in the U.S., with around 52,900 deaths Lifetime risk is about 1 in 24 for men and 1 in 25 for women
Colorectal cancer remains the 2nd leading cause of cancer death in the U.S. when sexes are combined

Rising Rates Among Younger Adults

Since the mid-1990s, incidence rates have increased by 1% to 2% per year among individuals under age 55
ACS reports that 1 in 5 cases now occur in people under 50, and that young-onset cancer is frequently diagnosed at a more advanced stage (e.g. only ~1 in 3 early-stage diagnoses)
This makes colorectal cancer the leading cause of cancer death in men under 50 and the second in women of that age group

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Treatment Overview

Treatment depends on stage and may include:
- Surgery (resection of the tumor; polypectomy for precancerous lesions),
- Chemotherapy, targeted therapy, and radiation (usually for rectal cancers or advanced disease),
- Immunotherapy, especially for microsatellite instability–high (MSI-H) tumors typical in Lynch syndrome patients

Prevention Strategies

Lifestyle interventions can significantly lower risk:
- Maintain healthy weight, engage in regular physical activity, eat fiber-rich diets with fruits, vegetables, and whole grains, minimize red and processed meats, quit smoking, and limit alcohol intake to ≤1 drink/day for women or ≤2 for men
Aspirin, when taken daily for at least two years, may lower risk—but only with guidance due to bleeding risks, especially in older adults.
Removal of high-risk polyps during colonoscopy is an effective preventive measure