Adenocarcinoma of the colon is the most common type of cancer that begins in the large intestine, forming in the glands that line the inner walls of the colon. This disease typically affects older adults, though cases among younger people are rising. Understanding the risk factors, symptoms, and preventive measures can help in early detection, when treatment is most successful.
What is Adenocarcinoma of the Colon
Adenocarcinoma of the colon represents a specific type of cancer that develops in the glandular cells, which are specialized cells that produce mucus and digestive fluids in the lining of the colon. The colon, also known as the large intestine, is the first and longest part of the large intestine and serves as the main component of the digestive system responsible for absorbing water and nutrients while storing waste material before it leaves the body.[1]
When doctors and healthcare providers discuss colon cancer, they are usually referring to colorectal adenocarcinoma, as this form accounts for the vast majority of cases. Over 95% of colorectal cancers are adenocarcinomas, making it by far the most common type of tumor originating in the mucus-producing glands of the colon or rectum.[2][14]
The disease typically begins as small clumps of cells called polyps that form on the inner lining of the colon. While most polyps are not cancerous, some types can gradually transform into cancer over time. This transformation process is usually slow, often taking between 10 to 15 years for a colon polyp to develop into cancer. This extended timeframe provides a valuable window for detection and removal of precancerous polyps through screening tests, potentially preventing cancer from ever developing.[1][3]
As the disease progresses without treatment, cancer cells that begin in the innermost layer of mucous membrane can work their way through additional layers of tissue and muscle in the colon wall. Eventually, the cancer may spread beyond the colon to other parts of the body through the lymphatic system or bloodstream, reaching organs such as the liver, lungs, brain, or bone.[2]
Epidemiology
Colon cancer stands as the third most frequently diagnosed cancer worldwide and the second most common cause of cancer-related deaths globally. In 2020, approximately 1.8 million cases of colorectal cancer were diagnosed around the world, along with over 900,000 deaths attributed to the disease.[4][14]
In the United States alone, colorectal cancer represents a major public health concern. According to the Centers for Disease Control and Prevention, it is the third most common cancer diagnosed among Americans. In 2023, there were an estimated 150,000 new cases of colorectal cancer diagnosed and 53,000 colorectal cancer-related deaths in the United States.[7][14]
The disease shows distinct patterns across different demographic groups. Males are slightly more likely to develop colon cancer compared to females. Among racial and ethnic groups in the United States, African-Americans face the highest risk of developing colorectal cancer compared to all other races. The group at highest overall risk includes women who are age 50 years or older, though men in this age range are also at elevated risk.[1][12]
While colon cancer has traditionally been considered a disease of older adults, a concerning trend has emerged over the past 15 years. The number of people between the ages of 20 and 49 diagnosed with colon cancer has been increasing by approximately 1.5% each year. Medical researchers continue to investigate why this shift is occurring among younger populations.[12]
Causes
The exact causes of colorectal adenocarcinoma remain unclear, but researchers have identified that the disease develops when changes or mutations occur in the DNA of cells in the colon. DNA makes up your genes and contains the instructions that tell cells how to function, grow, and divide. When these instructions become damaged or altered, cells can begin to grow out of control and form tumors.[1][2]
For colon cancer to develop, many different genes typically need to undergo mutations. The disease does not usually result from a single genetic change but rather from an accumulation of multiple alterations over time. This multi-step process helps explain why colorectal cancer development is often a slow process that can take many years.[1]
There are two main types of gene mutations that can affect the development of colorectal adenocarcinoma. The first type is inherited gene mutations, which pass from one generation to the next through family genes. These inherited mutations are responsible for a relatively small proportion of cases. It is estimated that Lynch syndrome, an inheritable genetic disorder, results in about 5,000 new cases of colorectal cancer each year in the United States.[1][14]
The second and more common type is acquired gene mutations. These mutations are not related to family genes but instead happen during a person’s lifetime due to various environmental and lifestyle factors. Acquired gene mutations cause the majority of cases of colorectal adenocarcinoma. These changes can occur due to exposure to carcinogens, chronic inflammation, or simply as part of the aging process.[1]
Colorectal adenocarcinoma is not an infectious disease and cannot be transmitted from person to person. However, understanding the role of both inherited and acquired genetic changes helps explain why some individuals develop the disease while others do not, even when exposed to similar risk factors.[9]
Risk Factors
Numerous factors can increase a person’s likelihood of developing colorectal adenocarcinoma. A risk factor is anything that increases the chance of getting a disease, though having one or more risk factors does not guarantee that a person will develop colon cancer. Conversely, some people with no known risk factors do develop the disease.[9]
Age represents one of the most significant risk factors for colorectal cancer. The disease typically affects older adults, with risk increasing substantially after age 50. However, as noted earlier, younger adults are increasingly being diagnosed. Current guidelines recommend that most men and women at average risk should start screening at age 45.[3][7]
Family history plays a crucial role in determining risk. Having a first-degree relative, such as a parent, sibling, or child, with a history of colon or rectal cancer significantly increases a person’s risk. Similarly, having a personal history of colon, rectal, or ovarian cancer places individuals at higher risk for developing or redeveloping colorectal cancer.[9]
Certain medical conditions substantially elevate risk. People with a personal history of high-risk adenomas—colorectal polyps that are 1 centimeter or larger in size or that have cells that look abnormal under a microscope—face increased risk. Having colon polyps in general, which are masses of cells similar to tumors in the colon, also raises the likelihood of cancer development.[1][9]
Individuals with chronic inflammatory conditions of the bowel, such as ulcerative colitis or Crohn disease lasting 8 years or more, have elevated risk due to the ongoing inflammation in the intestinal lining. This chronic irritation can lead to cellular changes that may eventually become cancerous.[1][9]
Lifestyle factors significantly influence risk as well. Smoking cigarettes or using tobacco products is associated with increased colorectal cancer risk. Drinking three or more alcoholic drinks per day has been linked to higher risk. Obesity or being overweight puts additional strain on the body and is connected to greater cancer risk.[1][9]
Dietary habits also matter. A diet high in fat and low in fiber may increase risk, as can eating too much red meat and processed meats. Not getting enough physical exercise contributes to risk, as a sedentary lifestyle has been linked to higher rates of colorectal cancer.[1]
People with type 2 diabetes face increased risk, possibly due to shared risk factors such as obesity and diet, as well as the metabolic changes associated with diabetes itself. Having inherited gene mutations, such as those that cause familial adenomatous polyposis (FAP) or Lynch syndrome (hereditary nonpolyposis colorectal cancer), dramatically increases lifetime risk.[1][9]
Symptoms
One of the challenges with colorectal adenocarcinoma is that many people experience no symptoms at all in the early stages of the disease. When colon cancer is small or has not yet grown significantly, it may not cause any noticeable changes in how a person feels. This absence of early warning signs is why screening tests are so important—they can detect cancer before symptoms appear.[1][3]
When symptoms do develop, they often depend on the cancer’s size and location within the colon. It is crucial to understand that many of these symptoms can be caused by conditions other than cancer, such as infections, hemorrhoids, or irritable bowel syndrome. However, any persistent or concerning symptoms should prompt a visit to a healthcare provider.[2]
Blood in the stool or bleeding from the rectum is among the most common symptoms that should never be ignored. People might notice blood on toilet paper after wiping, see blood mixed in with their stool, or observe that their stool appears dark or bright red. Sometimes the amount of blood may be so small that it is not visible to the naked eye. While many conditions can cause blood in the stool, including hemorrhoids and anal tears, this symptom always warrants medical evaluation.[2][12]
Changes in bowel habits that persist represent another important warning sign. This might include new onset of constipation or diarrhea that does not resolve, or experiencing narrow stools. Some people feel as if they still need to have a bowel movement even after using the bathroom—a sensation of incomplete evacuation or fullness in the bowels.[1][2]
Abdominal pain or discomfort is frequently reported. This pain might be persistent, with no known cause, or it might be severe enough to significantly impact daily activities. The pain can occur in various locations in the abdomen and may be accompanied by cramping or bloating.[1][2]
General symptoms affecting overall health may also occur. Unexplained weight loss—losing weight without trying or without changes to diet and exercise—can be a red flag. Persistent tiredness or weakness that does not improve with rest may develop. Some people experience a loss of appetite or feel full after eating only small amounts of food.[1][2]
Bloating that lasts for more than a week, gets progressively worse, or is accompanied by other symptoms like vomiting should prompt medical attention. These symptoms, while nonspecific, may indicate a growing tumor that is affecting normal digestive function.[2]
Prevention
While not all cases of colorectal adenocarcinoma can be prevented, several strategies can significantly reduce risk. Prevention efforts focus on both lifestyle modifications and screening to detect and remove precancerous polyps before they develop into cancer.[7]
Regular screening represents the most powerful tool for prevention. Screening tests can find polyps so they can be removed before turning into cancer. Screening also helps find colorectal cancer at an early stage when treatment is most effective. Current guidelines recommend that people at average risk should begin screening at age 45. For individuals with certain risk factors, such as inflammatory bowel disease or a family history of colon polyps or cancer, screening may need to start at a younger age.[7][18]
Tobacco cessation is crucial. Smoking or vaping may increase colorectal cancer risk, and quitting tobacco use is strongly advised for anyone who currently smokes. Healthcare providers can offer support and resources to help people successfully quit tobacco.[2][18]
Limiting alcohol consumption can help reduce risk. Heavy alcohol use has been linked to increased risk of developing colorectal cancer. Doctors recommend drinking alcohol in moderation only or avoiding it altogether.[2][18]
Adopting a healthy diet may play a protective role. Eating a diet rich in fruits, vegetables, and whole grains while limiting red meat and processed meats has been associated with reduced colorectal cancer risk. A high intake of red and processed meats has been shown to increase the risk, so replacing these with plant-based proteins, fish, or poultry may be beneficial.[2][18]
Regular physical activity has been linked to reduced colorectal cancer risk. An active lifestyle contributes to maintaining a healthy weight and may directly affect colon function and inflammation levels. People should aim to incorporate regular exercise into their routines and speak with their doctor about an exercise program appropriate for their fitness level.[2][18]
Maintaining a healthy body weight is important because being overweight or obese can increase colorectal cancer risk. Eating healthfully and exercising regularly work together to help achieve and maintain a healthy weight. For some individuals struggling with weight management, medical support programs may be helpful.[2][18]
Pathophysiology
Understanding how adenocarcinoma of the colon develops and affects normal body functions helps explain both its symptoms and the rationale behind different treatment approaches. The pathophysiology describes the mechanical, physical, and biochemical changes that occur as the disease progresses.[4]
The colon wall consists of several distinct layers. The innermost layer, called the mucosa, contains specialized glandular epithelial cells that produce and release mucus and other digestive fluids. These cells normally divide in a controlled manner to replace old or damaged cells. In adenocarcinoma, mutations cause these glandular cells to begin dividing uncontrollably.[12]
The disease typically follows a progression known as the adenoma-carcinoma sequence. It usually begins when normal epithelial cells in the colon lining undergo genetic changes that cause them to form polyps—small growths protruding from the inner surface of the colon. Most polyps remain benign throughout a person’s lifetime. However, certain types of polyps, particularly adenomatous polyps, have the potential to accumulate additional genetic mutations over time.[3][7]
As these precancerous cells accumulate more mutations, they can transform into cancer cells that invade deeper layers of the colon wall. The colon wall consists of the mucosa, a layer of supporting tissue, a muscle layer, and an outer layer. Early-stage cancer remains confined to the innermost layers, but as it progresses, cancer cells can work their way through the tissue and muscle layers.[12]
When adenocarcinoma cells penetrate through the full thickness of the colon wall, they can spread to nearby lymph nodes. The lymphatic system, which normally helps fight infection by filtering fluid through lymph nodes, can inadvertently provide a pathway for cancer cells to travel. Once in the lymphatic system, cancer cells may establish new tumors in lymph nodes or travel to distant parts of the body.[2]
Cancer cells can also enter blood vessels and use the bloodstream as a highway to reach other organs. The liver is a particularly common site of spread because blood from the colon flows directly to the liver through the portal vein before circulating to the rest of the body. Other frequent sites of spread include the lungs, brain, and bones.[2]
As tumors grow within the colon, they can cause symptoms through several mechanisms. A tumor may bleed, leading to blood in the stool. It can partially block the passage of stool through the colon, causing changes in bowel habits or a feeling of incomplete evacuation. Large tumors can physically compress or invade surrounding structures, causing pain. The cancer can also produce substances that affect appetite and metabolism, contributing to weight loss and fatigue.[12]
The biological behavior of colorectal adenocarcinoma varies based on its genetic characteristics. Recent research has identified different molecular subtypes of the disease. For example, tumors with high microsatellite instability (MSI-H) or DNA mismatch repair deficiency (dMMR) have distinct characteristics and may respond differently to certain treatments, particularly immunotherapy. These genetic features reflect defects in the cellular machinery that normally repairs DNA damage, leading to accumulation of numerous mutations.[14]
Understanding these pathophysiological processes has led to more targeted approaches to treatment and helps explain why early detection through screening is so valuable. By interrupting the disease process at the polyp stage, before cancer develops, or by detecting cancer when it remains confined to the inner layers of the colon wall, intervention can occur before the cancer has the opportunity to spread to lymph nodes or distant organs.[7]
