Colon Cancer
Colon cancer is a cancer arising in the colon, the longest portion of the large intestine. The vast majority of colon cancers are adenocarcinomas that develop over years from precancerous polyps, which is why screening is so effective at prevention and early detection. Colon cancer is one of the most common cancers overall in both men and women, but it is also one of the most treatable when found early. Symptoms may include changes in bowel habits, blood in the stool, unexplained weight loss, fatigue from anemia, or abdominal pain—though early cancers often cause no symptoms. Treatment depends on stage and molecular characteristics and typically involves surgery, often combined with chemotherapy, immunotherapy, or targeted therapies. Evaluation combines colonoscopy with biopsy, imaging for staging, and molecular testing to guide treatment.
What is it?
The colon is the longest portion of the large intestine and is divided into several sections: the cecum (the first portion, where the small intestine empties into the large intestine), the ascending colon (going upward on the right side of the abdomen), the transverse colon (crossing the upper abdomen from right to left), the descending colon (going downward on the left side of the abdomen), and the sigmoid colon (a curved segment leading to the rectum). The colon’s primary functions are to absorb water and electrolytes from the digested material passing through it and to store waste until elimination.
Colon cancer is a cancer that arises from cells in the colon wall. The vast majority (over 95%) of colon cancers are adenocarcinomas, which develop from the glandular cells that line the inner surface of the colon. Other, less common types include neuroendocrine tumors (carcinoids), lymphomas, gastrointestinal stromal tumors (GISTs), and metastases from other cancers—these are discussed on separate pages. When people refer to “colon cancer,” they are typically referring to adenocarcinoma.
Colon cancer typically develops slowly, over 10 or more years in most cases, from precancerous polyps through what is known as the adenoma-carcinoma sequence. Normal cells in the colon lining accumulate genetic changes and progress through stages: from small benign polyps to larger and more atypical polyps (adenomas) to invasive cancer. This slow, stepwise progression is what makes screening so effective—removing precancerous polyps during colonoscopy can prevent cancer from developing, and early cancers are often highly curable. Most colon cancers arise from adenomatous polyps (particularly certain types such as tubular, tubulovillous, and villous adenomas), and a smaller subset arises through a different pathway involving serrated polyps.
Colon cancer can be distinguished by its location within the colon, which has important clinical implications. Right-sided colon cancers (in the cecum, ascending colon, and often the proximal transverse colon) tend to have different molecular characteristics, symptoms, and outcomes than left-sided colon cancers (distal transverse colon, descending colon, and sigmoid colon). Right-sided cancers often bleed slowly, causing iron-deficiency anemia over time, and typically present with fatigue rather than obvious changes in bowel habits. Left-sided cancers more often cause changes in stool caliber, constipation or diarrhea, obstructive symptoms, or visible blood in the stool. Right-sided cancers more often have mismatch repair deficiency and microsatellite instability, while left-sided cancers more often have specific other molecular alterations. Rectal cancers—those arising in the rectum, which is the last portion of the large intestine below the sigmoid colon—are a distinct entity that shares many features with colon cancer but has some important differences in evaluation and treatment (see also our Bowel Adenocarcinoma page).
Several risk factors contribute to colon cancer risk. Age is the most important risk factor, with the majority of cases occurring in adults over 50, though rates of early-onset colorectal cancer (in adults under 50) have been increasing in recent decades. This trend has prompted lowering of the recommended screening age from 50 to 45 for average-risk adults in the United States. Family history significantly increases risk, particularly with first-degree relatives affected at a young age. Inherited syndromes—including Lynch syndrome (hereditary nonpolyposis colorectal cancer, HNPCC), familial adenomatous polyposis (FAP), MUTYH-associated polyposis, and others—account for a small but important proportion of cases and require specific evaluation, screening, and family counseling. A personal history of colorectal polyps, colorectal cancer, or inflammatory bowel disease (particularly ulcerative colitis with extensive colonic involvement or long-standing Crohn’s colitis) increases risk. Lifestyle factors including obesity, physical inactivity, high consumption of red and processed meats, low intake of fruits and vegetables and dietary fiber, heavy alcohol use, and smoking contribute to risk. Type 2 diabetes is also associated with increased risk.
Symptoms of colon cancer depend on the location and stage of the tumor. Common symptoms include changes in bowel habits (persistent diarrhea, constipation, or change in stool caliber), blood in the stool (which may be visible or detected only on stool testing), unexplained weight loss, fatigue (often from iron-deficiency anemia due to slow blood loss from the tumor), abdominal pain or cramping, sensation of incomplete emptying after bowel movements, and, in some cases, a palpable abdominal mass or symptoms of anemia (weakness, shortness of breath, pale skin).
Right-sided colon cancers tend to bleed slowly and often present with iron-deficiency anemia and fatigue rather than obvious changes in bowel habits, and may reach substantial size before causing symptoms. Left-sided colon cancers more often cause changes in bowel habits, obstructive symptoms (constipation, sensation of incomplete emptying, change in stool caliber), or visible blood in the stool. Early cancers often cause few or no symptoms, which is why screening is so highly recommended.
In some patients, colon cancer presents acutely with complications such as bowel obstruction (from tumor mass or luminal narrowing), bowel perforation (severe abdominal pain, fever, generalized illness), significant bleeding, or, less commonly, other complications. These presentations often occur in patients who have not been up to date on screening.
Screening is highly effective for colon cancer and is one of the strongest reasons for the observed decrease in colon cancer incidence and mortality in populations with high screening rates. Screening approaches include:
– Colonoscopy—the gold standard test that allows visualization of the entire colon and removal of polyps during the procedure; typically performed every 10 years in average-risk adults, starting at age 45 in the United States
– Fecal immunochemical test (FIT)—a stool test that detects small amounts of blood; typically performed annually
– Multi-target stool DNA test (such as Cologuard)—detects both blood and specific DNA alterations associated with cancer or advanced polyps; typically performed every 3 years
– CT colonography (virtual colonoscopy)—a CT-based test that produces images of the colon; typically performed every 5 years in appropriate patients
– Flexible sigmoidoscopy—an endoscopic test that visualizes the lower portion of the colon and rectum; less commonly used in current practice
– Blood-based tests for circulating tumor DNA are emerging options that have received recent regulatory approval and may play an increasing role in screening
Different screening tests have different strengths and are appropriate in different situations. Adults at higher risk (family history of colorectal cancer, personal history of polyps, inflammatory bowel disease, or hereditary syndromes) often require earlier initiation of screening, more frequent screening, and, in some cases, tailored surveillance protocols.
Diagnosis of colon cancer is typically made by colonoscopy, which allows direct visualization of the tumor and biopsy for tissue diagnosis. Colonoscopy also identifies any additional polyps or lesions and allows their removal.
Staging is essential for treatment planning and involves several components. CT of the chest, abdomen, and pelvis is the primary imaging test for staging and evaluates for spread to lymph nodes, liver, lungs, or other sites. PET/CT is used in specific circumstances, particularly when there are equivocal findings on CT or when planning treatment of oligometastatic disease. MRI may be used in specific settings, particularly for evaluation of possible liver metastases.
Blood tests include complete blood count (which may show anemia), kidney and liver function tests, and tumor markers such as CEA (carcinoembryonic antigen), which can be elevated in colon cancer and useful for monitoring during and after treatment. CEA is not specific to colon cancer and can be elevated in other conditions, so it is used in the context of other information rather than as a standalone diagnostic test.
Molecular testing of the tumor is increasingly important and guides treatment decisions. Testing typically includes:
– Mismatch repair (MMR) protein status or microsatellite instability (MSI) status—identifies tumors that may be highly responsive to immunotherapy and also raises suspicion for Lynch syndrome
– RAS mutations (KRAS, NRAS)—guide selection of EGFR-directed therapies in metastatic disease
– BRAF mutations—identify tumors with different behavior and guide specific targeted therapy combinations
– HER2 status—identifies tumors that may respond to HER2-directed therapies
– NTRK fusions and other rare alterations that may guide specific targeted therapies
For patients with suspected hereditary syndromes (based on young age at diagnosis, family history, multiple primary cancers, or specific tumor features such as MMR deficiency), evaluation may include genetic counseling and germline genetic testing.
Colon cancer staging uses the TNM (tumor, node, metastasis) system, which describes the depth of invasion into the bowel wall (T), the presence and extent of lymph node involvement (N), and the presence of distant spread or metastasis (M). Stages range from stage 0 (carcinoma in situ, confined to the innermost layer) to stage IV (metastatic disease with spread to distant sites). Prognosis and treatment vary significantly by stage.
Important to Know
Management of colon cancer is highly individualized based on stage, molecular characteristics, tumor location, patient factors, and preferences. Care is best delivered by multidisciplinary oncology teams that include colorectal surgeons or surgical oncologists, medical oncologists, gastroenterologists, radiologists, pathologists, geneticists (when relevant), palliative care specialists, and, when needed, other specialists.
For localized colon cancer, surgery is the mainstay of curative treatment. The specific procedure depends on the tumor’s location:
– Right hemicolectomy for cancers in the cecum, ascending colon, and often the proximal transverse colon
– Extended right hemicolectomy for cancers in the transverse colon
– Left hemicolectomy for cancers in the descending colon
– Sigmoid colectomy for cancers in the sigmoid colon
– Subtotal or total colectomy for cancers in specific settings (such as multiple simultaneous cancers or hereditary syndromes)
The surgical procedure typically involves removal of the affected segment of colon along with a wide resection of the associated mesentery containing lymph nodes and blood supply, followed by reconnection of the remaining bowel ends. Modern surgery is often performed using minimally invasive techniques (laparoscopic or robotic-assisted), which offer some advantages over open surgery in appropriate patients. For very early cancers (particularly those confined to a polyp), endoscopic resection may be appropriate in selected cases with careful pathologic examination.
For stage I colon cancer (limited invasion, no lymph node involvement), surgery alone is typically curative in most patients, and adjuvant treatment is generally not needed.
For stage II colon cancer (deeper invasion but no lymph node involvement), the role of adjuvant chemotherapy is more nuanced. Adjuvant chemotherapy may be considered in patients with high-risk features (such as bowel obstruction or perforation at presentation, inadequate lymph node evaluation, high-grade tumor, or specific molecular features), while patients with lower-risk stage II disease often do well with surgery alone. Mismatch repair status is particularly important, as patients with MMR-deficient stage II tumors generally do well with surgery alone and may not benefit from certain chemotherapy regimens.
For stage III colon cancer (spread to nearby lymph nodes), adjuvant chemotherapy after surgery is standard and significantly improves outcomes. Regimens typically include combinations of fluoropyrimidines (intravenous 5-fluorouracil with leucovorin, or oral capecitabine) with oxaliplatin, such as FOLFOX or CAPOX. Duration has traditionally been 6 months but growing evidence supports shorter courses (3 months) for lower-risk stage III patients based on the IDEA collaboration and other studies. For MMR-deficient stage III colon cancer, growing evidence supports the role of immunotherapy in the adjuvant setting.
For stage IV (metastatic) colon cancer, treatment approaches have evolved significantly and include several components:
Systemic chemotherapy regimens (typically containing fluoropyrimidines, oxaliplatin, and/or irinotecan, in various combinations such as FOLFOX, FOLFIRI, and FOLFOXIRI) form the backbone of treatment.
Targeted therapies matched to molecular characteristics include:
– EGFR-directed therapies (such as cetuximab or panitumumab)—used only in RAS wild-type tumors, and typically more effective in left-sided tumors
– VEGF-directed therapies (such as bevacizumab, aflibercept, ramucirumab)
– BRAF-directed therapies for BRAF V600E-mutant tumors, often combined with EGFR-directed therapy (encorafenib plus cetuximab is a standard combination)
– HER2-directed therapies for HER2-amplified tumors (such as trastuzumab-based regimens)
– Newer targeted therapies for specific molecular alterations
Immunotherapy with immune checkpoint inhibitors has transformed treatment of tumors with mismatch repair deficiency or high microsatellite instability (MMRd/MSI-H), which occur in approximately 4–5% of metastatic colon cancers. These tumors are highly responsive to immunotherapy, and it has moved to first-line treatment in the metastatic setting (based on trials such as KEYNOTE-177) and, increasingly, earlier disease settings including neoadjuvant treatment before surgery.
For patients with limited metastatic disease (particularly to the liver or lungs)—so-called oligometastatic disease—surgical resection of metastases, thermal ablation, stereotactic body radiation therapy (SBRT), or other local treatments can offer meaningful long-term outcomes in appropriately selected patients. Multidisciplinary discussion is essential for these decisions.
For patients presenting with acute complications such as bowel obstruction, perforation, or significant bleeding, urgent management is needed. This may include emergency surgery, endoscopic placement of a stent to relieve obstruction as a bridge to surgery or as palliative treatment, or other approaches depending on the specific situation and patient factors.
For patients with confirmed or suspected Lynch syndrome, coordinated care includes:
– Treatment of the current cancer following appropriate principles
– Genetic counseling and family screening
– Long-term surveillance for other Lynch-associated cancers (endometrial, urinary tract, gastric, ovarian, small bowel, and others)
– Consideration of preventive strategies in appropriate patients
For patients with FAP or other polyposis syndromes, care involves specialized surveillance and, in appropriate patients, prophylactic colectomy or other risk-reducing interventions.
Long-term follow-up after treatment is important because of the risk of recurrence. Standard surveillance typically includes clinical assessment, CEA levels, CT imaging of the chest, abdomen, and pelvis, and colonoscopy at defined intervals. Surveillance schedules are individualized based on stage, treatment, and other factors, and are often intensive in the first several years after treatment.
For patients considering fertility preservation before treatment, coordination with reproductive medicine is important, particularly for younger patients receiving chemotherapy.
Nutritional support, physical rehabilitation, mental health support, and support for family and caregivers are important components of comprehensive cancer care.
For patients with advanced disease, palliative care is an important component of treatment and can be integrated alongside cancer-directed therapy at any stage.
Care during pregnancy in patients with colon cancer is complex and requires coordination with oncology, obstetrics, maternal-fetal medicine, and surgery. Treatment decisions balance the mother’s oncologic needs with fetal safety.
Care is best coordinated by multidisciplinary oncology teams. Imaging, laboratory, pathology, and molecular findings are interpreted alongside the patient’s symptoms, examination, family history, and broader clinical context rather than in isolation.
Patient education plays an essential role. Understanding the diagnosis, the specific characteristics of the tumor including molecular features, the rationale for the recommended treatment plan, the importance of surveillance, the significance of hereditary syndromes when identified, treatment side effects, and warning signs of complications or recurrence all contribute to better outcomes.
Red flag symptoms include severe abdominal pain, symptoms of bowel obstruction (severe abdominal pain, distention, vomiting, inability to pass stool or gas), significant gastrointestinal bleeding (large amounts of visible blood, black tarry stools, or lightheadedness with bleeding), high fever with signs of severe infection, sudden severe pain (which may suggest perforation), signs of sepsis, severe difficulty breathing (which may suggest lung metastases or pulmonary embolism), new severe back pain with weakness, numbness, or bowel or bladder changes (which may indicate spinal cord compression from metastases), unintended significant weight loss, jaundice, or rapid clinical deterioration. These warrant prompt or urgent medical evaluation.