Colorectal cancer

What it is

Colorectal cancer is cancer of the colon or rectum and accounts for about 10% of all cancer cases worldwide. It usually starts as a growth in the inner lining of the large bowel, often from a polyp, and many polyps take about 10 to 15 years to become cancerous. This long precancerous phase is one reason screening and polyp removal can prevent some cancers, not just find them early. Colorectal cancer is also called bowel cancer or colorectal carcinoma. It is one of the most common cancers globally and a major cause of cancer death, but outcomes are much better when it is found at an early stage.

The colon and rectum are parts of the large intestine. Cancers in these sites are grouped together because they share many risk factors, symptoms, and treatment principles, although rectal cancer can differ from colon cancer in staging and local treatment. In India, colorectal cancer is less common than some other cancers such as breast, oral, or cervical cancer, but it is increasingly seen in urban settings and in younger adults as diet, obesity, inactivity, and lifespan patterns change.

A simple way to think about colorectal cancer is:

Site	Where it starts	Common note
Colon cancer	Most of the large intestine	Often managed first with surgery if localised
Rectal cancer	Last part of the large intestine, near the anus	May need radiation and chemotherapy in addition to surgery

How it works

Most colorectal cancers develop through a stepwise process in which normal cells acquire genetic and epigenetic changes over time. A common pathway is the adenoma-carcinoma sequence, where a benign adenomatous polyp gradually becomes cancer. Another pathway involves serrated polyps. Not every polyp turns into cancer, but some do, which is why colonoscopy matters.

Important biological mechanisms include changes in genes and signaling pathways that control cell growth, DNA repair, and programmed cell death. Inherited syndromes can strongly raise risk. The best known are Lynch syndrome, which involves mismatch repair defects, and familial adenomatous polyposis, which causes many polyps and a very high lifetime risk if untreated.

Risk rises with age, especially after 50, but colorectal cancer can occur earlier. WHO notes a rising burden in some adults aged 30 to 50 years. Lifestyle factors linked with higher risk include diets high in processed or red meat, low intake of fruits and vegetables, physical inactivity, overweight or obesity, smoking, and alcohol use. Personal history of colorectal polyps, inflammatory bowel disease, or prior colorectal cancer also increases risk.

Evidence and uses

The main evidence-based uses of screening are to detect colorectal cancer early and to find and remove precancerous polyps. Colonoscopy is the best-known test because it can both detect lesions and remove many polyps during the same procedure. Classic long-term evidence has shown that colonoscopic polypectomy reduces later colorectal cancer risk.

Common symptoms and signs include:

1. Change in bowel habits such as diarrhoea, constipation, or narrower stools
2. Blood in the stool, either bright red or dark
3. Abdominal pain, cramps, or bloating that persists
4. Unexplained weight loss
5. Fatigue or iron deficiency anaemia from chronic blood loss

Early colorectal cancer may cause no symptoms at all. That is why screening is important even when a person feels well.

Treatment depends mainly on stage, tumour location, and molecular features. A simplified overview is below:

Stage or setting	Main treatment approach
Localised colon cancer	Surgery is usually the main treatment
Localised rectal cancer	Surgery plus, in selected cases, chemotherapy and radiation
Higher-risk stage II or stage III	Surgery plus adjuvant chemotherapy in many patients
Metastatic disease	Systemic therapy such as chemotherapy, targeted therapy, and sometimes immunotherapy

Modern treatment may also use biomarker testing. Tumours with deficient mismatch repair or high microsatellite instability may respond better to certain immunotherapies. RAS, BRAF, HER2, and other markers can help guide treatment choices in advanced disease.

Diagnosis / how it's measured

Diagnosis usually starts with a clinical history, physical examination, blood tests, and visualisation of the bowel. Colonoscopy with biopsy is the standard way to confirm the diagnosis. Imaging is then used for staging.

Typical work-up may include:

Test	What it helps with
Colonoscopy	Finds the tumour, allows biopsy, can remove some polyps
Biopsy	Confirms cancer under the microscope
CBC and iron studies	May show anaemia from chronic bleeding
CEA blood test	Baseline tumour marker for follow-up in some patients
CT scan	Looks for spread to liver, lungs, lymph nodes, and other sites
MRI pelvis	Especially important for rectal cancer staging

Doctors stage colorectal cancer using the TNM system, which looks at how deeply the tumour has grown into the bowel wall, whether lymph nodes are involved, and whether it has spread to distant organs. Stage strongly affects prognosis and treatment planning.

Screening approaches vary by country and health system. They may include stool-based tests and colonoscopy. In India, access and uptake are uneven, and there is no single nationwide population screening programme comparable to some high-income countries. People with symptoms, strong family history, inherited syndromes, or inflammatory bowel disease usually need earlier and more individualised evaluation.

When to see a clinician

See a clinician promptly if you have blood in the stool, a persistent change in bowel habits, unexplained iron deficiency anaemia, ongoing abdominal pain, or unintentional weight loss. These symptoms do not always mean cancer, but they should not be ignored, especially if they last more than a few weeks.

Urgent assessment is also important if there is bowel obstruction, severe abdominal swelling, vomiting, inability to pass stool or gas, or heavy rectal bleeding. People with a first-degree relative with colorectal cancer, especially at a younger age, should ask whether they need earlier screening or genetic evaluation.

Limitations and open questions

Not all colorectal cancers follow the same biological pathway, and outcomes vary widely by stage, tumour site, and molecular subtype. Although screening clearly helps, the best strategy for younger adults and for settings with limited resources is still being studied. Evidence also continues to evolve on why early-onset colorectal cancer is increasing in some populations.

Another limitation is that symptoms are nonspecific. Common problems such as piles, irritable bowel symptoms, or anaemia from other causes can delay diagnosis. Access to colonoscopy, pathology, MRI for rectal cancer, and biomarker testing is also uneven in many parts of India.

Research is active in prevention, microbiome links, blood-based screening tests, and more precise use of targeted therapy and immunotherapy. These advances are promising, but they do not replace timely evaluation of symptoms and standard screening where available.