Colorectal cancer: Full guide

What is colorectal cancer?

Colorectal cancer is cancer that starts in the colon or the rectum. The colon is the long part of the large intestine. The rectum is the last few inches of the intestine, just above the anus. Together they are called the large bowel.

Most colorectal cancers start as a polyp. A polyp is a small growth on the inside lining of the bowel. Over many years, some polyps can turn into cancer. This is why screening tests that find and remove polyps can prevent cancer.

Colon cancer and rectal cancer are closely related. They are often treated with the same medicines. But the rectum sits deep in the pelvis, close to other organs. So rectal cancer is often treated with radiation and chemotherapy before surgery, while colon cancer usually starts with surgery.

Colorectal cancer is common in both men and women. It is becoming more common in adults under 50. Screening for people at average risk now starts at age 45.

Some people get colorectal cancer because of a gene change passed down in their family. The most common one is called Lynch syndrome. Your care team may suggest genetic testing for you and your family.

Types

Colon cancer

Cancer that starts in the colon. Most colon cancers are a type called adenocarcinoma, which starts in the cells that make mucus.

Rectal cancer

Cancer that starts in the rectum. Because the rectum is in a tight space in the pelvis, treatment often includes radiation, chemotherapy, and surgery. How close the tumor is to the anus affects the surgery options.

Right-sided vs left-sided colon cancer

The right side of the colon is the first part food waste passes through. Right-sided and left-sided cancers can act differently and may respond differently to some medicines. Your care team will tell you which side your cancer is on.

MSI-high or dMMR cancer

In a small group of colorectal cancers, the cells cannot fix mistakes in their DNA. These cancers often respond very well to immunotherapy. About 1 in 5 of them are linked to Lynch syndrome.

Mucinous and signet ring cell cancer

These are less common forms of adenocarcinoma. They make a lot of mucus. Your pathology report will say if your cancer is one of these.

Hereditary colorectal cancer

A small number of cancers are caused by an inherited gene change. Examples are Lynch syndrome and familial adenomatous polyposis (FAP), which causes hundreds of polyps. Family members may need earlier and more frequent screening.

Appendix cancer

A rare cancer of the appendix, a small pouch attached to the colon. Some types are treated much like colon cancer.

Tests you may have

Colonoscopy

A doctor uses a thin, flexible tube with a camera to look at the whole colon and rectum. They can take a small tissue sample (biopsy) and remove polyps. It also checks for a second tumor elsewhere in the bowel.

Biopsy

A small piece of tissue is taken and looked at under a microscope. This is how cancer is confirmed. The same tissue is used for gene and marker tests.

CT scan

A detailed X-ray of the chest, belly, and pelvis. It checks whether cancer has spread to places like the liver, lungs, or lymph nodes.

Pelvic MRI (for rectal cancer)

A scan that uses magnets to make detailed pictures of the rectum and the tissue around it. It is the main test to stage rectal cancer. It helps your team decide whether you need radiation or chemotherapy before surgery.

Endorectal ultrasound

A small probe placed in the rectum uses sound waves to see how deep a rectal tumor has grown. It is most useful for small, early tumors.

CEA blood test

CEA is a protein some colorectal cancers release into the blood. It is checked at diagnosis and during follow-up. A rising level can be a sign the cancer is growing, but it is not perfect.

Tumor gene and marker tests

Lab tests on your tumor tissue look for MMR/MSI status and, if the cancer has spread, gene changes like RAS, BRAF, and HER2. These results help choose the best treatment.

Germline (inherited) genetic testing

A blood or saliva test that looks for gene changes you were born with, like Lynch syndrome. The results can matter for you and your relatives.

DPYD test

A blood test done before some chemotherapy pills or infusions (5-FU or capecitabine). It checks whether your body can break down these drugs safely. Some people need a lower dose or a different drug.

ctDNA blood test

A blood test that looks for tiny bits of tumor DNA. After surgery, it may show whether cancer cells remain. How best to use it to guide treatment is still being studied.

What the stage means

The stage tells you how far the cancer has grown and whether it has spread. It is based on how deep the tumor goes into the bowel wall, whether lymph nodes have cancer, and whether it has spread to other organs. Lymph nodes are small bean-shaped glands that are part of the immune system.

For colon cancer, the final stage is often known only after surgery, when the tumor and lymph nodes are checked under a microscope. For rectal cancer, the stage is usually set before treatment, using MRI.

Stage is one important piece of information, but it is not the whole story. Your gene test results, your overall health, and your own goals all matter too.

Stage 0

Very early cancer found only in the inner lining. It can often be removed during a colonoscopy.

Stage 1

The cancer has grown into the bowel wall but not through it, and lymph nodes are clear. Surgery alone is usually the treatment.

Stage 2

The cancer has grown through the bowel wall or into nearby tissue, but lymph nodes are clear. Some people need chemotherapy after surgery, depending on risk features. Rectal cancer at this stage often gets treatment before surgery.

Stage 3

The cancer has spread to nearby lymph nodes, but not to distant organs. For colon cancer, chemotherapy after surgery lowers the chance of the cancer coming back. For rectal cancer, chemotherapy and often radiation are given before surgery.

Stage 4

The cancer has spread to distant organs, such as the liver, lungs, or the lining of the belly (peritoneum). Treatment is usually medicine given through the whole body. Some people with only a few spots in the liver or lungs can have them removed with surgery, with the goal of cure.

Tumor markers and gene tests

Your tumor tissue should be tested for certain markers. These results can change your treatment in a big way. Every colorectal cancer should be tested for MMR/MSI. If the cancer has spread, it should also be tested for RAS, BRAF, and HER2.

Some results tell your team which medicines will likely work and which ones will not. Ask your care team for your results and what they mean for you.

MMR / MSI

MMR stands for mismatch repair, the system that fixes mistakes in DNA. If it is broken, the tumor is called dMMR or MSI-high (MSI-H). These cancers often respond very well to immunotherapy. Most colorectal cancers are pMMR or MSS, which means the repair system works.

Lynch syndrome

An inherited gene change in the DNA repair system. A dMMR tumor can be a sign of Lynch syndrome, so more testing may be advised. If you have Lynch syndrome, your relatives may need testing and earlier screening.

RAS (KRAS and NRAS)

RAS genes act like an on switch for cell growth. If your tumor has a RAS change, medicines called anti-EGFR antibodies (cetuximab or panitumumab) will not work. About half of spread colorectal cancers have a RAS change.

KRAS G12C

A specific type of KRAS change found in a small number of colorectal cancers. There are targeted pills for it, given together with an anti-EGFR antibody, usually after other treatments.

BRAF V600E

A gene change found in a small group of colorectal cancers. It can make the cancer grow faster. There is now a targeted treatment combination for it, and it can be used as a first treatment for spread cancer.

HER2

A protein that some colorectal cancers have too much of. If your tumor is HER2-positive, there are HER2-targeted medicines that may help.

NTRK fusion

A rare gene change. If it is found, there are pills that target it directly.

Tumor side (left or right)

Not a gene test, but it matters. For spread cancer without RAS or BRAF changes, left-sided tumors often do better with anti-EGFR antibodies. Right-sided tumors often do better with bevacizumab.

Main treatments

Surgery for colon cancer

The surgeon removes the part of the colon with the cancer plus nearby lymph nodes. The two ends of the bowel are then joined. This is the main treatment for colon cancer that has not spread.

Surgery for rectal cancer

The standard surgery is called total mesorectal excision (TME). It removes the rectum and the fatty tissue around it. Many people keep their normal bowel opening. For tumors very close to the anus, the anus may need to be removed, and a permanent colostomy (an opening on the belly where stool leaves the body into a bag) is made. Some people need a short-term ostomy while they heal.

Chemotherapy

Medicines that kill fast-growing cells. Common ones for colorectal cancer are 5-FU (given by pump), capecitabine (a pill), oxaliplatin, and irinotecan. They are often combined, with names like FOLFOX, CAPOX, and FOLFIRI. After colon cancer surgery, chemotherapy may lower the chance of the cancer coming back.

Radiation therapy

High-energy beams aimed at the tumor to kill cancer cells. It is mostly used for rectal cancer, often before surgery, to shrink the tumor and lower the chance it comes back in the pelvis. It can be a short course (about 1 week) or a long course (about 5 to 6 weeks) with chemotherapy.

Total neoadjuvant therapy (TNT)

For many people with stage 2 or 3 rectal cancer, all the chemotherapy and radiation are given before surgery. This helps more people finish their treatment. In some people, the tumor disappears completely. Some people with lower-risk rectal cancer may be able to skip radiation if the tumor shrinks well with chemotherapy.

Watch-and-wait

If a rectal tumor seems to be completely gone after treatment, some people can skip surgery and be watched very closely instead. This means frequent exams, scopes, and MRI scans. In some people the tumor grows back, and surgery can usually still be done then. This approach is best done at a center with experience.

Immunotherapy

Medicines that help your immune system attack the cancer, such as pembrolizumab, nivolumab, ipilimumab, and dostarlimab. They work very well for dMMR/MSI-high cancers, but they usually do not help other colorectal cancers. For spread dMMR cancer, immunotherapy is now preferred over chemotherapy as a first treatment. For early dMMR rectal cancer, immunotherapy alone has made the tumor disappear in many people, so some can avoid surgery, radiation, and chemotherapy. This approach is still new and is best done at an experienced cancer center.

Targeted therapy

Medicines aimed at a specific part of the cancer. Bevacizumab blocks blood vessel growth to the tumor. Cetuximab and panitumumab block a growth signal called EGFR, and only work if there is no RAS or BRAF change. Other targeted drugs treat BRAF, HER2, or KRAS G12C changes.

Treatment for spread (stage 4) cancer

Most people get a combination of chemotherapy and a targeted drug, chosen by gene test results and tumor side. After some months, treatment is often lightened to a gentler maintenance plan. If the cancer grows, there are several more options, including trifluridine/tipiracil with bevacizumab, regorafenib, and fruquintinib.

Surgery or local treatment for spread cancer

If cancer has spread only to a few spots in the liver or lungs, removing them may offer a chance of long-term control or cure. Ask to be seen by a team with experience in this. Radiation, stents, or an ostomy may also be used to ease symptoms like a blocked bowel or bleeding.

Common side effects and what helps

Numbness and tingling (neuropathy)

Oxaliplatin can cause numbness, tingling, or pain in the hands and feet. It is often worse with cold, so wear gloves, avoid cold drinks, and do not touch ice right after treatment. Tell your team early. They may shorten or stop the oxaliplatin to protect your nerves.

Diarrhea

Common with chemotherapy, radiation to the pelvis, and some targeted drugs. Drink plenty of fluids and use the anti-diarrhea medicine your team recommends. Call if you have many loose stools a day, cannot keep fluids down, or feel dizzy.

Tiredness (fatigue)

Feeling very tired is common. Gentle daily activity, like walking, can help. Plan rest breaks and accept help with chores. Being active after treatment has also been linked to better outcomes.

Low blood counts and infection

Chemotherapy can lower the white blood cells that fight infection. Wash your hands often and avoid people who are sick. A fever can be an emergency during chemotherapy.

Hand-foot syndrome

Capecitabine and regorafenib can make palms and soles red, sore, or peeling. Use thick moisturizing cream, wear soft shoes, and avoid hot water and rubbing. Tell your team if walking or using your hands becomes hard.

Skin rash

Cetuximab and panitumumab often cause an acne-like rash on the face and chest. Your team may start preventive creams and an antibiotic pill before the rash appears. Use sunscreen and gentle, fragrance-free skin products.

Mouth sores

5-FU and capecitabine can cause sore mouth. Rinse with a salt and baking soda rinse, use a soft toothbrush, and avoid spicy or acidic foods.

Immune side effects

Immunotherapy can cause the immune system to attack healthy organs. This can cause diarrhea, rash, cough, tiredness, or hormone problems. These can show up even months later. Always tell any doctor that you have had immunotherapy.

Bowel and sexual changes after rectal treatment

Radiation and rectal surgery can cause urgency, frequent stools, or trouble controlling stool. They can also affect sex, erections, and fertility. Ask about fertility before treatment starts. Pelvic floor therapy and diet changes can help bowel control.

Living with an ostomy

Some people have a temporary or permanent ostomy bag. An ostomy nurse (WOC nurse) can teach you how to care for it and find the right supplies. Many people return to work, travel, exercise, and intimacy with an ostomy.

Living with and after colorectal cancer

After treatment, you will have regular follow-up visits. These often include exams, CEA blood tests, CT scans for a few years, and colonoscopies. The first follow-up colonoscopy is usually about 1 year after surgery. If you had watch-and-wait for rectal cancer, your checkups will be much more frequent.

Staying active, keeping a healthy weight, eating more fiber, vegetables, and whole grains, and limiting red and processed meat, alcohol, and smoking are good for your overall health. Exercise after colon cancer treatment has been linked to better outcomes.

Some side effects, like numbness in the feet or changes in bowel habits, can last a long time. Tell your team about them. There are often ways to help.

If your cancer is linked to an inherited gene change, your children, brothers, sisters, and parents may need genetic counseling and earlier screening. Even without a gene change, close relatives usually should start screening earlier than age 45. Ask your team what your family should do.

If your cancer has spread, many people live for years with ongoing treatment, and new options keep coming. Palliative care is extra support for symptoms and stress. It can be given at any stage, alongside cancer treatment. Emotional support, support groups, and counseling can help you and your caregivers too.