Colon cancer and rectal cancer: how diagnosis and treatment differ
By Dr Wong Nan-Yaw, MBBS (NUS), FRCS (Edinburgh). Published . 6 min read.
Colon and rectal cancer start the same way and are both diagnosed by colonoscopy and biopsy. The difference is location: the rectum lies in the pelvis near the anus, so rectal cancer needs detailed pelvic imaging and is more often treated with radiotherapy or chemoradiation before surgery (NCI, NHS). Dr Wong Nan-Yaw is a colorectal surgeon who treats both.
Where are the colon and the rectum?
The colon is the main part of the large bowel. The rectum and anal canal form its last section, ending at the anus (US National Cancer Institute). Cancers starting in either place are called colorectal or bowel cancer, but position matters, because the rectum sits deep in the pelvis, close to the anus and to other pelvic organs.
The two cancers share risk factors, including a close relative with colorectal cancer, a past history of large or abnormal polyps, long-standing ulcerative colitis or Crohn's disease, smoking, heavy alcohol use and obesity (NCI). They also share symptoms: blood in the stool, a lasting change in bowel habit, tummy pain, weight loss and tiredness from anaemia (NHS).
How is the diagnosis made for each?
Both are diagnosed the same way. A colonoscopy looks along the whole large bowel, and a biopsy taken during the test confirms the diagnosis under the microscope. A rectal examination may also be done. The tumour tissue can be tested for faults in DNA repair genes, which can point to an inherited condition such as Lynch syndrome and guide treatment.
The NHS describes colonoscopy as the main test for bowel cancer, with a biopsy taken during the procedure. Some results are available on the day; others take days or weeks.
Once cancer is confirmed, further tests usually follow, such as a CT scan, an MRI or PET scan, blood tests and genetic testing (NHS). The NCI notes that some people choose to get a second opinion on the diagnosis and treatment plan, which needs copies of the reports, slides and scans.
How does staging differ between colon and rectal cancer?
Staging finds out how far the cancer has spread, and it decides the treatment. Both cancers are staged with CT scans of the chest, abdomen and pelvis and blood tests such as CEA. Rectal cancer staging adds a close look at the rectum itself, for example with MRI or an ultrasound probe placed in the rectum (NCI).
| Colon cancer | Rectal cancer | |
|---|---|---|
| First test | Colonoscopy and biopsy | Colonoscopy and biopsy, often with a rectal examination |
| Scans for spread | CT scan; sometimes PET or MRI | CT scan, plus MRI of the rectum and sometimes endorectal ultrasound |
| Usual first treatment for early stages | Surgery to remove the affected section | Surgery, or local removal for very early tumours |
| Treatment before surgery | Sometimes chemotherapy, to shrink the tumour | Often chemoradiation, short-course radiotherapy or chemotherapy for stage II and III |
| Radiotherapy | Mainly if the cancer has spread | Commonly part of treatment |
| Stoma | Sometimes needed; temporary or permanent | Temporary or permanent, depending largely on how close the tumour is to the anus |
Sources for this table: NCI patient summaries for colon and rectal cancer treatment, and NHS pages on bowel cancer tests and treatment. Individual plans vary with the stage, tumour features and your general health.
How is colon cancer usually treated?
Surgery is the main treatment for colon cancer that has not spread. The surgeon removes the section of colon containing the cancer with nearby lymph nodes, then joins the healthy ends, a step called an anastomosis. For stage III cancer, where lymph nodes are involved, surgery may be followed by chemotherapy to lower the risk of the cancer returning.
According to the NCI, treatment of stage I and II colon cancer may include resection and anastomosis, and stage III treatment adds chemotherapy after surgery in many cases. Very early cancers found within a polyp can sometimes be removed during colonoscopy.
If the bowel ends cannot be joined safely, a stoma is made so that waste passes into a bag on the abdomen. The NCI notes this is sometimes needed only until the bowel has healed and can then be reversed, but may be permanent if the entire lower colon is removed.
Why does rectal cancer often need treatment before surgery?
Treatment given before surgery can shrink a rectal tumour, make it easier to remove completely, and help preserve bowel control afterwards (NCI). This is why many people with stage II or III rectal cancer have chemoradiation, short-course radiotherapy or chemotherapy first. The NHS lists radiotherapy for cancer in the rectum, or cancer that has spread.
These are the options the NCI lists for stage II and III rectal cancer. Which is suitable depends on the tumour's position and stage on MRI, its genetic features and your own priorities, and it is planned with your specialist treatment team (NHS).
- Chemoradiation followed by surgery
- Short-course radiotherapy followed by surgery and chemotherapy
- Chemotherapy alone followed by surgery, for some lower-risk cancers
- Chemoradiation followed by close monitoring, with surgery if the cancer returns
- Immunotherapy for tumours with a defect in DNA repair genes
Will I need a stoma after rectal cancer surgery?
Not always. After removing a rectal cancer, the surgeon joins the remaining bowel when it is safe to do so. A temporary stoma is sometimes made to protect the join while it heals, and is later reversed. A permanent colostomy may be needed if the cancer is too close to the anus and the whole rectum must be removed (NCI).
The NHS notes that a colostomy or ileostomy may be temporary or permanent depending on the operation. Ask your surgeon how close the tumour is to the anus and what that means for your own operation.
Sources
Medically reviewed by Dr Wong Nan-Yaw
MBBS (NUS), FRCS (Edinburgh). 33 years in medicine (since 1993), 23 years as an accredited specialist (since 2003) and 18 years in private practice at Mount Elizabeth (since 2008). Last reviewed 6 November 2026.
How this article is prepared
Statements are checked against the sources listed above. This article is general education and does not replace a consultation. It is reviewed and updated when guidance changes. In an emergency, call 995.