The colon and rectum make up the large intestine, which is the final part of the digestive or ‘gastrointestinal’ system. Colorectal cancer is cancer that develops in the colon or rectum. Doctors may also call colorectal cancer ‘bowel cancer’, ‘colon cancer’ or ‘rectal cancer’, based on where the cancer started.
Colorectal cancer starts when cells lining the large intestine and bowels grow too fast and the body cannot control their growth. This happens because of errors that develop in the genes, the chemical inside cells that tells them how to behave. If the broken cells carry on growing, this can make a lump of cancer cells called a tumour.
More than 9 out of 10 colorectal cancers start in the lining of the large intestine and bowel, which contains special cells that produce a slippery substance called mucus to help the poo travel through the digestive system. These colorectal cancers are called ‘adenocarcinomas’, which means a cancer that starts in cells of the body that produce fluids.
Other types of colorectal cancer, which do not happen very often, are ‘sarcomas’ and ‘lymphomas’. These rare types of colorectal cancer are treated in a different way than adenocarcinomas.
It is not fully understood what leads to the development of colorectal cancer, but there are some things that can increase a person’s chances of getting the disease, called ‘risk factors’. Some risk factors are: 
The symptoms of colorectal cancer can be different from person to person, but some examples of the main symptoms are: 
If a person has symptoms of colorectal cancer, their doctor will send them for a short hospital examination that looks at the back passage and some of the large bowel using a long, thin, flexible tube attached to a very small camera and light. This procedure is called a ‘flexible sigmoidoscopy’. The camera sends images to a screen and can be used to take small tissue samples for further analysis (called ‘biopsies’).
A small number of cancers can only be diagnosed after more tests. A longer tube will be used to look at more of the bowel in a test called a ‘colonoscopy’. If the colonoscopy tube cannot be passed all of the way around the bowel, it may be necessary to scan the bowel and rectum to create 3D images using ‘CT colonography’.
If a diagnosis of colorectal cancer is made, then more tests will usually be done to check whether the cancer is only in the bowels, or has spread to other parts of the body. Once the tests have been done, colorectal cancer is normally ‘graded’ into stages depending on a number of things, including whether the cancer has spread to nearby lymph nodes or other parts of the body.
A tumour limited to the bowel might be called ‘early’ and is often removed by surgery. If the cancer has spread to other parts of the body (e.g. the liver or bones), it will be called ‘advanced’. Colorectal cancer most commonly spreads to the liver, but sometimes it might spread to other organs like the lungs or brain.
If a person is diagnosed with advanced colorectal cancer, their chances of surviving for 5 years or more are much lower than those of a person with an early stage of the disease.  Around 1 in 10 patients with advanced colorectal cancer will survive for 5 years or more, compared with around 9 in 10 patients with an early stage of the disease.
There are quite a few treatments for advanced colorectal cancer. Most of these treatments will probably not cure the disease  but many treatments can slow the cancer down and help people to live longer. The type of treatment a patient gets will depend on things like:
Patients with advanced colorectal cancer might have surgery, but this is only used when the cancer has not spread at all or has only spread to very limited parts of the body. Surgery is sometimes done after treatment with chemotherapy, which can be used to try to make the tumour smaller and slow down the disease.
Patients with advanced colorectal cancer will normally be given chemotherapy to try to help keep the cancer under control. There are many different chemotherapy drugs that can be used to treat advanced colorectal cancer, and these all work by killing cells that are growing very quickly, including cancer cells. Some drugs can be taken as a tablet and some drugs (or mixtures of drugs) are injected directly into the bloodstream through a patient’s vein.
‘Targeted therapies’ are drugs that are made to slow down cancers by attacking the way cancer cells grow. There are two main types of targeted therapies for colorectal cancer, and these are used along with chemotherapy to make them better at fighting the cancer.
Colorectal cancers often produce more of a ‘cell messenger’ called ‘VEGF’, a chemical that calls for new blood vessels to grow and help feed the tumour. Some targeted therapies called ‘VEGF inhibitors’ can stop VEGF working and help to starve the tumour.
Some colorectal cancer cells have a lot of a molecule called ‘EGFR’ on the outside of the cell, and this can let the cancer cells grow quickly. Blocking this with drugs called ‘EGFR inhibitors’ can slow down the growth of cancers. The doctor would first do a genetic test to find out if EGFR inhibitors might work, as this treatment does not apply to all patients.
Tyrosine kinase inhibitors
Another type of targeted therapies for colorectal cancer are drugs called ‘TKIs’. These are drugs that stop the cell’s instructions telling it to carry on growing when it should not.
‘Cancer immunotherapy’ or CIT is one of the newest ideas for treating colorectal cancer. These drugs help the body to use its own immune system to fight the cancer. At the moment, these drugs are only prescribed if the doctor has already tried other treatments like chemotherapy or targeted therapies.
Additional tests are usually required to decide whether CIT might work for a person before giving it: currently, only 1 patient in 20 might benefit from CIT. Doctors hope that soon more people will be able to get help from CIT by adding these drugs to other cancer treatment drugs. This is known as ‘combination therapy’ and clinical trials are being conducted to test if more patients with advanced colorectal cancer can be helped by using this type of treatment.