The colon and rectum are parts of the digestive system, which processes everything you eat and drink. Colorectal cancer begins when healthy cells in the inner lining of the colon or rectum mutate and grow uncontrollably, forming a mass known as a primary tumor. Most colorectal cancers are thought to start as a polyp, or an abnormal growth in the lining of the colon or rectum. Cancer that begins in the colon is called colon cancer, and cancer that begins in the rectum is called rectal cancer.
Treatment options for colorectal cancer are surgery, radiation therapy, chemotherapy, targeted therapy and immunotherapy, which may be used alone or in combination.
Your doctor may use biomarker or genetic testing to determine the best treatment option for you (see Gastrointestinal Anatomy). Testing for biomarkers in colorectal cancer is used to plan treatment. The most common biomarkers in colorectal cancer are RAS, KRAS and NRAS mutations. Your doctor may also test for the BRAF mutation, human epidermal growth factor receptor-2 (HER2) overexpression and Lynch syndrome, which is an inherited disorder that increases your risk of colorectal cancer. Your doctor will also look specifically for DNA errors and mutations caused by high microsatellite instability (MSI-H) and deficient mismatch repair (dMMR) to determine eligibility for immunotherapy.
The immunotherapies approved for colorectal cancer are immune checkpoint inhibitors. These medications are indicated for people who have unresectable, metastatic MSI-H or dMMR colorectal cancer as first-line therapy or after it has progressed after treatment with chemotherapy. A combination of two immune checkpoint inhibitors is approved for children and adults who have MSI-H or dMMR metastatic colorectal cancer that has progressed after chemotherapy.