Colorectal Cancer
The colon and rectum are parts of the digestive system, which processes everything you eat and drink (see Figure 5). 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. Certain molecular and chromosomal abnormalities can now be identified with biomarker testing, and some treatments target these specific biological features in your type of colorectal cancer. Not all tumors will test positive for a biomarker.
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 look specifically for DNA errors and mutations caused by defective mismatch repair (dMMR) and microsatellite instability (MSI). When cells copy their DNA to a new cell, sometimes errors can occur. The body normally corrects these errors, but sometimes genetic mutations prevent the cells from correcting their mistakes, allowing altered or mutated cells to replace normal cells.
The first two immunotherapies approved for colorectal cancer are immune checkpoint inhibitors (see Exploring Immunotherapy). These medications are indicated for people who have unresectable, metastatic, microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) colorectal cancer that has progressed after treatment with chemotherapy. In July 2018, a combination of two checkpoint inhibitors was approved for children and adults who have MSI-H or dMMR metastatic colorectal cancer that has progressed after chemotherapy.