20 NCCN Guidelines for Patients ® : Colon Cancer, 2018 2 Treatment planning Tumor marker testing Tumor marker testing Just like each person’s DNA is unique, each person’s cancer is unique. This means that a treatment that helps one person might not help you. To find out if certain treatments might help you, your doctor may offer you tumor marker testing. This is also called biomarker (short for biological marker) testing. Tumor markers can be substances, like molecules or proteins, that are made by your body because you have cancer. Tumor markers can also be processes, such as the way your DNA “acts” that makes it unique. To find out if your cancer has any markers, the primary tumor removed during surgery is tested in a laboratory. MMR deficiency Some people have tumors that have developed a problem making them unable to fix damaged DNA. In normal cells, a process called MMR fixes errors that happen when the DNA divides and makes a copy of itself. If a cell’s MMR system isn’t working right, errors build up and cause the DNA to become unstable. This is called MSI ( m icro s atellite i nstability). There are two kinds of laboratory tests for this tumor marker. Depending on which method is used, if you have this genetic defect the result will either be MSI-H ( m icro s atellite i nstability h igh) or dMMR ( m is m atch r epair d eficient). Both results mean the same thing. NCCN experts recommend testing for this tumor marker in all people with colon cancer for two Figure 8 MRI MRI makes pictures of areas inside the body without using radiation. Not everyone with colon cancer will need an MRI. Your doctor may order it to help determine if you have colon or rectal cancer, or if results of other imaging tests were unclear.