Precision Medicine in Colorectal Cancer: Why Biomarkers Matter
Two people can hear the same words, "you have colorectal cancer," and still be facing very different diseases. Precision medicine, sometimes called personalized medicine, is built on that reality. It is an approach in which a person's tumor biomarkers are used to help diagnose and treat their cancer and to guide follow-up care.
Cancer treatment has long been guided by where a tumor starts and what type of cell it comes from. Precision medicine adds another layer: the tumor's molecular makeup. As understanding of cancer has grown, it has become clear that two colorectal tumors can be very different at the molecular level, while tumors in different parts of the body can share the same biomarkers. Knowing a tumor's biomarkers helps care teams choose treatments more likely to work, avoid ones unlikely to help or likely to cause severe side effects, and find clinical trials that fit. It all begins with one essential step: biomarker testing.
Every Cancer Has a Story to Tell
A biomarker, also called a biological marker, is a piece of information about a person's health. Blood pressure and blood type are biomarkers. In cancer, they are also known as tumor markers, and they can tell a medical team important information about a patient and their cancer.
In colorectal cancer, biomarkers include substances a tumor releases into the blood, such as circulating tumor DNA (ctDNA), the location of the tumor, and changes (mutations) in the tumor's genes. Together, they help care teams determine prognosis, guide treatment decisions, monitor treatment response, and look for recurrence after treatment.
Matching the Treatment to the Tumor
Targeted therapy uses drugs to target cancer cells directly and more precisely, ideally causing less harm to normal cells. Whether a targeted therapy may work for a given patient depends on what biomarker testing finds.
Tumors with wild-type (normal, non-mutant) KRAS and NRAS may be treated with EGFR inhibitors. Cancers with a BRAF V600E mutation may be treated with BRAF inhibitors, usually given with MEK inhibitors. Colorectal cancer with HER2 amplification or overexpression can be treated with HER2 inhibitors, and KRAS inhibitors have been approved for colorectal cancer with a KRAS G12C mutation. Even rarer findings, like NTRK fusion and RET abnormalities, also have targeted options. Plain-language explanations of more than 15 individual colorectal cancer biomarkers and their implications for treatment are available at knowyourbiomarker.org.
Unlocking the Immune System
Immunotherapy helps the body's own immune system kill cancer cells. In colorectal cancer, this means immune checkpoint inhibitors, and once again, biomarkers point the way.
Colorectal cancers with microsatellite instability (MSI-High) are particularly susceptible to immunotherapy, and immune checkpoint inhibitors are a first-line treatment for metastatic MSI-High disease. An MSI-High result also means the patient will need genetic testing for Lynch syndrome, a hereditary cancer syndrome. About 85% of colorectal cancers are microsatellite stable (MSS) and do not typically respond to these drugs. However, some people with MSS who also have a high tumor mutational burden (TMB-High) may respond, and clinical trials are exploring new combinations for MSS disease.
Why Knowing Is Worth the Wait
Biomarker results typically take one to three weeks. After a diagnosis, that can feel like forever. Yet more than 60% of colorectal cancer patients start treatment before their biomarker results arrive, which means decisions are being made without the full picture.
Closing that gap matters. Those results can unlock targeted therapies, clinical trial eligibility, and safer treatment plans. Some biomarkers, like DPYD and UGT1A1, can even flag a patient's risk of severe side effects from certain chemotherapy drugs. The wait is not hesitation. It is precision.
Testing for Every Patient
Everyone with colorectal cancer should have biomarker testing. All patients, at any stage, should be tested for MSS/MSI. People with stage IV (metastatic) disease should also be tested for KRAS, NRAS, BRAF, and HER2.
Every patient deserves that information. That is why patients and caregivers are encouraged to ask their care team before treatment begins: Has my tumor been tested? Which biomarkers? When will my results be ready? Precision medicine starts with a question, and the answer is worth the wait. Questions to ask the healthcare team are available at knowyourbiomarker.org.