Mary Jo was diagnosed with colon cancer at age 42 after undergoing regular colonoscopies because of her personal and family background. Between 2018 and 2021, she experienced five recurrences. Genetic testing confirmed that she has Lynch syndrome, an inherited condition that can increase the risk of colorectal cancer and several other cancers.
As part of her care, Mary Jo and her oncologist, John Strickler, MD, used circulating tumor DNA (ctDNA) to monitor for signs of cancer activity in her blood. After ctDNA results remained at zero for six months, her levels began to rise. That change helped signal that the cancer may be returning and prompted the care team to develop a new plan.
Mary Jo’s tumor was MSI-high, also called microsatellite instability-high. This biomarker can help guide treatment decisions in colorectal cancer. Because of the characteristics of her cancer, her team recommended immunotherapy rather than traditional chemotherapy. After three months of treatment, her ctDNA levels became undetectable. She completed one year of immunotherapy, and her ctDNA results have remained negative after treatment ended.
Every person’s cancer is different. Mary Jo’s story shows how genetic testing, tumor biomarker testing, ctDNA results, scans, and conversations with an oncology team can work together to help personalize care.
What you’ll hear
- Mary Jo’s experience being diagnosed with colon cancer at age 42
- Why she continued regular colonoscopies and underwent genetic testing
- Her diagnosis of Lynch syndrome and its relevance to cancer risk
- How ctDNA testing was used to monitor for signs of cancer activity in the bloodstream
- Why her rising ctDNA level prompted further discussion and planning with her oncology team
- What MSI-high means and why it can be an important biomarker in colorectal cancer
- Why her care team recommended immunotherapy rather than traditional chemotherapy
- How ctDNA results, imaging, and clinical follow-up informed her ongoing surveillance
- Mary Jo’s perspective on moving beyond fear and focusing on life after cancer
What is ctDNA?
Circulating tumor DNA, or ctDNA, refers to small fragments of DNA from cancer cells that can sometimes be detected in the bloodstream. A ctDNA test uses one or more tubes of blood to look for DNA fragments that match a person’s tumor.
For some people with colorectal cancer, ctDNA testing may provide additional information about possible residual cancer, recurrence risk, or disease activity. A result can sometimes become positive before cancer is visible on a scan. However, ctDNA testing does not replace CT scans, pathology findings, other laboratory studies, or medical judgment. Results should always be interpreted in the context of the individual patient’s cancer, treatment history, symptoms, and follow-up plan.
Dr. Strickler explains that testing is most useful when patients and care teams discuss its purpose in advance, including what a positive result, negative result, or changing trend may mean and what the next step could be.
Understanding MSI-high disease
MSI-high, short for microsatellite instability-high, is a tumor biomarker that may be identified through testing of the cancer. MSI-high tumors have many genetic changes, which can make them more visible to the immune system.
In colorectal cancer and other gastrointestinal cancers, MSI-high status can be especially important because it may indicate that a patient could benefit from immunotherapy. Immunotherapy uses the body’s immune system to recognize and attack cancer cells.
Mary Jo had Lynch syndrome, an inherited condition associated with a higher likelihood of MSI-high colorectal cancer. Her oncology team considered this information, along with her cancer history and ctDNA results, when recommending an immunotherapy-based treatment plan.
Not every person with colorectal cancer has an MSI-high tumor, and not every patient with MSI-high disease will have the same treatment plan or outcome. Biomarker testing and treatment choices should be discussed with an oncology team.
Why a personalized plan matters
For Mary Jo, ctDNA testing provided one part of a broader picture that included her history of recurrent cancer, Lynch syndrome, tumor biology, imaging, and ongoing clinical follow-up. Her rising ctDNA level helped her care team recognize the need to reassess her situation and determine the most appropriate next step.
After she began immunotherapy, her ctDNA results became undetectable within several months and remained negative after treatment ended. Her care team also continued to use scans and clinical follow-up to monitor her health.
A ctDNA result alone does not determine whether cancer has returned, whether treatment is needed, or which treatment is best. The value of testing lies in using the information thoughtfully, with a defined plan and shared decision-making between the patient and oncology team.
Questions to ask your care team
If you have colorectal cancer, a personal or family history of cancer, or questions about biomarkers and ctDNA testing, you may want to ask:
- Should I have genetic counseling or genetic testing for an inherited cancer syndrome such as Lynch syndrome?
- Has my tumor been tested for MSI or mismatch-repair deficiency, often called dMMR?
- What do my MSI or MMR test results mean for my treatment options?
- Is ctDNA testing appropriate for my cancer stage, treatment history, and surveillance plan?
- What would my care team do if my ctDNA result becomes positive or begins to rise?
- What would an undetectable ctDNA result mean in my situation?
- Would I still need CT scans, colonoscopies, laboratory tests, and regular appointments?
- Could immunotherapy be appropriate for my cancer?
- Are there clinical trials studying ctDNA-guided care, immunotherapy, Lynch syndrome, or MSI-high colorectal cancer that may be relevant to me?
Mary Jo’s experience highlights the importance of personalized care in colorectal cancer. Ask your oncology team whether hereditary cancer testing, tumor biomarker testing, and ctDNA monitoring may help inform your individual treatment or surveillance plan.
Patient information disclaimer
Mary Jo’s experience is personal and may not represent the experience of every person with colorectal cancer, Lynch syndrome, MSI-high disease, or recurrent cancer. ctDNA testing, biomarker testing, genetic testing, and immunotherapy are not appropriate for every patient. ctDNA results do not replace imaging, pathology, laboratory testing, routine follow-up, or individualized guidance from an oncology team. Patients should discuss their diagnosis, testing options, treatment choices, and surveillance plan with their doctor or another qualified healthcare professional.
This educational resource was made possible by Natera, OmniOncology, and The Personalized Medicine Foundation.
