For MSI-high patients responding to immunotherapy, a lingering mass on a CT scan may not be active cancer. A negative ctDNA test can help confirm that the visible lesion is likely just scar tissue, potentially averting unnecessary surgery.
Tumor-informed assays like Signatera sequence a patient's tumor to create a personalized test, making it highly sensitive but taking 3-4 weeks. Tumor-uninformed assays are faster (1 week) but less sensitive as they screen for a generic panel of cancer mutations.
Circulating tumor DNA (ctDNA) testing is described as unequivocally the most prognostic tool available for colorectal cancer. Patients who remain serially negative have a minimal recurrence risk, while a positive result almost universally predicts a future clinical recurrence by 6-8 months.
A powerful counseling technique for complex adjuvant therapy decisions is to ask patients: "If your cancer recurs, will you look back and regret the choice you're making today?" This forces patients to confront their own risk tolerance and helps them commit to a treatment path.
For every 10 Stage III patients receiving adjuvant chemo, 5 are already cured by surgery and 2-3 will recur regardless. This means therapy only benefits 2-3 patients, leading to significant overtreatment for the majority who endure toxicity without benefit.
With colorectal cancer rates rising in young adults, the long-term toxicity of oxaliplatin-induced peripheral neuropathy is a graver concern. A 30-year-old patient could face debilitating side effects for 40-50 years, fundamentally altering the risk-benefit calculation for adjuvant therapy.
Oncologists are more comfortable using a positive ctDNA test to escalate care (e.g., recommend chemo for a low-risk Stage II patient). However, they are more hesitant to use a negative test to de-escalate or withhold standard chemo for higher-risk patients, pending more definitive trial data.
The landmark DYNAMIC-2 study showed that using ctDNA to guide adjuvant therapy decisions in Stage II colon cancer cut chemotherapy use by 50% (from 30% to 15% of patients). This de-escalation was achieved without any negative impact on patient outcomes, validating the approach.