What does circulating tumor DNA (ctDNA) testing tell me about my Breast Cancer

Understanding Circulating Tumor DNA (ctDNA) Testing for Breast Cancer

Circulating tumor DNA testing—also called a "liquid biopsy"—is a blood test that detects tiny fragments of DNA shed by cancer cells into your bloodstream. Let me explain what this test can and cannot tell you about your breast cancer.

What ctDNA Testing Detects

According to the NCCN Guidelines for Patients: Metastatic Breast Cancer, ctDNA testing works by:

• Finding cancer DNA in your blood: When cancer cells die or break down, they release DNA fragments into your circulation. This test captures and analyzes those fragments.
• Identifying genetic mutations: The test can detect specific mutations in your tumor DNA—such as PIK3CA, AKT1, PTEN, ESR1, NTRK, and RET mutations—that may have targeted treatment options available.
• Monitoring disease over time: Serial testing (repeated tests over time) can track whether your cancer is responding to treatment or developing resistance.

How This Information Guides Care

For mutation detection: If ctDNA identifies a targetable mutation (like an NTRK gene fusion or RET fusion), this may open up specific treatment options. For example, according to NCCN guidelines, NTRK fusions might be treated with larotrectinib (Vitrakvi) or entrectinib (Rozlytrek), while RET fusions might respond to selpercatinib (Retevmo).

For treatment response monitoring: ctDNA can help your oncology team understand whether your current treatment is working by tracking changes in tumor DNA levels in your blood over time.

Important Limitations to Understand

According to expert perspectives shared in the CancerPatientLab webinar "An MD PhD Navigates Breast Cancer" with Dr. [removed] Lopez-Correa:

• Not all ctDNA tests are the same: Different tests provide different information. Some ctDNA tests detect genetic mutations but don't tell you about tumor burden (the amount of cancer in your body). Other specialized tests like Signatera can provide information about tumor burden and disease progression.

• Negative results don't mean "no cancer": Even if ctDNA testing is negative, cancer cells may still be present at microscopic levels that the test cannot detect. A negative result is reassuring but not definitive proof of no disease.

• Different from circulating tumor cells (CTCs): ctDNA testing is different from CTC (circulating tumor cell) testing. CTCs are whole cancer cells floating in your blood, while ctDNA is just DNA fragments. For breast cancer, CTCs are often negative even in metastatic disease, making them less useful for detecting mutations.

• Timing matters: The value of ctDNA testing depends on when it's done—at diagnosis, during treatment, or during follow-up—and your doctor needs to interpret results in the context of your complete clinical picture.

Questions to Ask Your Oncology Team

Since ctDNA testing is evolving, here are specific questions that will help you understand how it applies to YOUR situation:

1. "Are you recommending ctDNA testing for me? If so, what specific mutations are you looking for, and how will the results change my treatment plan?"

2. "Which ctDNA test are you using, and what information will it provide—just mutations, or also tumor burden?"

3. "How will you use ctDNA results to monitor my response to treatment, and how often will you repeat the test?"

4. "If my ctDNA test is negative, does that mean my cancer is gone, or could cancer still be present?"

5. "Are there any clinical trials I might be eligible for based on my ctDNA results?"

The Bottom Line

ctDNA testing is a valuable tool that can identify targetable mutations and help monitor treatment response—but it's one piece of information, not the whole picture. Your oncologist will combine ctDNA results with imaging studies, physical exams, and other blood tests to make decisions about your care. The test is most useful when you understand both what it can tell you and what it cannot.

---

This information is for educational purposes only. Always consult your healthcare team for personalized medical advice and decisions.