Understand blood tests and imaging after treatment
Learn what a residual-disease test can show, how to read it beside imaging, and what evidence a proposed response needs. The aim is to leave with a clearer question for the care team, even when results disagree.
A small blood-test number can feel like a direct count of cancer. A clear scan can feel like an all-body guarantee. Neither measurement has that reach. This guide connects each result to its actual sample, method and purpose, then shows how to combine information without pretending the gaps are gone.
The big ideas
- Cell-free DNA and circulating tumor DNA describe total material and tumor origin. Plenty of extracted DNA does not guarantee a detectable tumor signal.
- A tumor-informed assay tracks selected features from a reference tumor. Its scope is narrower than every possible molecular change.
- Units and detection limits travel with the result. A fraction, a concentration and a detection call have different meanings.
- Blood testing and magnetic resonance imaging (MRI) look through different windows. Neither replaces surgical pathology when tissue response is the question.
- Earlier detection, recurrence association and benefit from acting earlier are distinct claims. A useful plan names the evidence for the proposed action.
The map
Follow the numbered lessons below. Each result keeps its date, scope and limits attached.
Lessons
| Step | Lesson | Question it answers | Minutes | Learning aid |
|---|---|---|---|---|
| 1 | Read the blood result | What was sampled, tracked and detected? | 11 | Sample-to-report ledger |
| 2 | Compare blood and imaging | Why can two results disagree? | 12 | Two-window cartoon and four-pattern exercise |
| 3 | Ask what a result should change | Does evidence support the proposed next action? | 12 | Evidence ladder and fictional protocol |
Concepts you will use
Begin with cfDNA versus ctDNA, tumor-informed monitoring and units and limits.
Then use predictive values and base rates, prognostic versus predictive evidence, lead-time bias and validity and clinical utility. The multi-omics blood lesson is an optional companion.
Applied to Diana
Monitoring coordination owns her draw history and clinical plan. Results owns signed findings, while the clinical evidence reference owns dated assay and trial interpretation. This guide teaches a reusable reading method without copying those changing findings.
What is still uncertain
Different assays, draw schedules and disease settings can produce different detection rates. More frequent or earlier detection may add information while also adding uncertain results, anxiety or unnecessary procedures. Useful studies evaluate the entire strategy, including harms and patient outcomes.
A molecular result may be clinically useful for one indication and investigational for another. An approval, trial or result in another cancer does not automatically settle an early-breast-cancer question. Keep the disease setting and proposed intervention beside the evidence.
Sources and scope
Source check: October 9, 2026. Interpretation education with fictional exercises; expert and learner review pending. This guide does not set a surveillance schedule.
- FDA 2024 ctDNA guidance — assay and clinical-trial design.
- Magbanua et al. 2021, MRI and ctDNA in I-SPY2 — complementary measurements in a pilot cohort.
- Turner et al. 2023, c-TRAK TN — prospective detection and intervention feasibility in a defined early-TNBC setting.