# When the blood test is positive, hunt for the lesion with sensitive imaging

Source: https://onco.cc/ideas/idea-bio2-localise-the-mrd/  
OnCo record `idea-bio2-localise-the-mrd` (Idea). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

A positive blood test tells you cancer is back but not where. Combining whole-body MRI with modern PET tracers may find a single spot that can be zapped.

## Summary

Patients who are ctDNA-positive but scan-negative are currently given systemic therapy or nothing. Whole-body MRI plus a lineage-appropriate PET tracer (PSMA, somatostatin receptor, FAP or fibroblast-directed imaging) detects lesions well below conventional CT thresholds, which would allow ablative treatment of true oligorecurrence and spare the rest systemic therapy.

## Fields

- Kind: Idea
- Last checked: 2026-09-08
- Hypothesis: In ctDNA-positive, CT-negative patients, whole-body MRI plus a targeted PET tracer localises disease in at least 40%, and ablation of all detected lesions clears ctDNA in half of those patients.
- Rationale: PSMA PET already redefined biochemical recurrence in prostate cancer by finding lesions in patients previously classed as non-localisable, and metastasis-directed therapy in that setting delays systemic treatment.
- Proposed test: A prospective imaging cohort in ctDNA-positive colorectal and breast cancer measuring localisation rate; if the rate exceeds a pre-set threshold, proceed to a randomised trial of ablation plus systemic therapy versus systemic therapy alone.
- Maturity: early-clinical
- Actor: clinic

## Sources

- Bottleneck evidence (Dormant cells and minimal residual disease): Tie et al., ctDNA analysis guiding adjuvant therapy in stage II colon cancer (NEJM 2022): https://doi.org/10.1056/NEJMoa2200075

## Connected records

- ideas: [PSMA-PET-guided metastasis-directed therapy as a curative strategy in oligorecurrent prostate cancer](https://onco.cc/ideas/idea-psma-pet-guided-mdt/)
- cancers: [Colorectal cancer](https://onco.cc/cancers/colorectal/), [HR-positive / HER2-negative breast cancer](https://onco.cc/cancers/breast-hr-positive/), [Prostate cancer](https://onco.cc/cancers/prostate/)
- technologies: [FAPI PET](https://onco.cc/technologies/fapi-pet/), [MRD / molecular residual disease testing](https://onco.cc/technologies/mrd-testing/), [PSMA PET](https://onco.cc/technologies/psma-pet/), [SBRT / SABR (stereotactic radiotherapy)](https://onco.cc/technologies/sbrt/), [Somatostatin receptor PET (68Ga/64Cu-DOTATATE)](https://onco.cc/technologies/sstr-pet/), [Whole-body MRI](https://onco.cc/technologies/whole-body-mri/)
- terms: [Biochemical recurrence (BCR)](https://onco.cc/terms/biochemical-recurrence/), [Oligometastatic disease](https://onco.cc/terms/oligometastatic/)
- bottlenecks: [Dormant cells and minimal residual disease](https://onco.cc/bottlenecks/b-dormancy-mrd/), [Metastasis is understood least and studied last](https://onco.cc/bottlenecks/b-metastasis-biology/)

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