Declaring a patient "cancer-free" is complex, carrying hope tinged with uncertainty. Why do some patients enjoy long-term remission while others face the challenge of relapse? Current studies increasingly link this variance to minimal residual disease (MRD). 

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MRD, also termed molecular or measurable residual disease, represents a small number of cancer-specific molecules that persist in the body post-cancer treatment. These molecules, such as proteins or DNA, are undetectable through standard imaging methods like PET or CT scans and may exist without causing any apparent clinical symptoms.

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ctDNA analysis

Circulating tumour DNA (ctDNA) is a key MRD biomarker in solid tumours. According to lung cancer researchers reporting in BMC Medicine, it enables early detection of cancer recurrence and aids in prognosis and treatment planning post-radio/chemotherapy, well before clinical or radiographic signs of disease recurrence.[1]

Liquid biopsies detect ctDNA in biological fluids, analysing the variant allele fraction (VAF) to quantify cancer cell DNA presence, as reported by Chinese researchers in Frontiers in Genetics.[2]

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Diagnostic approach

MRD testing employs tumour-informed and tumour-agnostic methods, as discussed in a 2023 review from Oncology Reports.[3] 

Tumour-informed ctDNA testing, using a bespoke tumour mutation profile of the patient, offers high sensitivity but is time-intensive. Conversely, tumour-agnostic testing uses broad mutation panels without prior tumour data, trading sensitivity for practicality in instances lacking tumour samples.

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Detection method

The Oncology Reports authors state that VAF may be as low as 0.01%–0.10% in post-surgical or early-stage cancers. Thus, sensitive and specific methods are needed to distinguish trace cancer-derived DNA from non-cancerous mutations in blood. PCR and next-generation sequencing (NGS) are employed to address this issue. Examples are cited in Frontiers in Genetics:

1. Droplet Digital PCR (ddPCR) divides DNA samples into droplets for individual analysis after PCR, achieving high sensitivity at a lower cost but limited to detecting about 0.1% of gene targets.
2. PCR amplicon-based NGS, capable of examining multiple DNA sequences simultaneously, uses unique molecular identifiers (UMIs) for increased accuracy. However, it is restricted to specific genomic regions and can exhibit allele amplification bias.
3. Hybridisation capture-based NGS uses biotinylated probes for DNA hybridisation, excelling in concurrent mutation detection with sensitivity up to one part per million.
4. Whole genome sequencing (WGS) and whole exome sequencing (WES) apply NGS to ctDNA, with WES being more cost-effective but limited to exonic regions.

Targeted methods, such as hybridisation capture-based NGS and PCR amplicon-based NGS, are generally preferred over broader sequencing approaches due to the low ctDNA concentration in bodily fluids.

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Role in solid tumours

Following the promising results of MRD testing in haematological malignancies, recent studies have also established its utility across multiple solid tumours. MRD analysis carries high sensitivity (88%–100%) and specificity (98%–100%) in detecting lung, breast, oesophagus, and bladder cancer relapse, with significant lead times over traditional radiographic imaging.[4]

The Frontiers in Genetics review highlights some noteworthy milestones of MRD testing:

Lung cancer

In post-surgery non-small-cell lung cancer (NSCLC) patients, MRD predicts recurrence risk, disease progression, prognosis, and survival rates. The 2023 meta-analysis reported in BMC Medicine, involving 1,251 patients, found that ctDNA was a reliable detector for relapse in NSCLC with high specificity (86%–95%) and moderate sensitivity (41%–76%).

Breast cancer

For early-stage, resectable breast cancer, MRD can monitor tumour load, guide therapy, and predict recurrence risk, especially in high-risk triple-negative cases. In advanced stages, ctDNA complements radiological assessments.

A 2024 meta-analysis of 5,779 patients showed that ctDNA presence correlates with reduced disease-free and overall survival.[5] ctDNA testing demonstrated variable sensitivity (31%–100%) and high specificity (70%–100%), offering a lead time of approximately 10.8 months before clinical recurrence.

Colorectal cancer

MRD helps select appropriate candidates with stage II and III colorectal cancer who might benefit from additional therapy. The GALAXY study, for instance, indicated that ctDNA levels declined more rapidly in patients receiving adjuvant chemotherapy, suggesting its utility in treatment monitoring.

Other solid tumours

1. MRD predicts outcomes in locally advanced, unresectable, or metastatic gastric cancers post-chemotherapy. 
2. Combining ctDNA and alpha-fetoprotein detection improves recurrence risk prediction post-surgery for early and intermediate-stage liver cancer patients.
3. In pancreatic cancer, ctDNA detects disease progression earlier than radiological imaging.
4. MRD helps determine the necessity of perioperative chemotherapy in upper tract urothelial carcinoma.

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Challenges and future prospects

Researchers from Washington University School of Medicine expressed their view of the current state of ctDNA assessment: "The most fundamental challenge in the analysis of ctDNA is its scarcity," they wrote.[6]

They noted that PhasED-Seq, a new technology, addresses some of these issues by detecting deficient ctDNA levels (within the part-per-million range). It is showing promise in lung and breast cancer, improving relapse detection over traditional methods. 

The Frontiers in Genetics authors identified other problems, such as that tumour-derived ctDNA load can vary across individuals and cancer types. Factors like low DNA shedding and metastasis location cause false negatives, while false positives can result from DNA fragments of clonal haematopoiesis of indeterminate potential (CHIP) or benign hematopoietic cells.

Additionally, inconsistencies in ctDNA results can be due to variable sample handling and lack of standardised testing, complicating the interpretation of results.

There is a growing interest in the role of epigenetic signals, like DNA methylation alterations and fragmentation patterns, to differentiate ctDNA from benign circulating cell-free DNA, with the potential for reducing false positives.[7]

Despite its challenges, post-resection MRD analysis can guide adjuvant chemotherapy regimens and reduce the risk of relapse.

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Disclaimer: This story is contributed by Alpana Mohta and is a part of our Global Content Initiative, where we feature selected stories from our Global network which we believe would be most useful and informative to our doctor members.