ESMO has released a report from the ESMO Precision Medicine Working Group on recommendations on the use of circulating tumour DNA assays for patients with cancer.
It has found that for patients with advanced cancer, ctDNA assays have sufficient evidence to be used routinely in clinical practice to genotype advanced cancers to direct molecularly targeted therapies. This approach may be used in routine clinical practice, provided the limitations of the assays are taken into account.
Methodology behind the recommendation
The European Society for Medical Oncology (ESMO) Precision Medicine Working Group (PMWG) convened a group of experts to address the critical considerations, with a focus specifically on assays for ctDNA detected in blood as a liquid biopsy (LB) analyte. The group reviewed the many different technical aspects of ctDNA assays, in order to provide a standardized requirement for decision making, when handling ctDNA assays. The experts also assessed the evidence of ctDNA as a tool in many different phases of cancer care, as well as the type of cancers, seeing potential in clinical trial designs and even cancer screenings.
Challenges in conducting a ctDNA analysis
Plasma DNA constitutes DNA fragments bound to proteins that protect the fragments from degradation in blood. While a healthy person’s plasma DNA arises from the body’s natural blood stem cell, a cancer patient’s plasma DNA can contain a variable fraction from the cancerous tumour (i.e., ctDNA fraction).
Sizes are essential in providing information to distinguish normal from cancerous cells, and to define the site of origin or location of the tumour – where ctDNA fragments are smaller compared to normal plasma DNA. ctDNA also has a longer half-life, meaning that it takes longer for these tumour cells to clear the body, after it dies – making their exposure to the body longer.
While a ctDNA analysis has multiple potential clinical applications – from screening to detection of diseases, to prediction of relapses and monitoring – there is no one ctDNA assay that can match its potential. There are different specifics for each application, where a different amount, concentration, and quantitation of ctDNA plasma is required.
Variables to consider, and reporting standards to adhere to
Pre-analytical and analytical parameters impact the accuracy and reproducibility of a ctDNA analysis. Pre-analytical Variables include patient-specific factors affecting ctDNA release, sample volume, collection tube, storage conditions and processing. Physiological condition, inflammation, acute and chronic medical conditions are also factors to take note of.
The timing of plasma collection, and volume sampling, should also be carefully planned to ensure sufficient analyte is available to address the testing. Sample processes should always follow validated SOPs and be carried out in dedicated laboratory areas, to minimize the risk of contamination.
Analytical variables should also be taken into account including false negative results; Patients, with relatively advanced cancer, may have low levels of ctDNA in their plasma due to low ctDNA levels, preventing detection of the variant. Other factors include the amount of plasma DNA analysed, and the assay sensitivity that differs from variant to variant.
In the case of biological false positives, somatic alterations accumulate in normal tissues due to environmental causes, chemical exposures, or replication errors during cell division. These alterations can cause the cells to clone, increasing the potential development of cancer – ultimately resulting in false positive ctDNA results.
Reporting standards depend on the ctDNA assay type, as well as the intended use of the test. ESMO recommends including pre-analytical parameters, and to utilise language that conveys the potential for discordance with tumour testing. It is also essential to alert clinicians to detected variants in ctDNA assays which maybe contributed by non-tumour origins.
Evidence for potential utilisation of ctDNA assays in clinics
Traditionally, tumour tissue biopsies are used to determine one’s cancer development, and form a precise medicine, and diagnosis for a potential cancer patient. Limitations for tissue biopsies include patient discomfort, risk, and morbidity, and even delays in obtaining results and restrict treatment options.
ctDNA or liquid biopsies is appropriate, especially when a tumour tissue is not available – being more accessible and providing a more accurate genotyping of the tumour cell compared to a tissue biopsy. Despite so, there are certain limitations of ctDNA testing – mainly higher rates of false negatives and positives, but also low tumour fraction – limiting reliable assessment.
Studies have reported excellent specificity and positive predictive values between ctDNA LB and tissue based testing with sensitive realtime or digital PCR/NGS platforms. This evidence gives confidence and validity for ctDNA’s clinical utility.
Studies for the ctDNA testing has shown potential for clinical applications, though further interventional studies could define and assess the testing for improved accuracy, outcome as well as the quality of life in areas such as advanced cancer, and early-stage cancer monitoring.
Source: ESMO
The research was initiated by the PMWG and has been published on the ESMO scientific journal Annals of Oncology. For more information, please click here.
