Cancer incidence is increasing worldwide and in the UK it is estimated that 1 in 2 people born after 1960 will be diagnosed with some form of cancer in their lifetime. Could circulating tumour (ct) DNA technology help the health system meet the challenge of this cancer burden, and what needs to be done to maximise its utility?
As tumour cells die they release fragments of DNA into the patient’s bloodstream - this is known as circulating tumour (ct) DNA
Clinicians can use ctDNA to learn about the genetics of a patient’s tumour; this information can aid cancer management
ctDNA is extracted from a blood sample and analysed using genomic technologies including PCR, exome sequencing and whole genome sequencing
Taking a blood sample - a ‘liquid biopsy’ - is less invasive than a solid tumour biopsy, and so can be used throughout treatment, as well as for patients in whom a tissue biopsy is difficult or impossible
Liquid biopsies have several other advantages over traditional biopsies, such as capturing tumour diversity. But they also have disadvantages, one being that they cannot be used for diagnosis since the tumour tissue of origin cannot be determined
ctDNA liquid biopsies can be used to make treatment decisions, such as whether patients are eligible for certain therapies, based on the genetic profile of their tumour
As cells within the body die and are broken down, fragments of DNA are released into the bloodstream. If a patient has cancer, their bloodstream can contain both DNA from their normal cells and DNA from the cells in their tumour - circulating tumour DNA.
Analysis of ctDNA requires sensitive scientific techniques that are able to detect very small amounts of ctDNA among the non-cancerous DNA in a blood sample. These include next generation sequencing technologies such as exome sequencing and whole genome sequencing, and other methods such as PCR.
Making treatment decisions
Monitoring emergence of resistance mutations
Should a patient stop responding to a therapy, a liquid biopsy can be used to determine if the tumour has developed mutations that confer resistance to that therapy.
Monitoring treatment response and relapse
Liquid biopsies can also help to monitor treatment effectiveness, since if a treatment is successful ctDNA levels ultimately fall to undetectable levels.
Liquid biopsies could potentially be used for screening purposes, particuarly in high risk groups. However, this is a long way from implementation due to challenges including the risk of overdiagnosis and lack of evidence to show that early detection of cancer in this way results in better outcomes for patients.
Increase accessibility of testing
All of the tests outlined in the previous section can also be carried out on solid tumour samples but solid tumour biopsies can be difficult or impossible to carry out, sometimes causing harmful side-effects. Liquid biopsies are less invasive and easier to carry out, meaning that more measurements can be made without placing too much of a burden on patients. This makes cancer genetic testing accessible to more patients and also enables clinicians to build up a more accurate picture of how tumour genetics change over time.
Capture tumour diversity
Cancer is a genetically diverse (heterogenous) disease, and capturing this diversity can be challenging, particuarly if tumours have spread. Liquid biopsies can potentially capture tumour heterogeneity more accurately than isolated solid biopsies, as they measure ctDNA from multiple sites in the body.
Some cancers release more ctDNA than others and so are more amenable to the use of ctDNA technology, for reasons that are not yet fully understood.
It is not yet clear whether using ctDNA testing improves patient care and outcomes. Work is being carried out to collect this information. Currently ctDNA testing is only used when the result can be used in clinical decision making.
While ctDNA testing can be used to offer more personalised treatment, it is not clear if a more personalised approach saves the health service money.
Solid tumour biopsies remain the gold standard for diagnosis; liquid biopsies are unsuitable since the tumour’s tissue of origin cannot be determined. Currently, ctDNA testing complements conventional methods.
While research and early-phase clinical trials have demonstrated that ctDNA testing can be used to monitor tumours and make treatment decisions in a range of cancers, clinical application is currently limited. Liquid biopsy is currently being offered or is being developed in a small number of UK laboratories to determine EGFR status in non-small cell lung cancer, in order to make treatment decisions in some patients.