Cell-free DNA (cfDNA)
Currently the standard approach for cancer diagnosis is the examination of tumour tissue through either removing cells through a small needle (fine needle aspiration cytology), or histological examination of a biopsy or surgical excision specimen. These procedures are invasive and involve some risk to the patient. In some cases, biopsy is also not possible due to the location of the tumour.
Cell-free DNA (cfDNA) is released from normal cells and tumours by programmed cell death (apoptosis) and comprises small fragments of nucleic acid that are not associated with cells or cell fragments. This cfDNA is present in the liquid part of the blood in all people. Tumours containing ~ 50 million malignant cells releases sufficient DNA for the detection of circulating cell-free tumour DNA (ctDNA) in blood  and this is below the limit of resolution of radiology studies. Tumours are able to be detected when they are approximately 7 – 10 mm in size and contain ~1 billion cells. As the volume of the tumour increases, the number of apoptotic and dead cells increases due to increased cellular turnover.
The development of sensitive and specific tests for the detection of circulating cell-free DNA on blood samples means that patients with tumours can now be monitored over time to assist in the assessment of response to treatment. The testing is specific to each individual’s tumour and forms part of personalised medicine. The evaluation of the clinical utility of these tests, defined as the measure of net health benefits, is on-going.
Circulating tumour DNA fragments (ctDNA) contain identical genetic defects to those seen in the primary tumour itself . Because ctDNA fragments are released from all parts of the tumour the ctDNA is in fact a liquid biopsy .
Potential applications for ctDNA include:
- Monitoring of tumour burden  [5, 6] [4, 5, 7-10].
- Monitoring of minimal residual disease
- ctDNA is a potential marker of residual disease after surgery and should be measured after the surgery but before the commencement of adjuvant therapy (generally 6-8 weeks after surgery) .
- In the case of curative surgery for cancer, we currently have no means to identify patients who are cured from those patients that still have residual disease. Currently predicting which patients are disease free is based on clinical and pathology criteria.
- Measurement of ctDNA has been used for bowel cancer where a group pf patients were followed for 2-5 years after treatment. ctDNA was able to detect minimal residual disease after surgery .
- A similar study has also been performed for breast cancer.
- Monitoring of molecular resistance
- Resistance to treatment results from the acquisition of new molecular changes in the cancer cells.
- Liquid biopsies can be used to monitor the development of resistance to therapy during treatment. This has been demonstrated for leukaemia, lung cancer, bowel cancer and malignant melanoma .
- This understanding of the mechanisms of resistance can be used to plan combination treatment and institute alternate therapies.
- Monitoring of tumour heterogeneity
- ctDNA analysis can provide an overview of all the cells in a patient’s tumour simultaneously and this takes into account variations in different cells in the tumour and may provide an early indication if cells are becoming resistant to therapy.
- In addition, mutations in different pathways in the tumour cells can be used to plan combination treatment and this approach can prevent resistance developing in the tumour cells and improve response .
- Early diagnosis of tumours
- Whilst more work is needed to develop these assays for the early detection of cancer, in certain cases when tissue is not available, or used in conjunction with other clinical tests, this blood tests is very specific and are increasing in sensitivity.