Certain fragments of DNA shed by tumours into the bloodstream can potentially be used to non-invasively screen for early-stage cancers, monitor responses to treatment and help explain why some cancers are resistant to therapies. For most tumours, a tissue biopsy is quite challenging in that it is costly, painful, or potentially risky for the patient. All these are good reasons to learn about cancer through blood and to get excited about the possibility of carrying out liquid biopsies.
The development of non-invasive methods to detect and monitor tumours continues to be a major challenge in oncology. Cell-free circulating tumour DNA (ctDNA) and circulating tumour cells (CTCs) are plasma sources of tumour DNA that have been investigated for non-invasive detection and monitoring of patient tumours but have not been analysed or directly compared across multiple tumour types. Although the current Food and Drug Administration (FDA)-approved liquid biopsy measures intact CTCs to give a prognosis of overall survival, the potential predictive value of ctDNA is much more exciting. ctDNA liquid biopsy allows us to understand specifically what kind of molecular changes are happening in the tumour in real time, which is a very big step beyond where CTCs are today in clinical terms.