A new blood test for women with breast cancer could identify more women who will benefit from the targeted treatment Herceptin without the need for uncomfortable biopsies.

The new 'liquid biopsy' uses cutting-edge genetic techniques to detect breast cancer DNA in the bloodstream, and was developed by a team at The Institute of Cancer Research, London, and The Royal Marsden NHS Foundation Trust.

The technique can detect whether breast cancers are being driven by too many copies of the HER2 gene - known as HER2 amplification - and could be used to pick out women who might benefit from Herceptin and other similar drugs, but are not currently receiving them.

Researchers believe it could be adapted to a range of other cancers and drug targets, allowing doctors to monitor genetic changes in tumours through a blood test rather than needing invasive biopsies.

The research is published in Clinical Cancer Research and was funded by the National Institute of Health Research, with additional support from Cancer Research UK and the Dr Mildred Scheel Foundation for Cancer Research.

Currently, if a patient's breast cancer relapses after initial treatment, doctors perform tumour biopsies to determine which treatments the cancer is most likely to respond to. But biopsies are uncomfortable, intrusive and only test part of a tumour - and it's not possible to biopsy cancer repeatedly.

Previous research has shown that blood samples from cancer patients contain trace amounts of DNA from their tumour cells, known as circulating free DNA. In this study, scientists investigated whether analysing circulating free DNA could detect gene amplifications known to cause cancer growth.

Researchers took blood samples from 58 patients with recurrent breast cancer to assess the potential of a technique called digital PCR to detect HER2 amplification - which signifies a particularly aggressive form of breast cancer, but one which can be treated with Herceptin.

The new technique was able to accurately identify HER2-positive breast cancer 64% of the time, and HER2-negative cancer 94% of the time.

Dr Nicholas Turner, Clinical Researcher at The Institute of Cancer Research and Honorary Consultant Oncologist at The Royal Marsden, said: "Herceptin has been effective at treating HER2-positive breast cancers, but the problem with cancer-causing genes like HER2 is that they can be acquired or lost as a tumour progresses, so at any point in time you might miss a tumour for which Herceptin may work.

"It's not possible to take multiple biopsies from patients through their treatment course, but this study shows that we can detect HER2-positive breast cancers through a blood sample. That could allow us to regularly monitor women with breast cancer using simple blood tests and potentially increase the number who are treated with Herceptin. The test is at this time at an early stage and does require further assessment in clinical trials before it could become widely available."

Professor Alan Ashworth, Chief Executive of The Institute of Cancer Research, said: "Personalised cancer treatments are becoming highly sophisticated and tailored to individual cancers so it's really important that doctors can make accurate assessments of the genetic make-up of tumours. This new liquid biopsy has exciting potential as a means of analysing tumour DNA in the blood stream, allowing clinicians to track genetic changes as they happen and adjust treatment to them. By assessing quickly and painlessly whether a particular gene is activated in breast cancer, doctors will be able to choose the best targeted therapy for their patients."

Dr Naureen Starling, Associate Director of Clinical Research and Medical Oncologist at The Royal Marsden, said: "This is a very exciting development using a new blood based assessment to ensure the right patients receive the right targeted therapy. At The Royal Marsden we are committed to ensuring that patients receive treatment that is tailored specifically to them and their tumour and by reducing the need for biopsies, there will be significant benefits for our patients."