Counting breast cancer cells in the blood may help predict a woman's survival chances and aid targeted treatment, a study has shown.
Patients with at least five circulating tumour cells (CTCs) detected straight after surgery had a four-fold increased risk of cancer recurrence, scientists found.
They were also three times more likely to die from their disease than women without CTCs.
But doctors believe it is not all bad news for women with circulating tumour cells. Identifying the cancer cells could make it possible to spot early disease changes and provide the most effective targeted treatment.
Circulating tumour cells are rare and difficult to detect. In cancer patients, one millilitre of blood contains an estimated 7.5 tumour cells compared with a few million white blood cells and a billion red blood cells.
The German scientists conducting the new study used advanced automated technology to find CTCs in blood samples. The blood test is much less invasive than the alternative way of detecting CTCs in bone marrow.
Researchers tested the blood of more than 2,000 patients all of whom had undergone surgery before starting chemotherapy treatment.
CTCs were detected in 21.5% of the blood samples, a much lower rate than that usually seen in advanced and spreading breast cancer.
Study leader Dr Bernadette Jager, from Ludwig Maximilian University Hospital in Munich, said: "Although there is currently no direct advantage to the patient of knowing her CTC status, this is already a step forward, and in future we believe that the presence of CTCs could be used as a marker for monitoring the efficacy of treatment.
"If this proves to be the case, it will also help us determine the best chemotherapy regime for each patient."
Dr Jager presented the findings at the European Breast Cancer Conference (EBCC) in Vienna.
CTC detection could be combined with routine blood tests and carried out frequently, she said.
The trial patients are having their CTC counts re-assessed after chemotherapy and then at intervals of two and five years.
The German team has also started a new study aimed at seeing if CTC detection can aid treatment.
Breast tumours that over-produce the protein Her-2 can be especially aggressive. But sometime a cancer can change its Her-2 status from negative to positive without this being immediately apparent.
The new study will look for Her-2 positive circulating tumour cells in women whose original Her-2 status was negative.
"When we find CTCs with a different Her-2 status from that of the primary tumour or the metastasis (cancer spread), we will evaluate the advantage of changing the therapy regimen," said Dr Jager.
"Therefore we are now investigating the benefit of a Her-2 targeted therapy in patients with Her-2 positive CTCs but a Her-2 negative primary tumour or metastasis; if the therapy is successful we can be sure that we are attacking the cancer in the right way and in the right place.
"If CTCs can be used as a direct treatment target, this will be a promising development and a further step on the road to enabling us to tailor treatment appropriately for individual patients."
Professor Michael Gnant from the Medical University of Vienna and Chair of the EBCC organising committee said: "Accurately identifying circulating tumour cells is an important step forward in our two decade-long quest to decipher these cells and the impact on treatment response and prognosis of early breast cancer patients."
For other incredible medical breakthroughs, take a look at HuffPost Lifestyle's round-up of life-changing scientific studies.