Scientists have found a way to predict which women with breast cancer have the most aggressive forms of the disease.
Tumours can be tested for the protein
A test which can be carried out on tumour tissue removed from the breast detects a chemical linked to the speed the tumour has been growing.
Researchers from the MRC Cancer Cell Unit at Cambridge University say the technique could help doctors give women the right level of treatment.
Dr Nick Coleman said that the finding was "good news" for women.
The research, published in the Journal of Clinical Oncology, focuses on a cell protein called Mcm-2, which can be found in breast tumours.
The first step in breast cancer treatment is to take a tiny tissue sample to diagnose the disease, and this could be tested at the same time for this chemical.
Different tumours have different levels of Mcm-2, and scientists worked out that those with certain levels tended to grow more quickly, and require higher doses of radiotherapy and chemotherapy, or a course of treatment that lasts longer.
Conversely, Mcm-2 levels could also be linked to a slow-growing tumour which perhaps might need a shorter course of treatment or a lower dose.
Dr Coleman said: "An accurate assessment of the outcome following surgery is key to choosing how much further therapy is needed for an individual patient.
"It is important that patients receive neither too little nor too much additional treatment for their tumours.
"The discovery has the potential to make a difference to the lives of thousands of women as breast cancer is the most common type of cancer to affect women in the UK."
Professor Robert Souhami, from Cancer Research UK, one of the research funders, said that the findings would hopefully prove "helpful" to doctors trying to work out how intensively to treat breast cancer patients.
However, he said that further research was now required to validate the results.