The findings - presented by researchers at the European Breast Cancer Conference in Spain - mean doctors should be able to tailor treatment to the patients who will benefit and avoid giving toxic drugs to those that will not be helped.
By conducting a review of four large breast cancer trials, the researchers found that an abnormality on chromosome 17, called CEP17, is a "highly significant indicator" that the tumour will respond to chemotherapy drugs called anthracyclines.
Anthracyclines are anti-tumour antibiotics that interfere with enzymes involved in DNA replication. They are widely used against a variety of cancers.
The researchers found that if patients with CEP17 were treated with anthracyclines, they were around two-thirds more likely to survive, and to survive without a recurrence of cancer, than those not treated with anthracyclines.
Our aim was to identify patients for whom anthracyclines provided benefit and to seek to ensure that future treatment was targeted to this group. The results suggest that only those patients with CEP17 tumours should receive anthracyclines.
The researchers say that the existence of a readily available test for CEP17 means doctors could immediately start to better tailor chemotherapy to patient needs.