One third of breast cancer tumours change form when they spread, UK study shows.

Scientists from the Breakthrough Breast Cancer Research Unit at the University of Edinburgh analysed 211 tumours which had spread from the breast to the lymph nodes, in the armpit, where breast cancer cells usually spread to first. In the largest study of its kind they found that in 82 (39%) cases the disease in the lymph nodes had changed type.

Breast cancer is a complex disease with many different types which can be treated in different ways. Cancer cells that have spread to lymph nodes are often more difficult to treat than those in the breast and so it is vital women receive the most appropriate treatment. Breast cancer spreads to the lymph nodes in 40% of the 46,000 women diagnosed with breast cancer in the UK each year.

Researchers were surprised to find the disease changed in such a high proportion of patients, and in so many ways, when it had spread, it was reported in Annals of Oncology online.

For example, 20 tumours changed from oestrogen receptor (ER) negative to ER positive.

This change would mean hormone therapies such as tamoxifen, which would not have worked for the original tumour, could help treat the disease if it has spread. Other tumours changed from ER positive to ER negative, which suggests those patients may be given treatments which will not benefit them, and are therefore experiencing side effects unnecessarily.