A new UK study has revealed that almost 40 percent of breast cancer tumours change form as they spread.
This, according to Breakthrough Breast Cancer scientists in Edinburgh, could mean that patients require changes to their treatment regime as well.
For the study, they analysed 211 tumours, which had spread to the lymph nodes in the armpit-the place where breast cancer tends to migrate first.
Breast cancer often spreads to the lymph nodes and cells that travel in this way are often more difficult to treat than those in the breast - so it is vital that women receive the most appropriate treatment.
Researchers were surprised to find the disease changed in such a high proportion of patients, and in so many ways, when it had spread.
For example, 20 tumours changed from oestrogen receptor (ER) negative to ER positive-a change that 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 - experiencing side effects unnecessarily.
"We were surprised that such a high proportion of tumours change form when they spread beyond the breast," The BBC quoted lead researcher Dr Dana Faratian as saying.
"This suggests there is a need to test which type of disease a woman has in the lymph nodes, because it could radically alter the course of treatment she receives.
"We now need a clinical trial to see how these results could benefit patients," she added.
The study has appeared in Annals of Oncology.