A mammogram is used to detect breast lesions.
Nearly 40% of breast cancer tumours change form when they spread, a UK study shows.
The researchers say this could mean that patients require changes to their treatment regime.
They analysed 211 tumours which had spread to the lymph nodes in the armpit - the place where breast cancer tends to migrate first.
The study, by Breakthrough Breast Cancer scientists in Edinburgh, appears in Annals of Oncology.
Breast cancer is a complex disease with many different types which can be treated in different ways.
Breast cancer spreads to the lymph nodes in about 40% of the 46,000 women diagnosed with breast cancer in the UK each year.
Cancer cells which spread 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.
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 - experiencing side-effects unnecessarily.
Lead researcher Dr Dana Faratian said: "We were surprised that such a high proportion of tumours change form when they spread beyond the breast.
"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."
Professor David Harrison, director of the Breakthrough Breast Cancer Research Unit, said: "This research may show why some women whose cancer has spread to the lymph nodes do not respond to treatment.
"With an additional test we may be able to treat women more effectively and also make more efficient use of NHS resources."
The researchers stress that a clinical trial needs to be carried out to fully evaluate the benefits of testing cancer cells in the lymph nodes before it can be approved for use on the NHS.
Breast cancer accounts for nearly one in three of all female cancers and one in nine women in the UK will develop breast cancer at some point in their lifetime.