Breast cancer tumours known as ductal carcinomas are graded from I to III, where grade I is the least aggressive.

Doctors had thought that grade I tumours eventually grew into grade III tumours.

But the new research strongly suggests that the different grades are actually distinct forms of breast cancer.

The finding could lead to better-targeted research to tackle the disease.

Several types of breast cancer

Dr Rebecca Roylance and colleagues conducted the research at the Imperial Cancer Fund's Human Cytogenetics and Breast Pathology Laboratories.

Dr Roylance said: "It was about 10 years ago that people began to question the meaning of these grades.

New treatments could look to target specific types of tumour (Photo: NASA Ames Home Page)

"Researchers in the Hedley Atkins-ICRF Breast Pathology Laboratory found that if you have a primary tumour that is treated and then recurs, it tends to come back as the same grade.

"This isn't what you'd expect if tumours were progressing."

Her team compared the genetic make-up of 40 grade I tumours with that of 50 grade IIIs.

They found that genetic changes found on grade I tumours were not always found on grade III tumours.

While grade III tumours may well have more genetic changes and losses than grade I tumours.

But if there is progression, then all the differences found on grade I tumours should also exist on grade III.

This was not always the case - for example, 65% of grade I tumours had lost part of chromosome 16, but this was the case in only 16% of grade III tumours.

Reasons for the differences

Dr Roylance said: "It's unclear why some women get a grade I and others a grade III tumour, but it's likely there will be more than one reason.

"Grade I tumours occur more often in older people, and it could be that their causes are quite distinct from those of grade III tumours, which are relatively more common in the young.

"Breast cancer has usually been investigated as one disease, so these findings have implications for past research on this group of tumours.

"Our study provides a framework for the further study of the molecular basis of breast cancer."

Dr David Miles, of the ICRF's Breast Biology Group at Guy's Hospital in London, said the research could focus future treatments.

He said: "We hope that in future the ability to profile tumours in terms of their genetic material will give us a better understanding of their behaviour.