The research has the potential to revolutionise mammogram screening programs across the world and breast cancer treatment – and even to change the very definition of what we call cancer, when it comes to the breast.

The study of more than 100,000 women, published in the prestigious JAMA Oncology journal, represents the most extensive data collection ever of the early-stage cancers, known as ductal carcinoma in situ, or DCIS.

It followed the women for more than 20 years, finding those who develop DCIS have only a very small chance of actually dying from breast cancer, and treatment may not make a difference to their outcomes.

Cancer Council Australia medical adviser and breast cancer expert Susan Fraser said the research was likely to change medical practice in the near future – with women treated for DCIS based on their own set of risk factors, not just the fact abnormal cells were found.

In DCIS, abnormal cells are found only within the milk duct, and no cancer is seen anywhere else in the breast.

"It looks like DCIS is not just one disease, it's a group of diseases," Dr Fraser said. "We will have to start looking at things like the grade and the size, whether it is oestrogen positive or negative, if it's in a very young person or a much older person and if it is very small.

"It will take a while to change medical practice but I think women will have to ask the questions about what the pathology is, and what the evidence is for treatment."

Dr Fraser said the advent of wide-spread mammography had saved lives, but it had also lead to a huge rise in the proportion of DCIS cancers diagnosed, from about 5 per cent to as many as 25 per cent of all breast cancers. About 1200 DCIS cases are diagnosed each year in Australia.

"We have to look at the costs to the patients of giving them radiotherapy," she said. "If patients aren't living any longer then it's worth questioning why we put them through it."

She said a "change of name" from cancer to something else may help women who could benefit from not invasively treating DCIS cope with focusing instead on lifestyle or other changes rather than active treatment.

Professor of surgical oncology at the University of Western Australia Christobel Saunders cautioned that it would be some time before doctors could be sure enough exactly which women with DCIS would benefit most from ongoing monitoring and lifestyle changes, and which would definitely need active treatment.

"I think we really need to do some trials of how best to treat lower-risk DCIS – we just don't know enough about who should and who shouldn't get treatment, unfortunately," she said. "But we are getting better at knowing."

"The message for women at the moment is that they still will need to get treatment, but the good news is for the majority of them, the outcome will be excellent."

She said an international meeting on controversies in breast cancer management would be held in Melbourne in October, and this research would be useful to help formulate research on the best ways to treat DCIS, and whether in some cases it should be called cancer.

"That is something that would need to be done across the world, not just in Australia," she said.

In an accompanying editorial in JAMA Oncology, breast cancer expert Laura Esserman​ said the study meant that for the lowest-risk DCIS, doctors should trial lifestyle and dietary changes and removing alcohol for breast cancer prevention, along with avoiding some post-menopausal hormone therapy. Such women were so low-risk they did not need to be specifically targeted for screening or early breast cancer detection programs.