Data presented at a major conference found differences across the UK in how hospitals treat ductal carcinoma in situ (DCIS), which is diagnosed in more than 4,600 women in the UK every year.
DCIS means that cells inside some of the ducts of the breast have started to turn into cancer cells.
These cells are contained inside the ducts and have not started to spread into the surrounding breast tissue. Many women undergo surgery to remove the area of DCIS, together with a border of healthy tissue around it.
This is called breast conservation surgery or lumpectomy, and can be followed by radiotherapy.
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Today's research, published at the European Breast Cancer Conference in Glasgow, analysed data from more than 8,300 women whose DCIS was detected as part of the UK NHS Breast Screening Programme.
They found that of the 6,633 women who had breast conservation surgery, 799 (12%) went on to need a mastectomy.
Furthermore, a fifth (21%) of all the 2,479 women who had a mastectomy (including those who had it as a first course of treatment and those who had one following failed breast conservation) had tumours that were smaller than 20mm (0.8in) in diameter.
These would normally have been better treated with a lumpectomy, researchers said.
Dr Jeremy Thomas, a consultant pathologist at the Western General Hospital in Edinburgh, said: "These two groups of women - those having a mastectomy after failed breast conservation and those having a mastectomy for tumours smaller than 20mm - accounted for 49% of all mastectomies."
He said the "wide variation between hospitals" was arising from assessments made by teams of radiologists, pathologists, surgeons, oncologists and nurses when deciding on treatment.
Of 57 hospitals that had the highest number of patients, the proportion of failed breast conservation surgery in these hospitals ranged from 3% to 32%, while the proportion of mastectomies for small tumours ranged from 0% to 60%.
Dr Thomas said: "It would appear from our data that, in some hospitals, the discussions in the multidisciplinary teams are not looking in enough detail at the results from the mammograms and pathology in order to make the right decision about the best surgical treatment for these women.
"DCIS accounts for about 20% of the cancers detected and managed by the NHS Breast Screening Programme and overall our data show that the NHS Breast Screening Programme is working very well in what we would all regard as one of the most challenging parts of breast screening practice.
"After all, 88% of breast conservations are successful and 80% of mastectomies are carried out for tumours bigger than 20mm wide. Nevertheless, our data show there is a range of outcomes in different hospitals and we need to explore why that is the case and disseminate the highest standards across the programme."
Baroness Morgan, chief executive of Breast Cancer Campaign, said: "These results highlight a variation in practice which needs to be addressed to ensure that all patients who are given a diagnosis of DCIS receive the highest possible standard of care and most appropriate treatment, regardless of the hospital they are in.
"We look forward to seeing how these results can inform practice to ensure that these variations are no longer an issue."