Women undergoing treatment for breast cancer should be warned that 20% of patients who choose to only have part of the breast removed are likely to need a second operation, scientists have said.
Researchers made the suggestion after they found that one in five English women who underwent breast conserving surgery needed further treatment.
There are 45,000 women diagnosed with breast cancer each year in England. Of these, 58% chose to have part of the breast removed - breast conserving surgery - rather than a mastectomy which sees the whole breast removed.
When combined with radiotherapy, breast conserving surgery produces similar survival rates to those achieved with mastectomy alone. But because some tumours are difficult to detect, breast conserving surgery may result in their inadequate removal and lead to another operation.
The study, published in the British Medical Journal, examined the re-operation rates of 55,297 women who had primary breast conserving surgery in 156 NHS trusts in England between April 2005 and March 2008.
Of these women, 11,032 needed a second operation within three months. Among women who had breast conserving surgery as a re-operation, one in seven needed further surgery, researchers found. More than 80% of the the 55,000 women were suffering from isolated invasive cancer, 12% had isolated carcinoma in-situ - or pre-cancerous disease - and 6% had both invasive and in-situ disease.
Re-operation was more likely among women with pre-cancerous disease compared to those with isolated invasive disease. Around 40% of women who had a re-operation underwent a mastectomy. Re-operation rates also varied between NHS trusts - in some trusts re-operation rates were below 10% while in others they were above 30% - but the authors stress that further research is required to understand its cause.
Dr Emma Pennery, clinical director at Breast Cancer Care, said: "Surgery is the first treatment for most people with breast cancer and some people will be offered the choice between a mastectomy and breast conserving surgery. It's really important that women are aware of all the potential benefits and drawbacks when they make this decision so their choice is informed.
Macmillan Cancer Support head of evidence Dr Siobhan McClelland said: "Nobody wants to go through the emotional strain of unnecessary surgery or mastectomies. This is why breast conserving surgery has become more popular.
"But it is essential that oncologists ensure that breast cancer patients are made aware of all of their treatment options and their possible results before any decision is made. However, this should not undermine what the surgeons are doing - it also shows that 80% of breast conserving surgery is successful."