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 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.
"Just over half of women diagnosed as having breast cancer in England now select breast conserving surgery as their primary treatment," the authors write. "Cosmetic outcomes after surgery for breast cancer are an important consideration, and women should be made aware of the local rates of re-operation after primary breast conserving surgery, along with the likelihood of proceeding to mastectomy."
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.
"While the majority of people who choose breast conserving surgery won't need another operation, it's important everyone knows beforehand that it is a possibility, as we know from calls to our helpline that finding out at a later stage can lead to a great deal of anxiety. There can also be physical impacts of further surgery such as delaying other treatment and increased chances of asymmetry, as well as emotional and practical issues, such as time off work and childcare while in hospital."