Breast tumours may develop years later from cells left over from surgery designed to preserve a diseased breast, scientists believe.

Surgeons used to remove the entire breast (a mastectomy) when ridding the body of abnormal cells growing in the milk duct.

The cells pose no immediate threat, but it is thoght that eventually they may lead to the development of breast cancer.

In recent times, surgeons have favoured a technique known as lumpectomy, in which only the diseased part of the breast is removed.

The new research has highlighted that this technique is only effective if surgeons successfully remove all the abnormal cells. If it is not thorough tumours can develop years later.

It also emphasises the advisability of a course of follow-up radiotherapy after surgery.

A team from the University of California, San Francisco examined the tissues of ductal carcinoma in situ (DCIS) tumours in 18 women.

DCIS is a neoplasm, or growth, of non-invasive tumour cells in the milk ducts that have not broken through to invade the rest of the breast.

New treatment

Before the 1980s, treatment of DCIS was primarily by mastectomy.

More recently, lumpectomy - often accompanied by radiation - has been the recommended treatment for such tumours.

However, following lumpectomy DCIS tumours recur in 8% to 10% of cases. The rate depends on the type of follow-up treatment.

The recurrence of DCIS following mastectomy is only 1% to 2%.

Frederic Waldman, professor in laboratory medicine and lead author of the study, said: "The question was, 'Are the tumours really coming back or are the women who have pre-invasive cancer more likely to have a different tumour in the same breast or in the opposite breast?'".

The researchers isolated DNA from tissue samples to analyse the genetic changes in the 18 tumours and to compare those changes to the recurrent lesions.

In 17 patients, the chromosomal changes in the initial abnormal cells were very similar to those in the recurring tumour.

Only one patient's second tumour was a new growth.

Professor Waldman said: "This is an important confirmation that these recurrences are the same tumours, sitting there quiescent and coming back at a much later time.

"It gives us more of an understanding of what is going on when there are recurrences and it shows us that most of the time it's the same DCIS tumour coming back when we get local recurrences, even if they are many years later."

Dr Kate Law, head of clinical programmes at the Cancer Research Campaign, said: "Whether a woman opts for a mastectomy or lumpectomy is a matter of personal choice.

"There is still a risk even if lumpectomy is followed up with radiotherapy, but it is a manageable risk.

"The advantage of mastectomy is that it does get rid of all the abnormal cells and reconstructive techniques are now very good."

The study results will be published in the Journal of the National Cancer Institute.