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The card inviting Margaret Corrie for her first mammogram arrived just a few weeks before her 50th birthday. She popped along to a mobile breast screening unit and went away convinced she would hear nothing more.

But two weeks later, doctors explained they had found what looked like a cerous abnormality. It transpired that Margaret had a 1cm Grade 2 (it hadn't spread to surrounding areas) lobular cancer and the surgeon recommended a full mastectomy which was carried out with a reconstruction in July. She also had to go through ten months of chemotherapy.

Margaret is now fully recovered, and credits the screening with saving her life.

Another tremendous success, then, for the breast screening programme which was introduced across the UK in 1988.

Today, around one and half million women, mostly between the ages of 50 and 70, are screened in the UK each year and the programme saves the lives of around 1,300 women annually.

Early diagnosis

The perceived wisdom is that if you have cancer, any cancer, surely it's best to catch it as early as possible so that it can be more effectively treated and you have the highest chance of survival.

But there is a growing body of respected medical and research opinion which shows things are not that clear-cut. Last week, a paper published in the Journal of the Royal Society of Medicine reiterated the conclusion that breast cancer screening may be doing nearly as much harm as it does good.

"I understand why people find this hard to believe," says Dr Karsten Jorgensen, a research scientist with the independent Nordic Cochrane Centre in Denmark, who has carried out research into the harms and benefits of breast screening.

By comparing survival rates from an area of Denmark that has had no screening against areas that have, The Cochrane Centre found that far from the 1,347 lives saved each year by screening that is claimed by the NHS, the figure is more like 500. That is, for every life saved, 2,000 women have to be screened.

A case of false positives

Further, Dr Jorgensen and his team claim that of those women screened, around one in four will receive an incorrect diagnosis — a false positive — during her screening lifetime. This is when a woman will be told that their mammogram has thrown up something suspicious which will need further investigation, such as a biopsy, only to be told that there is no cancer.

Even worse, he says, thousands of women in the UK are being overdiagnosed each year — that is, they are being treated for cancers which they do not have.

These figures are backed up by Australian research published in 2010 which found that one in three of all invasive cancers diagnosed by screening falls into this category. This means every year in the UK, up to 7,000 women receive unnecessary surgery, plus possible chemotherapy and radiotherapy, with all the long-term health risks such as lymphoma, infections and stroke risk, for a cancer which may never have existed.

Dr Emma Pennery, clinical director of the charity Breast Cancer Care, says: "Women should be properly informed about the risks of the screening programme besides the benefits. For example, receiving a false positive may be a completely acceptable risk to one woman if she thinks she may have an early detection of cancer. But to another woman this may bring unimaginable strain."

"Screening picks up cancers early, which gives women more choices about treatment," says consultant breast surgeon Rajiv Vashisht, who practises at The Clementine Churchill Hospital and the West Middlesex University Hospital.

"Overdiagnosis is something we all dread but if the results show a problem, we have to assume the worst and treat our patients accordingly," he adds.

So, what is the best option for a woman offered screening?

"If you have a high risk of breast cancer — a family or genetic risk — you should be regularly scanned," says Professor Jane Maher, chief medical officer of Macmillan Cancer Support and an oncologist.

"Other women should make sure they understand the risks before screening, and the best source of unbiased information is cancer charity websites. Next, decide in advance what you will do if you get a positive result. With the majority of breast cancers there is no need for immediate action. Think about your options, possibly ask for a second opinion, read up on the different treatment options."

She adds: "Don't just accept what you are being told. Ask questions until you are satisfied you know all the answers."