One in three women diagnosed with breast cancer after a screening test will have slow-growing forms of cancer that could safely go untreated, according to new research. Although breast cancer screening saves lives, it also means that some women go through the worry and side effects of unnecessary treatment.
What do we know already?
Screening programmes, like screening for breast cancer with regular mammograms, help to detect cancer early. For some women, this allows for life-saving treatment. However, screening comes with a downside that's often ignored. As well as helping to detect dangerous cancers, mammograms can also pick up harmless cancers that grow so slowly they never cause symptoms in a woman's lifetime, or don't grow at all. Doctors call this overdiagnosis.
It's impossible to tell the difference between dangerous and harmless cancers. Doctors have to treat them all. So, the price of screening is the possibility of having drastic treatments, such as surgery or radiotherapy, which you may not need.
That's not to say there's no point in screening for cancer. It's a question of weighing the benefits against the risks. If screening saved 100 lives for every person who had unnecessary treatment, it would make sense for people to take part in the programme. If the numbers were reversed, you might well conclude it wasn't worth the risk.
To help women weigh up the benefits and risks, researchers have tried to work out how many women screened for breast cancer end up being treated unnecessarily.
What does the new study say?
The researchers looked at five countries which have introduced breast cancer screening: the UK, Canada, Australia, Sweden, and Norway. They looked at trends for the number of women diagnosed with breast cancer before and after screening was introduced. They also looked at whether breast cancer rates in older women started to fall several years after screening was introduced, which is what you'd expect if cancers were being detected earlier in women.
The researchers calculated that for every two cases of dangerous breast cancer picked up by screening, one woman would be offered unnecessary treatment for a cancer that would never have harmed her.
How reliable are the findings?
Researchers are now fairly certain that breast cancer screening harms some women, owing to the unnecessary treatment they're offered. But how often does it happen?
The current study used statistical methods. We'd get a more accurate picture of the risks and benefits by comparing women who had screening with women who didn't. A study published three years ago did exactly that, and came up with slightly different results. Like the current study, it found that some women were treated unnecessarily for breast cancer, but there was a much smaller margin. Only one woman was treated unnecessarily for every 10 who benefitted from treatment.
If I attend breast cancer screening, what are the benefits?
Here's what we know based on all the research done so far: assuming 1,000 women started screening at the age of 50, and had a mammogram every year, one would have her life saved through early detection and treatment of dangerous cancer.
What are the downsides of breast cancer screening?
In the same group of 1,000 women, between two and 10 would be given treatment they don't need for a slow-growing cancer. This could mean painful surgery, along with side effects from radiotherapy or chemotherapy.
Between 10 and 15 women in the same group would have their breast cancer diagnosed earlier because of screening, but researchers say this makes no difference to their chances of surviving. The outcome for these women would have been the same even if they hadn't been treated until later on.
Between 100 and 500 women would have a false alarm, and go through the worry of more tests before getting the all-clear. About half of these women would undergo an unnecessary biopsy, where breast tissue is removed and checked for cancer.
Where does the study come from?
The researchers were based in Denmark. Their study was published in the BMJ (British Medical Journal), which is owned by the British Medical Association.
What should I do now?
In the UK, women between 50 and 70 are offered mammograms every three years. Women over 70 can continue being screened if they wish, but aren't invited routinely.
Whether you accept the invitation for screening or not is your decision. You may decide that the benefits outweigh the risks. However, if you're not convinced, there's no need to feel pressured into taking part. Iona Heath, a doctor writing about her own decision to refuse screening, says, "It is not wrong to say yes, but neither is it wrong to say no".
In the future, changes to the way mammograms are done could potentially reduce the numbers of women diagnosed and treated unnecessarily. For example, trials could be done to find out whether it's safe to ignore lumps below a certain size.
If you find a lump or notice any other changes in your breast, it's still extremely important to see a doctor as soon as you can.
From:
Jørgensen KJ, Gøtzsche PC. Overdiagnosis in publicly organised mammography screening programmes: systematic review of incidence trends. BMJ. 2009; 339: b2587.
© BMJ Publishing Group Limited ("BMJ Group") 2009
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