(NaturalNews) In countries with public breast cancer screening programs, one in every three diagnosed with invasive breast cancers would never have produced symptoms in a patient before she died of other causes, a new study has revealed."Screening for cancer may lead to earlier detection of lethal cancers but also detects harmless ones that will not cause death or symptoms," wrote the researchers, from the Nordic Cochrane Center in Denmark, in the"The detection of such cancers, which would not have been identified clinically in someone's remaining lifetime, is called overdiagnosis and can only be harmful to those who experience it."Researchers analyzed breast cancer diagnosis rates among both screened unscreened women in Australia, Canada, Norway, Sweden and the United Kingdom for at least seven years before and after the public breast cancer screening programs in those countries began. As expected, they found that breast cancer diagnosis rates in every country increased in conjunction with the introduction of screening programs. Breast cancer rates among older women did not undergo a corresponding decrease, however  suggesting that rather than detecting cancers earlier, screening was merely detecting cancers that would otherwise never have produced a detectable effect on a woman's life.When all forms of breast cancer were taken into account, the rate of overdiagnosis after public screening programs were introduced ranged from a low of 46 percent (in Sweden) to a high of 59 percent (in Canada), with an average overdiagnosis rate of 52 percent. When only invasive breast cancers were taken into account  cancers that have spread beyond the mammary tissue and are more likely to be lethal, and thus more likely to be treated aggressively  the average rate of overdiagnosis was still 35 percent, or more than one in three.This was the second time that this research team had found evidence that overdiagnosis is a serious consequence of public screening programs."[The study] means that screening for cancer , in this case breast cancer, is a much closer call than has been previously advertised," wrote Gilbert Welch of the Dartmouth Institute for Health Policy in an accompanying editorial. "It has the opportunity to help some women but it also has the consequence of leading others to be treated needlessly for cancer and that's not a trivial thing."Because no tests exist that can predict how aggressive or dangerous a cancer will be, all women diagnosed with breast cancer are referred to similar treatment programs, many of which  such as chemotherapy, radiation and breast surgery  carry serious and even dangerous side effects.Screening advocates insisted that the benefits of screening still outweigh the risks of overdiagnosis."Without screening, women would face the prospect of having to wait for a visible symptom of cancer, such as a lump, to become apparent before treatment could start," said Emma Pennery of Breast Cancer Care.Sarah Cant of Breakthrough Breast Cancer agreed, but said that women should be given clear information about screening in order to make informed decisions.Welch also believes that better information is essential, saying that doctors should show women a simple statistical table quantifying the relative risks and benefits of screening for them, based on their own risk profile."Mammography undoubtedly helps some women but hurts others," he said. "No right answer exists, instead it is a personal choice."Researchers do not know how many lives are saved for every case of overdiagnosis, with estimates ranging between one in two and one in 10.Welch noted, however, that "the amount of overdiagnosis is a function of the mammographer's threshold to recommend biopsy.""The time has come for a randomized controlled trial to test higher thresholds, such as only recommending biopsy for breast masses larger than a certain size," he wrote.Sources for this story include: news.bbc.co.uk; www.cancerpage.com