Mammograms Don’t Reduce Cancer Deaths, Says Large Study
Tens of thousands of women followed
Researchers at the University of Toronto followed 89,835 women between the ages of 40 and 59 years old among 15 breast cancer screening centers throughout Canada. During the initial five years of the study half the women (44,925) received a mammogram examination each year and half the women (44,910) received one physical examination during the five year period.
Within that five-year period, the mammography diagnosed 666 breast cancers, and 3,250 were diagnosed in the 25-year follow up. Of the single physical examination group, 524 women were diagnosed with breast cancer in the first five years, and 3,133 were diagnosed in the 25-year follow up period.
Yet in the five years of screening, 180 women in the mammography group died of breast cancer while only 171 women died in the physical examination group.
The researchers identified 142 cancer cases they considered “over-diagnosed.” They also found that this excess diagnosis calculated to one over-diagnoses breast cancer per 424 women receiving mammography, or 22% of all mammography-detected cancer cases – or 106 over-diagnosis cases for 484 good diagnoses.
The researchers wrote:
“In conclusion, our data show that annual mammography does not result in a reduction in breast cancer specific mortality for women aged 40-59 beyond that of physical examination alone or usual care in the community. The data suggest that the value of mammography screening should be reassessed.”
As to whether the mammography provided any conclusive benefit, the researchers wrote:
“However, after excluding the prevalent breast cancers from the mortality analysis, the data do not support a benefit for mammography screening,”
Did the Mammograms cause more cancer?
Another interesting finding of the study was that among those receiving mammography, there were a total of 4,789 deaths from all causes, while the group not receiving mammography had 4,688 deaths from all causes – 10.44% versus 10.66%.
Many have conjectured that this might be the result of the mammography group receiving higher levels of radiation over the five year period – which could result in a total dose of about 2 millisieverts over the five year period – as one traditional mammogram dose was about 0.4 millisieverts (mSv).
Is this enough to increase risk of cancer? This depends upon other doses of radiation the person may also be given during that period, including dental X-rays, other X-rays or CT-scans, how much they fly, and where they live – as different locations have different background radiation levels.
In the broad analysis, yes, the increased radiation could add to the radiation burden of a person, and add to their increased risk of cancer and other radiation-related conditions. But, in the context of the radiation that most people receive from background radiation (which averages about 3 mSv when the high and low ranges are calculated in the U.S.) along with other ionizing EMF sources, the difference may or may not be enough to increase cancer risk.
In this study, the notion that the mammography in itself produced more breast cancer was examined and eliminated by the researchers, who wrote:
“It has also been suggested that women in the screening group might have been at higher a priori risk of developing breast cancer than women in the control group. After the screening period ended, however, breast cancer was diagnosed in 5.8% of women in the mammography arm and in 5.9% of women in the control arm, showing that the risk of breast cancer was identical between the compared groups.”
The bottom line is that not only is the risk of over-diagnosis a real reason to be concerned about mammography, but there is the extra dose of radiation to be weighed into the equation.
Two-view mammograms versus new mammogram technology
When the mammograms of the University of Toronto were given – in the 1980s – mammography consisted of a two-view X-ray. This has been shown to emit an effective dose of radiation from 0.25 mSv to 0.5 mSv, which has been shown to increase the lifetime risk of cancer death for a 40 year-old woman by 1.3 to 1.7 women in 100,000.
Today’s mammograms typically utilize breast-specific gamma imaging and positron emission mammography to produce multiple views of the tissues from different gradients.
Even among those centers not using these recent technologies, digital breast tomosynthesis is typically used, which have increased effective doses that range from 0.5 mSv to 1.0 mSv, increasing the top of the lifetime cancer death risk range up to 2.6 women in 100,000.
Furthermore, research from the University of Colorado’s School of Medicine determined that the newest gamma imaging system delivers an effective dose of 5.9 to 9.4 mSv. This dosage is calculated to produce a lifetime increased risk in cancer deaths by 26-39 per 100,000 for women aged 40 years old during exposure.
The research also found that the positron emission mammography technology yields an effective dose of 6.2 to 7.1 mSv, which yields an increased risk of cancer death by 30 cases for every 100,000 women 40 years old receiving the dose.
Thus we can conclude that the lower increased risk delivered (1.3-1.7 per 100,000 women) with the two-view X-ray technique may not have equated to noticeable cancer cases from the 1980s, but the increased doses received by today’s mammogram technologies may require further consideration.
As always, check with your doctor.
Miller AB, Wall C, Baines CJ, Sun P, To T, Narod SA. Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial. BMJ. 2014 Feb 11;348:g366. doi: 10.1136/bmj.g366.
Hendrick RE. Radiation doses and cancer risks from breast imaging studies. Radiology. 2010 Oct;257(1):246-53. doi: 10.1148/radiol.10100570.