The role of screening mammograms is in the news once again, with the publication of a new Canadian study that reported the 25-year follow-up of nearly 90,000 women aged 40-59, half of whom were randomly assigned to receive yearly mammograms for 5 years.
The investigators concluded that, “Annual mammography in women aged 40-59 does not reduce mortality from breast cancer beyond that of physical examination or usual care when adjuvant therapy for breast cancer is freely available.” Furthermore, “Overall, 22% (106/484) of screen detected invasive breast cancers were over-diagnosed.”
Every study has strengths and limitations, as does this latest, and expect to see and hear controversy as these are debated in the media. But, when all of these studies are considered together, what do we know?
1. Breast cancer is a potentially deadly disease.
- In this study of 89,835 women followed for 25 years, 9477 (10.6%) died of all causes. Breast cancer was the cause of death in 1005 women (1.1%)
2. Women have longer survivals if they are diagnosed with smaller breast cancer tumors. And mammography can find some tumors that are too small to be felt.
- In this study, the survival at 25 years was 77.1% if the tumor was <2 cm but 54.7% if the tumor was >2 cm.
- There were 212 women diagnosed by mammography only (with tumors that were not felt on physical exam, and on average only 1.4 cm in size, compared to an average 2.1 cm for palpable tumors), and their survival at 25 years was highest, at 79.6%.
Some of that longer survival may be due to lead-time bias, length bias, and over-diagnosis. Lead-time bias means that the date of death from breast cancer didn’t change, but the number of days alive after the diagnosis of breast cancer is greater because it was diagnosed earlier. Length bias is due to finding more cases of breast cancer that are slowly progressive, so that the types of patients found with screening are different than the types of patients found without screening. Over-diagnosis refers to finding breast cancers that would have otherwise not ever been found and not lead to disease or death. In this study, it is estimated that 106 of the 484 cases of breast cancer (22%) found during screening were cases of over-diagnosis.
3. The definition of an effective breast cancer screening test is that it is associated with a reduction in death from breast cancer.
- In this study, there was no reduction in the rate of death from breast cancer among women randomized to screening mammography. There were 500 breast cancer deaths among 44,925 mammography-screened women (1.1%), and 505 breast cancer deaths among the 44,910 controls (1.1%).
- There are other large studies with different designs that do seem to show a reduction in breast cancer death with mammography, and hence controversy.
The bottom line is that mammography is an imperfect screening test. But for a woman who, in discussion with her doctor, chooses to maximize her chance of finding and treating breast cancer (and understanding the risks of over-diagnosis), mammography remains a good choice. The American Cancer Society recommends yearly mammography beginning at age 40, and the US Preventative Services Task Force recommends every other year mammography beginning at age 50.
It is very important to remember that in this study, women who were not assigned to mammography still had access to high-quality care that included physical exams (baseline, and yearly when aged 50-59), education about breast health, and access to adjuvant therapy if they were diagnosed with breast cancer. The rate to which women in this Canadian study were able to maintain these “usual” care elements and follow-up were very high, and may have been one of the reasons that the benefit of adding mammography was not shown. For example, in this study, the average tumor size diagnosed in the mammography group was 1.98 cm, compared to only 2.10 cm in the controls! Other studies showing a mortality benefit to mammography have reported average tumor sizes in the control groups of 2.8cm (over half a centimeter larger).
This observation stresses the critical importance of delivering high-quality cancer-directed care (with or without routine screening mammography). LIVESTRONG remains committed to supporting patients in their personal decisions about cancer screening and policies that maximize access to quality cancer care today, while we wait for better cancer screening tests in the future. If you have questions about cancer screening the LIVESTRONG Cancer Navigation team can help – www.livestrong.org/we-can-help or call 1-855-220-7777.
- Dr. Brandon Hayes-Lattin is the LIVESTRONG Foundation’s Senior Medical Adviser.