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Mammograms in Women Age 40 to 49: Results of The Canadian Breast Cancer Screening Study
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What is the problem and what is known about it so far?
Breast cancers are often found when a woman or her doctor feels a lump in the breast. Mammograms are special x-rays of the breast that can identify breast cancers before lumps are felt. The goal of screening mammograms is to identify breast cancers before they have spread. Recently, there has been much debate about whether mammograms decrease rates of death from breast cancer. The controversy is greatest for mammograms in women age 40 to 49 years. The Canadian Breast Cancer Screening study is a large study of screening (mammograms and breast examinations by doctors) for women in that age group. Seven to 10 years after the first screening visit, screening had not reduced deaths from breast cancer. However, some experts suggested that it would take longer than 10 years to demonstrate benefit from screening.
Why did the researchers do this particular study?
The researchers report results of the Canadian Breast Cancer Screening Study that were obtained an average of 13 years after the women entered the study.
Who was studied?
50,430 Canadian women age 40 to 49 years who had no previous breast cancer and no mammogram for at least 12 months before they entered the study.
How was the study done?
The researchers enrolled women between 1980 and 1985 and randomly assigned them to have either yearly mammograms and breast examinations by doctors for the next 5 years or to receive usual care. “Usual care” meant that mammograms and breast exams were done only if the patient's doctor suggested them. Women in both groups received instructions on how to examine their own breasts. The researchers then contacted the women yearly by mail for 5 years and examined cancer registries thereafter to see who developed breast cancer. They compared the frequency of death due to breast cancer in the women in the mammogram group to the frequency in the usual care group.
What did the researchers find?
105 of the 25,214 women in the mammogram group and 108 of the 25,216 in the usual care group died of breast cancer. The numbers of invasive (cancer that had spread into surrounding areas) and in situ (cancer that had not spread) cancers were similar in both groups. These results suggest that mammograms and regular breast examinations do not reduce breast cancer deaths.
What were the limitations of the study?
Although this study included more women than other studies on this topic, it had too few patients to detect reductions in breast cancer deaths smaller than 20%.
What are the implications of the study?
This study suggests that mammograms decrease the breast cancer death rate in women 40 to 49 years of age by less than 20%.
Article and Author Information
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The summary below is from the full report titled “The Canadian National Breast Screening Study-1: Breast Cancer Mortality after 11 to 16 Years of Follow-up. A Randomized Screening Trial of Mammography in Women Age 40 to 49 Years.” It is in the 3 September 2002 issue of Annals of Internal Medicine (volume 137, pages 305-312). The authors are AB Miller, T To, CJ Baines, and C Wall.
- Copyright ©2004 by the American College of Physicians
