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2004 Update on Summary of the Evidence

Posted by medconsumers on January 26, 2004

It is not clear whether the benefits of mammography outweigh the risks for women in their 40s. Not all of the clinical trials have consistently shown that mammography screening reduces your chances of dying of breast cancer. Of the seven large trials in which women were randomly assigned to have periodic mammograms or not, five trials showed a lower breast cancer death rate among all mammography-screened women, ages 40 to 69 years. But two found no lifesaving benefit to mammograms for women of any age. Pool the results of all seven trials for women ages 50 and older, and the breast cancer mortality for those having mammograms drops by 22% in relation to the unscreened women. Pool the results for women in their 40s, and their breast cancer mortality drops by 15% in comparison to their counterparts not given mammograms. Here’s what that means to you as an individual: If you are in your early 40s, your risk of dying of breast cancer in the next ten years is 0.3%. If you start mammograms and continue having them for the next ten years, you will, at best, reduce that risk of dying of breast cancer by one-tenth of 1%.

This modest benefit should be weighed against mammography’s potential for causing some women to undergo unnecessary treatment with radiation therapy or mastectomy. Clinical trials have documented mammography’s ability to find cancers that will never produce symptoms or become life threatening. One of the two trials that found no mortality reduction benefit for mammography was conducted in Canada . It is the only one specifically designed to answer the long-standing question of mammography’s value to women in their 40s. All the women in this trial were given skilled professional breast exams, but only half of them had regular mammograms.

There were 40 more cases of nonpalpable invasive breast cancer and 42 more cases of ductal carcinoma in situ (a non-invasive cancer that some experts believe is just a risk factor for breast cancer) detected in those screened with mammography than in those given solely a professional breast exam. Contrary to expectations, their rate of breast cancer deaths after 13 years was no different from that of the women not given regular mammograms. This not only shows that the technology leads to the unnecessary treatment of some cancers that would have remained dormant, but it also detects a type of breast cancer so slow-growing that women will be successfully treated regardless of when tumors are found.

An in-depth review of all seven mammography-screening trials, conducted in 2001 by the Nordic Cochrane Centre, concluded that the two showing no breast cancer mortality-reduction benefit to mammography were of a higher quality than the five trials that did. This review also looked at the question of whether mammography saves lives when deaths from all causes are taken into account. This is an important consideration because some women die of breast cancer treatment-related causes. For example, some trials showed an increase in cardiovascular deaths after radiation therapy among younger mammography-screened women. The Nordic Cochrane Centre concluded that mammography screening does not reduce the overall death rate. This conclusion has been widely challenged by other researchers; it is important for women to know about it nonetheless. Although mammography screening is thought to lead to less drastic treatment, the Nordic analysis of all seven trials found 20% more mastectomies among the women given mammograms.

Unnecessary breast biopsies are another well-known consequence of mammography screening. The rate of false alarms, causing emotional trauma, is particularly high among women under the age of 50. If 10,000 women under the age of 50 have a mammogram, 640 will have an abnormal mammogram and 1,280 extra tests will be performed on them. 150 will go on to have a surgical biopsy, of which 17 turn out to have invasive breast cancer. However, only one death, at best, will have been prevented.

Keep in mind that the statistics quoted here come from clinical trials in which high-quality mammography screening is done under the best of circumstances, and results may differ where it concerns mammography as it is performed and interpreted in the day-to-day practice of medicine. While your health practitioner may strongly recommend them, the decision whether to undergo regular mammograms is entirely up to you.

Maryann Napoli
January 26, 2004

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