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Screening Mammography: Does Overdiagnosis Overshadow Prevention of Advanced Breast Cancer?

Summary and Comment |
November 21, 2012

Screening Mammography: Does Overdiagnosis Overshadow Prevention of Advanced Breast Cancer?

  1. Andrew M. Kaunitz, MD

Analysis of U.S. data spanning 30 years suggests many mammographically detected tumors are destined to be harmless.

  1. Andrew M. Kaunitz, MD

Screening to lower cancer mortality should enable earlier detection of malignancies destined to be fatal while also facilitating early treatment of screen-detected cancers. U.S. investigators analyzed three decades of federal data to assess the long-term effects of screening mammography. Breast cancer rates from 1976 through 1978 (when mammography was uncommon) were used to estimate baseline incidence; data from 2006 through 2008 were used to estimate current incidence. To minimize confounding effects of menopausal hormone therapy, the transitory increase in incident breast cancers from 1990 through 2005 was not included. Models for determining the excess in screen-detected early-stage breast cancer as well as the reduction in diagnoses of late-stage cancer included the “best-guess” estimate, in which the underlying incidence of breast cancer was assumed to rise by 0.25% annually (the known percent change in women who were younger than 40).

Incidence of early-stage breast cancer rose from 112 (baseline) to 234 (current) cases per 100,000 women. During the same 30-year interval, incidence of late-stage disease declined by 8 cases per 100,000. Overdiagnosis (i.e., identification of tumors destined not to progress to advanced disease) attributable to screening mammography affected an estimated 1.3 million women (including >70,000 women in 2008 alone, when overdiagnosis accounted for 31% of tumors identified in women 40 and older). During the study period, breast cancer mortality fell by 28% among women 40 and older and by 42% in women younger than 40, a group in which screening was not prevalent.

Comment

By promoting early diagnosis of breast cancer, screening mammography can save lives. However — and consistent with other reports (JW Womens Health Sep 22 2010 and JW Womens Health Aug 11 2011) — this study suggests that screening's contribution to the decline in breast cancer mortality is surpassed by improvements in treatment, and that the benefits of screening mammography are smaller and the harms associated with overdiagnosis greater than have been previously appreciated. This viewpoint is more in line with the 2009 U.S. Preventive Services Task Force recommendation (JW Womens Health Nov 16 2009) that women initiate screening mammography later (i.e., after age 50) and less frequently (i.e., biennially) than has been specified in guidelines from other organizations, including the American Cancer Society, the American College of Obstetricians and Gynecologists, and the American College of Radiology (JW Womens Health Feb 4 2010). In the future, comprehensive genetic analysis of breast tumors could allow cancers to be distinguished according to their potential to cause advanced disease (JW Oncol Hematol Oct 9 2012). Until then, the pros and cons of mammography should be incorporated into the counseling that women receive as they decide whether and when to be screened.

Citation(s):

Reader Comments (2)

Andrew M Kaunitz

Thank you for your feedback regarding my JWWH summary of Welch’s NEJM article. In suggesting that these findings support the USPSTF mammography guidelines, my point was that the findings are likely to steer clinicians and women in the direction of starting mammograms later and having them less frequently than has been recommended by other organizations, including ACS, ACOG, and ACR. I agree that this point did not come through clearly enough; accordingly, we have revised and republished the summary.

Competing interests: Editor-in-Chief, JWWH

Richard Ganz, MD

This is Welch's last paragraph in article summarized by Kaunitz in Nov 22 Women's Health.

Our study raises serious questions about the value of screening mammography. It clarifies that the benefit of mortality reduction is probably smaller, and the harm of overdiagnosis probably larger, than has been previously recognized. And although no one can say with certainty which women have cancers that are overdiagnosed, there is certainty about what happens to them: they undergo surgery, radiation therapy, hormonal therapy for 5 years or more, chemotherapy, or (usually) a combination of these treatments for abnormalities that otherwise would not have caused illness. Proponents of screening should provide women with data from a randomized screening trial that reflects improvements in current therapy and includes strategies to mitigate overdiagnosis in the intervention group. Women should recognize that our study does not answer the question “Should I be screened for breast cancer?” However, they can rest assured that the question has more than one right answer.

How can this be construed as supporting biennial mammography starting at 50?

Competing interests: None declared

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