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

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

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.

— Andrew M. Kaunitz, MD

Published in Journal Watch Women's Health November 21, 2012

Citation(s):

Bleyer A and Welch HG. Effect of three decades of screening mammography on breast-cancer incidence. N Engl J Med 2012 Nov 22; 367:1998. (http://dx.doi.org/10.1056/NEJMoa1206809)

• Medline abstract (Free)

Reader Remarks:

Review and add to remarks on this article

• Biennial mammography
  Richard Ganz, MD, Healdsburg CA, 14 Dec 2012 2:14 PM EST
  Specialty: Unspecified
  This is Welch's last paragraph in article summarized by Kaunitz in Nov 22 Women's Health.
  
  Our study raises serious questions... [more]
• Reply: Biennial mammography
  Andrew M Kaunitz, 14 Dec 2012 2:14 PM EST
  Specialty: Gynecology
  Thank you for your feedback regarding my JWWH summary of Welch’s NEJM article. In suggesting that these findings support the... [more]

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