Practical Review in Chronic Diseases Free Clinical Brief

Cancer - Clinical Brief

Overdiagnosis, Overtreatment -- Results of Current Screening Protocols

The reduction in breast cancer mortality after the implementation of screening mammography is predominantly the result of improved systemic therapy.

Author: Welch HG, Prorok PC, et al

Background: How important is mammography? Does it meet standard screening criteria? The generally accepted goal of mammography is to detect malignant tumors while they are small, and to prevent the growth of small tumors into large tumors. Effective screening should result in significant reductions in incidence of large tumors with an associated reduction in mortality. What does the U.S. Surveillance, Epidemiology, and End Results (SEER) program data show?

Objective: To evaluate the contribution of screening mammography in the reduction of breast cancer mortality by using trends in malignant breast tumor size.

Methods: Data from the SEER program, 1975 to 2012, were used to calculate the tumor-size distribution, size-specific incidence of breast cancer, and the size-specific cancer case fatality rate among women aged ≥40 years. The study interval includes times before and after widespread adoption of mammographic screening. It was assumed that the disease burden of breast cancer did not change during the study interval.

Results: With screening mammography, detection of large tumors decreased by 30 cases of cancer per 100,000 women (from 145 to 115 cases of cancer per 100,000 women), and the incidence of small tumors increased by 162 cases of cancer per 100,000 women (from 82 to 244 cases of cancer per 100,000 women). There was a much greater increase in detection of small tumors than a reduction in increase of large tumors. The implication is that only 30 of 162 additional small tumors per 100,000 women that were diagnosed were expected to progress to become large, which implied that the remaining 132 cases of cancer per 100,000 women were overdiagnosed (ie, cases of cancer were detected on screening that never would have led to clinical symptoms). The potential of screening mammography to lower breast cancer mortality is reflected in the reducing incidence of larger tumors. However, with respect to only these large tumors, the reduction of size-specific case fatality rate suggests that better treatment caused at least two-third reduction in breast cancer mortality.

Conclusions: Women were more likely to have an overdiagnosed breast cancer than to have an early detection of a tumor that was destined to become large. The reduction in breast cancer mortality after the implementation of screening mammography was primarily due to an improved systemic treatment.

Reviewer's Comments: Registry-based studies from Scandinavian countries have echoed the conclusions of this study. Our screening paradigm leads to much unnecessary treatment and is not optimum. Rather than anatomy-based criteria for detection using mammography, we need to find a way to detect biologic characteristics of the breast that favor aggressive tumor development. In most of the cases, breast cancer appears to be a result of unknown interactions between environmental and genetic factors. Much work is needed to further our understanding of the incipient conditions that increase the risk of neoplasia.(Reviewer–John D. Jacobson, MD).

Article Reviewed: Breast-Cancer Tumor Size, Overdiagnosis, and Mammography Screening Effectiveness.

Welch HG, Prorok PC, et al: N Engl J Med; 2016;375 (October 13): 1438-1447.

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