Though screening mammography starting at age 40 to 50 has been the standard of care for more than a third of a century, we are still sorting out the correct way to use it. The technique is another example of the ready-fire-aim approach that characterizes the introduction of new approaches to diagnosis and treatment by the medical profession. The October 13, 2016 0f the New England Journal of Medicine presents a paper and an accompanying editorial that attempt to sort out the proper use and the result of screening mammography. Below is the abstract of the paper:


The goal of screening mammography is to detect small malignant tumors before they grow large enough to cause symptoms. Effective screening should therefore lead to the detection of a greater number of small tumors, followed by fewer large tumors over time.


We used data from the Surveillance, Epidemiology, and End Results (SEER) program, 1975 through 2012, to calculate the tumor-size distribution and size-specific incidence of breast cancer among women 40 years of age or older. We then calculated the size-specific cancer case fatality rate for two time periods: a baseline period before the implementation of widespread screening mammography (1975 through 1979) and a period encompassing the most recent years for which 10 years of follow-up data were available (2000 through 2002).


After the advent of screening mammography, the proportion of detected breast tumors that were small (invasive tumors measuring <2 cm or in situ carcinomas) increased from 36% to 68%; the proportion of detected tumors that were large (invasive tumors measuring ≥2 cm) decreased from 64% to 32%. However, this trend was less the result of a substantial decrease in the incidence of large tumors (with 30 fewer cases of cancer observed per 100,000 women in the period after the advent of screening than in the period before screening) and more the result of a substantial increase in the detection of small tumors (with 162 more cases of cancer observed per 100,000 women). Assuming that the underlying disease burden was stable, only 30 of the 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 (i.e., cases of cancer were detected on screening that never would have led to clinical symptoms). The potential of screening to lower breast cancer mortality is reflected in the declining incidence of larger tumors. However, with respect to only these large tumors, the decline in the size-specific case fatality rate suggests that improved treatment was responsible for at least two thirds of the reduction in breast cancer mortality. 


Although the rate of detection of large tumors fell after the introduction of screening mammography, the more favorable size distribution was primarily the result of the additional detection of small tumors. Women were more likely to have breast cancer that was overdiagnosed than to have earlier detection of a tumor that was destined to become large. The reduction in breast cancer mortality after the implementation of screening mammography was predominantly the result of improved systemic therapy.

The following are excerpts from the editorial that commented on the study:

In this issue of the Journal, Welch et al add to the growing literature regarding overdiagnosis by describing the shift in size distribution of breast-cancer tumors since screening mammography was introduced in the United States. When data from women who received a diagnosis of breast cancer in the late 1970s were compared with data from those who received a diagnosis in the early 2000s, the incidence of large tumors decreased by 30 cases of cancer per 100,000 women (which suggests that screening has had the desired effect), and the incidence of small tumors increased by 162 cases of cancer per 100,000 women. Assuming that the underlying burden of clinically meaningful breast cancer was unchanged, these data suggest extensive overdiagnosis of small tumors (i.e., only 30 of the 162 additional small tumors per 100,000 women that were diagnosed were expected to become large). Welch et al. also propose that the reduction in breast-cancer mortality for large tumors after the introduction of screening mammography reflected improved cancer treatment more than screening

The National Academies of Sciences, Engineering, and Medicine recently deemed improvement of the diagnostic process “a moral, professional, and public health imperative.” Rigorous analytic methods are required for the development of disease nosologies, and physicians need more sophisticated tools to improve diagnostic precision and accuracy. At the patient level, we need better methods of distinguishing biologically self-limited tumors from harmful tumors that progress.

It is clear, and has been for some time, that screening mammography has had effects different from those expected and desired when the technique was first introduced. Yet the message has not been received by both practicing physicians and the public they serve. The fear of medical malpractice litigation is one of the most immutable barriers stopping a revision in the standard of care which now mandates screening mammography. This is not new information, that the world’s most influential medical journal sees fit to publish both a paper and an editorial asking for a reassessment of current screening mammography guidelines, shows the inertia and politicization that grips this issue. I don’t see meaningful change in front of us. The cost, both psychic and financial, of our current practice is astronomical.  “Stop me before I screen again,” seems an unanswered plea.