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Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial

BMJ 2014; 348 doi: https://doi.org/10.1136/bmj.g366 (Published 11 February 2014) Cite this as: BMJ 2014;348:g366

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Re: Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial

Dear Sir:
United States (U.S.) radiologist critics of the Canadian National Breast Screening Study promote a “talking point” that the quality of the Canadian screening mammography was poor or substandard. Therefore, it would be useful to compare the Canadian study results with 21st century screening mammography in the U.S.

Table 2 of the study shows that the 212 non-palpable invasive cancers in the screening arm (mammography screen-detected) had a mean size of 1.4 cm. The 454 palpable cancers in the screening arm (physical exam-detected) had a mean size of 2.1 cm. The overall average size was 1.9 cm, versus 2.1 cm in the control group.1

The Breast Cancer Surveillance Consortium reports that the mean size for invasive cancers detected after 2.1 million screening mammography exams (asymptomatic women) in the U.S. from 2004 to 2008 was 1.8 cm. The median size was 1.4 cm. For 360,000 diagnostic exams in symptomatic women, the mean and median sizes were 2.6 and 2.1 cm.2

How is it possible that mammography screen-detected invasive cancers in the “substandard” Canadian study (1.4 cm) were smaller than is typical today in the U.S. (1.8 cm)?

Overdiagnosis would also be lower with “substandard” mammography, especially without recently adopted digital technology or computer-aided detection. The Canadian study invasive overdiagnosis estimate of 22% equals excess cancers divided by any cancer detected in the screening arm (106/484). As briefly noted but quite alarming, if the denominator is replaced with only mammography screen-detected cancer, overdiagnosis increases to 50% (106/212). This is double the estimate from the Malmo trial.3

Will the authors provide overdiagnosis estimates including in situ lesions? These lesions represent about one third of U.S. screen-detected cancers. 4 The recent cumulative overdiagnosis estimate for screen-detected cancer including in situ lesions in the U.S. is 50%, assuming that 60% of all breast cancer is detected by screening mammography. 5

Yours truly,
John D. Keen, MD
References:
1. Miller AB, Wall C, Baines CJ, Sun P, To T, Narod SA. Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial. BMJ;348:g366.

2. Breast Cancer Surveillance Consortium: National Cancer Institute:BCSC Data & Statistics. Accessed February 14, 2014, at http://breastscreening.cancer.gov/data/benchmarks/.

3. Welch HG, Schwartz LM, Woloshin S. Ramifications of screening for breast cancer: 1 in 4 cancers detected by mammography are pseudocancers. BMJ 2006;332(7543):727.

4. Keen JD. Promoting screening mammography: insight or uptake? J Am Board Fam Med 2010;23(6):775-82.

5. Bleyer A, Welch HG. Effect of three decades of screening mammography on breast-cancer incidence. N Engl J Med 2012;367(21):1998-2005.

Competing interests: I receive income from reading mammograms.

17 February 2014
John D. Keen
Radiologist
Stroger Cook County Hospital
Chicago, Illinois, US