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

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

The recent publication by Miller et al. [1] indicating no mortality benefits from mammography as part of the Canadian National Breast Screening Study is fraught with bias and the conclusions deeply suspect.

The study ignores pre-invasive cancers which have a high survival rate and are often detectable by mammography. It is reasonable to assume that if they had been included in the analysis then the mammography arm of the trial would have demonstrated higher survival rates than those reported. It is not safe to assume that all pre-invasive cancers are indolent (ie. that they would never go on to harm the patient), especially since the standard model of malignant tumour growth has invasive cancers first developing through a pre-invasive stage.

The study’s results contradict the authors’ conclusion that mammography is not assisting in saving women’s lives. The section of the Results titled “Breast cancer survival” indicates that “The 25 year survival was 70.6% for women with breast cancer detected in the mammography arm and 62.8% for women with cancers diagnosed in the control arm” which the authors demonstrate to be a statistically significant difference. This demonstrates a real benefit to women surviving breast cancer thanks to receiving mammographic screening. Had pre-invasive cancers been included in this study the difference in 25 year survival is liable to have been even larger. Concluding that mammographic screening provides no benefit with respect to saving women’s lives based on the analysis presented [1] is unfounded and dangerous.

[1] A. B. Miller, C. Wall, C. J. Baines, P. Sun, T. To, S. A. Narod, “Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial,” British Medical Journal, 2014;348:g366.

Competing interests: No competing interests

14 February 2014
Jacob Levman
Researcher
University of Oxford, Institute of Biomedical Engineering
Parks Road, Oxford, UK, OX1 3PJ
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