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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

Rapid Response:

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

If existing data is consistent with conflicting interpretations, if it does not command consensus amongst reasonable disinterested qualified people, then the problem is not the quality of the analyses but the quality of the data.

Data that cannot clinch the matter is no basis for a screening programme.

It is uncontroversial that if a woman attends mammography screening, no conscientious practitioner can give her useful information about what her results mean.

Nobody can tell her that a negative mammogram means she doesn’t have cancer, still less that she won’t have it tomorrow.

If the mammogram shows something suspicious, diagnostic tests may yield a negative result. All anyone can say is that the bit that caused concern probably wasn’t cancerous, according to this competent pathologist, on balance. They can’t say she has no cancer, only that nothing else showed on the mammogram, and they can’t say that she won’t have cancer tomorrow.

Diagnostic tests may show cancer. Whether invasive or noninvasive, nobody can tell her that it will progress without treatment since some, but they don’t know how many, and they don’t know which, do not. They can tell her that estimates of numbers vary widely. So they don’t know if she has progressive cancer or not, and on this basis it’s time to decide about treatment.

She’ll want to know whether, if her cancer is progressive, earlier treatment will prolong her life. Nobody can tell her there is a good chance it will because nobody knows if there is any chance it will. Nobody can honestly say that anyone declining screening, or declining treatment at point of diagnosis after screening, will die sooner because they have not had earlier treatment.

They can tell her the wide range of estimates of the other treatment outcomes: life shortened, no difference to lifespan, unnecessarily treated. But they can’t say which will affect her. They can say how different the situation is now from when the poor to middling quality trials were carried out, making the estimates inapplicable to her present predicament. They can tell her about more recent research, but they’d have to say they can’t be more precise because the experts are still – er – debating – its implications and it would be misleading to give only one side of the debate.

She will wonder if anything she has heard constitutes information. For all she’s been through, she knows no more about her situation with regard to breast cancer than before she was screened. She could have read the tea leaves.

A person who declines screening is not risking death. Screening, if it worked, would offer a chance of longer life than fate decrees at a price, so declining it would be forgoing a chance of extra time and not paying the price, not risking earlier death. As things stand, being screened is risking grave harm for no known benefit. Declining it is opting out of Russian roulette.

No conscientious expert would deny these things.

Competing interests: Diagnosed through screening

14 March 2014
Miriam Pryke
PhD student
King's College London
WC2R 2LS