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The efficacy of earlier detection of breast cancer by routine screening of women with mammography was clearly demonstrated in several randomized controlled trials (RCTs) conducted in the 1970s-1990s (1). The benefit is understood to come from the ability to treat smaller and/or earlier stage screen-detected cancers, the key words being “earlier” and “treatment”. This is logical – no oncologist, given the choice would opt to treat more advanced disease.
Randomized trials are held up as the gold standard for quality of evidence. But this is only true if the intervention tested in the RCT is consistent with the current question of interest – i.e. what is the effectiveness of today’s breast cancer screening in women aged 40-74? Although the breast screening RCTs provided proof-of-principle of benefit, the screening techniques available at the end of the last millennium were primitive compared to what is used in screening programs today. Since that time there have been major improvements in both screening and therapy. In particular, the detectability of small invasive cancers has improved markedly.
Data from multiple organized screening programs have demonstrated that women aged 40 and over who avail themselves of screening mammography are from 30-45% less likely to die of breast cancer than their non-screened counterparts, two to three times the mortality reduction seen in the RCTs.(2,3) Much of the increased benefit holds, even after conservative corrections were applied for self-selection bias, one of the limitations of such observational studies. Recent studies have shown that earlier detection also decreases morbidity associated with breast cancer therapy, often reducing the need for mastectomy, axillary dissection and chemotherapy (4).
There are no RCTs attempting to evaluate the impact of modern screening combined with modern therapy. And there are unlikely to be any in the future because most scientists in the field are convinced that the question is already answered and feel that the cost and time delay in conducting another trial with modern screening are unjustified. Given the RCT results as a baseline, they accept the data from service screening programs that are consistent with a greater level of benefit than demonstrated by the RCTs. Further, it is unlikely that women would be willing to be randomized to a study arm that did not include screening.
Nevertheless, The Canadian Task Force on Preventive health Care (CTF) has chosen in 2011 and again in its current recommendations to ignore any data on benefit except that from the old RCTs. It refuses to consider modern data (some of it coming from recent Canadian studies), yet it places much emphasis on harms, without making the effort to put those into context with the benefits of reduced mortality and morbidity, a standard approach used in health services research. The CTF’s justification against screening women in their 40s is based on lower cancer risk in this group (in fact 24% of life-years lost and 14% of breast cancer deaths (5) come from cancers arising over that age range) and harms that are estimated only in fuzzy terms and in some cases incorrectly.
Even the title of the Recommendations is misleading, limiting the recommendations to “… women not at increased risk …”. The greatest increase in breast cancer risk is in being a woman over 40. The next greatest attributable risk is having very dense breasts, a double-barrelled risk, because it is also more difficult to detect cancer in the dense breast with mammography – ultrasound is more accurate there. But despite a vast literature, density and ultrasound are dismissed out of hand by the CTF.
The Recommendations refer to an approach of “shared decision making between a woman and her health care provider” on the issue of whether and when to be screened. A great idea. But has it not always been a patient’s right to accept or reject medical advice? And it would have been more helpful if the CTF had provided accurate information to the physician to guide that advice.
Why does the CTF continue to make recommendations that are at odds with the science? The full reason is not clear to me, but the dogma that the only acceptable form of evidence is an RCT, even if that RCT doesn’t really speak to the question – is ludicrous. Physicians and the public should reject these recommendations. Given the very limited and cherry-picked consideration of modern data by the CTF, physicians cannot rely on its guidelines and have no other choice but to turn directly to the research literature. Our Federal Health Ministry should review and revamp the governance of the Task Force – a good idea that has gone off the rails.
1. International Agency for Research on Cancer. Breast cancer screening / IARC Working Group on the Evaluation of Cancer-Preventive Interventions, 2014. – 2nd edition. (IARC Handbooks of Cancer Prevention; Volume 15) Lyon.
2. Coldman A, Phillips N, Wilson C, Decker K, Chiarelli AM, Brisson J, Zhang B, Payne J, Doyle G, Ahmad R. Pan-Canadian study of mammography screening and mortality from breast cancer. J Natl Cancer Inst. 2014; Oct 1;106(11). pii: dju261. doi: 10.1093/jnci/dju261.
3. Broeders, M., Paci, E. The balance sheet of benefits and harms of breast cancer population-based screening in Europe: outcome research, practice and future challenges. Women's health (London, England) 2015; 11(6): 883-90.
4. Barth RJ Jr., Gibson GR, Carney PA, Mott LA, Becher RD, Poplack SP. Detection of breast cancer on screening mammography allows patients to be treated with less-toxic therapy. AJR. 2005;184:324–9.
5. Yaffe MJ, Mittmann N, Alagoz O, Trentham-Dietz A, Tosteson AN, Stout N. The effect of mammography screening regimen on incidence-based breast cancer mortality. J Med Screen. 2018 Dec;25(4):197-204. doi: 10.1177/0969141318780152. Epub 2018 Jul 26.