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Estimating net benefits and harms of screening mammography in women age 40-49
Submit responseIn the updated systematic review on the effects of screening mammography [SM] in women age 40-49, Armstrong et al [1] used relative effect measures to express most important benefits of SM, while at the same time they expressed most important harms of SM in terms of absolute effect measures. Notably, they estimated that SM is associated with relative-risk reduction in breast cancer mortality of 7-23%, while 30-200 women of age 40-49 will die in annual SM as a result of radiation-induced breast cancer [1]. It would be, however, more appropriate to express benefits and harms using the same effect measures.
It is particularly important to contrast net benefits and net risks in patients with and without SM. Of crucial importance for any prospective woman contemplating undergoing SM is to understand what outcome she can expect with and without SM. If individualized recommendations called by Qaasem et al is to become reality [2], estimates of net benefits and net harms of SM in women with different baseline risks of dying from breast cancer without undergoing SM should have been included in the recent ACP guidelines/systematic review. Had the authors presented data in such a way, they may have calculated that in the BEST case scenario for women at “average risk” of developing breast cancer, SM may save 46 lives/100,000 women screened over 10 years, at the cost of 30/100,000 lives lost. In the WORST case scenario, net benefits is actually negative, and more lives will be lost than saved [see below].
We believe that presenting data in this way may better help women understand benefits and harms of SM which need to be contrasted against their own individual risk for development of breast cancer. Only in this way can we truly exercise a women’s preferences and values and preserve their autonomy consistent with ethical and rational decision-making [3].
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Calculation of net benefits [4] (outcome: breast cancer deaths):
Net Benefits (B)= BC – BC*(1-RRR) – H
BC= Mortality from breast cancer without SM; RRR= relative reduction in breast cancer mortality due to SM; H= radiation-induced breast cancer deaths
Net harms (H) = Mortality without breast cancer- breast cancer mortality due to SM= 1-(1-H)= H
BEST CASE SCENARIO:
BC= 3.3/1,000 women (average risk)[5] RRR=23% [1] H=30/100,000 women [1]
B= 0.0033 -0.0033*(1-0.23)-0.00030 = 0.00046 (= 46/100,000 lives saved) H=0.00030 (30/100,000 lives lost) B/H=1.5 (If woman’s preferences are not taken into account, this B/H ratio translates into benefits of SM outweighing its harms only when probability of breast cancer exceeds 39.5%[4].)
WORST CASE SCENARIO
RRR=7% [1]
B= 0.0033 -0.0033*(1-0.07)-0.00030 = -0.000069 (negative net benefits; 69 lives lost/1,000,000 screened)
H= 200/100,000 women1
B= 0.0033 -0.0033*(1-0.07)-0.0020= -0.001769 (negative net benefits; 1769 lives lost/1,000,000 screened)
Literature
1. Armstrong K, Moye E, Williams S, Berlin JA, Reynolds EE. Screening Mammography in Women 40 to 49 Years of Age: A Systematic Review for the American College of Physicians. Ann Intern Med 2007;146(7):516-526.
2. Qaseem A, Snow V, Sherif K, et al. Screening Mammography for Women 40 to 49 Years of Age: A Clinical Practice Guideline from the American College of Physicians. Ann Intern Med 2007;146(7):511-515.
3. Djulbegovic B, Lyman G. Screening mammography at 40-49 years: regret or regret? Lancet 2006;368:2035-2037.
4. Djulbegovic B, Hozo I, Lyman G. Linking evidence-based medicine therapeutic summary measures to clinical decision analysis. MedGenMed 2000(January 13):http://www.medscape.com/Medscape/GeneralMedicine/journal/2000/v02.n01/mgm0113.djul/mgm0113.djul -01.html.
5. Moss SM, Cuckle H, Evans A, L. J, Waller M, Bobrow L. Effect of mammographic screening from age 40 years on breast cancer mortality at 10 year's follow-up: a randomised controlled trial. Lancet 2006;368:2053-60.
Conflict of Interest:
None declared