Case 2                                                     

You are seeing a 42-year-old asymptomatic woman in your clinic who has read in the newspaper that women should get mammograms starting from age 40. She wants to know how good is mammogram in screening for breast cancer in a woman of her age.

What do you answer to her?

Step 1. Formulating the question. In response to the question, you ask, "In an asymptomatic 42 year-old patient how effective is screening mammogram in early detection of breast cancer?" (question on screening)

Step 2. Identifying and retrieving relevant articles. You conduct a search using the Best Evidence CD Rom and type in the term "Screening Mammogram" and recover an abstract which seems to be addressing our question directly, "Canadian national breast screening study: 1. Breast cancer detection and death rates among women aged 40 to 49 years." (Can Med Assoc J. 147:1459, 1992)

Step 3. Appraising data critically. In a randomized controlled trial involving 50430 women with mean follow up of 8.4 years, where 25214 women were assigned to annual mammography and physical examination of breasts while 25216 women were assigned to single physical examination and annual follow-up through questionnaire. The mammogram group had 156 small invasive cancers (EER of 0.61%), compared to 116 in the control group (CER of 0.46%) giving an ARR of 0.15% and NNS (number needed to screen) of 666 patients for 7 years to detect one additional small invasive breast cancer. The 7 year survival rate was similar in both groups (90.2% in mammogram group and 89.9% in control group).

Step 4. Applying the evidence. You tell your 42-year-old patient that the evidence regarding routine screening mammogram for women in 40-49 years has not shown to decrease the mortality of breast cancer for up to 7 years of follow-up. Since the patient didn't have a high risk family history, and the NNS was 666, it is possible that a positive mammogram could possibly be a false positive results and lead to additional testing. You also indicate that a recent paper (NEJM 338:1089, 1998) indicates that the cumulative incidence of false positive mammogram for women 40 to 49 years after 10 screening mammogram was 56.2%. Your patient decides to wait for her mammogram till she is 50 years or there is more evidence that screening women of her age with her low risk profile definitely improves survival. 

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