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Breast cancer is the most common malignancy affecting American women excluding skin cancers. It is the second leading cause of cancer-related deaths, having been surpassed by mortality from lung cancer. The American Cancer Society estimated that there would be 182,460 new cases of breast cancer and 40,480 breast cancer-related deaths in 2008.1 Breast cancer mortality in the United States has declined substantially over the past 30 years, from 31.4 deaths per 100,000 women per year in 1975 to 25.9 deaths per 100,000 women per year in 2001.2 The reduction in breast cancer mortality has been attributed to advances in treatment options and the combination of increasing utilization of screening mammography and improved mammographic quality.3,4 Mammography remains the only study proven to detect early breast cancer and decrease breast cancer–related deaths.
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An additional important goal of screening mammography is to identify cancers when they are small, and subsequently at an earlier stage. In a study conducted by Barth and associates,5 the tumor size of patients who had breast cancer detected by mammography was 1.5 cm, which was statistically significantly smaller than the cohort group whose breast cancer was detected by physical exam and in which tumor size was on average 2.9 cm. The screening-detected cancer patients were also more likely to be node negative and less likely to receive chemotherapy. Similar results have been reported in populations with high mammographic screening rates, such as in Rhode Island.6 Ductal carcinoma in situ (DSIS), which is believed to progress to invasive carcinoma in certain patients, is clinically occult and only identified by mammography.
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The benefits from screening mammography for women age 40 to 70 years have been proven in 8 randomized controlled trials (RCTs) during the past 40 years.7-14 These trials were conducted in Europe (Edinburgh, Malmö, Gothenburg, Stockholm, Swedish Two County), Canada (includes 2 trials: age 40-49 and age 50-59), and the United States (Health Insurance Plan Project). Table 5-1 summarizes the results of these trials. All but one of the RCTs report statistically significant results as measured by mortality reduction as end-point. Reported reductions in breast cancer mortality range from -2% to 32%. In addition, long-term follow-up of 3 trials (HIP, Gothenburg, and Malmö) each found statistically significant reductions in breast cancer mortality.15-17 The only RCT to actually show mortality in the screening group is the Canadian study for women aged 40 to 49 years. This study has been highly criticized for the poor image quality and for including patients with clinically evident advanced breast cancer.18 The benefits of mammographic screening are in general accepted as valid. Controversy remains for screening women younger than the age of 40 years; in this group there are few data from RCTs. Therefore, decisions concerning screening practice in this age group must be based on less rigorous evidence and on a more individual basis.
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