Higher Rates of Mammography Screening Associated with Higher Rates of Breast Cancers, but not Associated with Decreased Breast Cancer Mortality

EBM Focus - Volume 10, Issue 28

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Reference: JAMA Intern Med 2015 Jul 6 early online (level 2 [mid-level] evidence)

Routine mammography for breast cancer screening is recommended beginning as early as age 40 for women with an average risk of breast cancer (CA Cancer J Clin 2012 Mar-Apr;62(2):129, USPSTF Screening for Breast Cancer 2009 Nov). As the rate of mammography increases, however, so does the potential for overdiagnosis and overtreatment. Overdiagnosis occurs if screening leads to diagnosis of cancers that would never have affected the patients’ health or longevity. Mammography has been associated with an increase in the detection of early stage breast cancer, but only a small reduction in the incidence of late-stage breast cancer (N Engl J Med 2012 Nov 22;367(21):1998). Furthermore, results on the benefit of mammography for the prevention of breast cancer-associated mortality have been mixed and the extent of overdiagnosis remains unclear (Cochrane Database Syst Rev 2013 Jun 4;(6):CD001877, BMJ 2014 Feb 11;348:g366). A recent retrospective cohort study evaluated 16,120,349 women ≥ 40 years old having screening mammography in the year 2000 in one of 547 United States counties reporting to Surveillance, Epidemiology, and End Results (SEER) database.

Of the > 16 million women screened, 55,809 (0.35%) were diagnosed with breast cancer and 10 years of follow-up data was available for 95.3%. The rate of screening, defined as the percentage of women ≥ 40 years old in each county having mammography in past 2 years, was determined for each county to examine the effect of different screening rates on breast cancer diagnosis and mortality. The median rate of screening was 62.2%, with a range of 39.1% to 77.8%. In the county-based analysis, each 10% increase in the rate of breast cancer screening was associated with an increase in breast cancer incidence (relative risk [RR] 1.16, 95% CI 1.13-1.19) without any decrease in 10-year breast cancer mortality. Further analysis of tumor size found each 10% increase in screening was associated with an increase in the incidence of small (≤ 2 cm) cancers (RR 1.25, 95% CI 1.18-1.32) as well as a smaller increase in the incidence of large (> 2 cm) cancers (RR 1.07, 95% CI 1.02-1.12). The incidence of stage 0-II breast cancer was also increased with screening, but there was no decrease in the incidence of stage III-IV breast cancer.

The goal of breast cancer screening is to identify cancers at an earlier stage by identifying asymptomatic lesions. Higher rates of mammography would be expected to increase the diagnosis of small breast tumors, and decrease the incidence of larger and later stage tumors. Finding and treating tumors earlier would be expected to result in a decrease in breast cancer-specific mortality. This retrospective cohort study did indeed find that higher rates of mammography were associated with the expected increase in the diagnosis of small breast tumors. However, it also found an increase in the incidence of large breast tumors with higher screening rates, contrary to expectations. Additionally, while higher screening rates increased the incidence of early stage (0-II) cancer, they did not decrease the incidence of late stage (III-IV) breast cancer. The lack of change in the incidence of late stage cancers may in part explain why screening did not decrease the 10-year breast cancer-specific mortality. Instead of preventing breast-cancer specific deaths, this large study suggests increased rates of mammography may be leading to an overdiagnosis of small, indolent tumors. Finding these tumors may lead to increased patient anxiety and distress along with potentially unnecessary treatment. For example, as was noted in last week’s EBM Focus, surgical management of women with low grade DCIS lesions detected by screening mammography may not improve survival compared to conservative management. Further studies are required to determine an optimal schedule for routine mammography screening and how best to manage women with small incidental tumors found by such screening.

For more information, see the topic Mammography for breast cancer screening in DynaMed.