The Utility of Screening Mammography

The Utility of Screening Mammography

This imaging method continues to be important in the early detection of breast cancer.

The overall number of cancer deaths in the U.S. fell for the second straight year from 2003 to 2004, and a marked decline in breast cancer mortality contributed to this notable achievement.¹ The Cancer Intervention and Surveillance Modeling Network (CISNET; Journal Watch Women’s Health Dec 27 2005) has estimated that 28% to 65% of the decline in breast cancer mortality from 1975 to 2000 can be attributed to the use of screening mammography in women older than 40.² However, a recent report from the Behavioral Risk Factor Surveillance System (BRFSS) suggests that the frequency of mammography screening in women 40 and older has decreased during recent years: In 2000, 76.4% of women 40 and older reported having a mammogram within the previous 2 years, while in 2005, only 74.6% did (test for trend, P <0.001).³ Although the reasons for this decline are unclear, the controversies associated with mammography may play a role. The trend is worrisome because it threatens to undermine the hard-fought gains against the most common malignancy that affects U.S. women.

THE BENEFITS OF SCREENING MAMMOGRAPHY

The first randomized controlled trial of screening mammography, the Health Insurance Program of New York (HIP) study, was conducted during the 1960s. The study group that received mammography (age range at study entry, 40–69) showed a 30% reduction in breast cancer mortality at 7 years of follow-up, which persisted as a 23% reduction at 18 years. The benefit emerged in a young subgroup (age range at study entry, 40–49) after 5 to 8 years of follow-up.⁴

In the recent Age Trial, U.K. investigators randomly assigned more than 160,000 women (age range at study entry, 39–41) to usual medical care (screening mammography not routinely offered before age 50) or annual screening mammography until age 48. At a mean follow-up of 10.7 years, the intervention group had a reduction in breast cancer mortality compared with the control group (relative risk, 0.83), even though this difference did not reach statistical significance.⁵ Results of a review and meta-analysis of randomized trials were consistent in showing a benefit of screening mammography in women older than 40 (overall RR, 0.84), although the average years of follow-up extended beyond age 50.⁶

CONTROVERSIES ABOUT SCREENING MAMMOGRAPHY

Breast cancer is less common in women younger than 50; therefore, to prevent one death from breast cancer during 14 years of observation, 1792 women in that age group must be screened compared with 1224 women overall.⁶ Moreover, mammography may be less efficacious in premenopausal women because of their tendency toward higher breast density and more-aggressive disease. The large number of screening mammograms needed to prevent one death has raised questions about risks to the healthy population. These risks include radiation carcinogenesis, which is more common at younger ages.⁷ Women 40 to 49 are also at greater risk for false-positive mammography results than are women older than 50.⁸

In terms of breast cancer mortality, the benefit of screening mammography has been clouded by debate questioning the impact of screening on all-cause mortality. Some investigators have argued that the randomized trials, when pooled, do not show a decrease in overall mortality in women who receive regular mammography. They contend, therefore, that screening is generally unjustified.⁹

ADVANCES IN BREAST IMAGING

Digital mammography is among the recent improvements in breast-imaging technology. Results of the Digital Mammography Imaging Screening Trial (DMIST; Journal Watch Women’s Health Nov 22 2005) showed that film-based and digital mammography can be complementary: Each imaging mode detected malignancies not seen with the other. Digital mammography was more useful for dense breasts, as the image-enhancement software could be adjusted to enhance subtle differences in intensity.¹⁰ An added benefit of digital mammography is the ease of data storage and retrieval. This is especially important when breast images taken at different times must be compared, for example, in determining false-positive results or in detecting the subtle changes seen in early malignancy.

The patterns seen on the mammographic image may help to better define a woman’s underlying risk for breast cancer. Higher breast density is associated with increased risk, probably because of a combination of factors. One factor is known as masking: Tumors may be harder to detect in dense breast tissue.¹¹ Palpable masses, even if not seen on mammography, continue to require further evaluation.

Breast magnetic resonance imaging (MRI) is another emerging method for detecting tumors. In younger women, this approach offers two advantages: the lack of ionizing radiation and the capability for enhanced imaging of dense breasts. However, this mode has a very high false-positive rate, is expensive, and requires specialized apparatus should a biopsy be necessary. Despite these drawbacks, women who are at very high risk, such as BRCA1 and BRCA2 carriers, may benefit from the increased sensitivity of breast MRI.¹² Women with breast implants may also want to consider breast MRI to assess for capsule rupture. Other technologies such as whole-breast ultrasound and thermography are continually undergoing technologic improvements, but they still have limitations that make them unlikely candidates for screening programs.

BREAST CANCER SCREENING GUIDELINES

Prominent healthcare advisory groups such as the American Cancer Society (ACS) call for regular mammography screening. Table 1 highlights the ACS recommendations for the early detection of breast cancer.

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Table 1. ACS Recommendations for the Early Detection of Breast Cancer¹³

These guidelines are worth following. Moreover, continued efforts within our healthcare system should be devoted to ensuring access to high-quality, affordable screening for breast cancer.

— Christine D. Berg, MD

Dr. Berg is Chief of the Early Detection Research Group, Division of Cancer Prevention, National Cancer Institute.

Published in Journal Watch Women's Health March 1, 2007

Citation(s):

1. Jemal A et al. Cancer statistics, 2007. CA Cancer J Clin 2007 Jan/Feb; 57:43-66.

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2. Berry DA et al. Effect of screening and adjuvant therapy on mortality from breast cancer. N Engl J Med 2005 Oct 27; 353:1784-92.

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3. Centers for Disease Control and Prevention (CDC). Use of mammograms among women aged $≥$ 40 years — United States, 2000-2005. MMWR Morb Mortal Wkly Rep 2007 Jan 26; 56:49-51.

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7. Berrington de González A and Reeves G. Mammographic screening before age 50 years in the UK: Comparison of the radiation risks with the mortality benefits. Br J Cancer 2005 Sep 5; 93:590-6.

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8. Elmore JG et al. Ten-year risk of false positive screening mammograms and clinical breast examinations. N Engl J Med 1998 Apr 16; 338:1089-96.

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9. Olsen O and Gøtzsche PC. Cochrane review on screening for breast cancer with mammography. Lancet 2001 Oct 20; 358:1340-2.

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10. Pisano ED et al. Diagnostic performance of digital versus film mammography for breast-cancer screening. N Engl J Med 2005 Oct 27; 353:1773-83.

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11. Boyd NF et al. Mammographic density and the risk and detection of breast cancer. N Engl J Med 2007 Jan 18; 356:227-36.

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12. Warner E et al. Surveillance of BRCA1 and BRCA2 mutation carriers with magnetic resonance imaging, ultrasound, mammography, and clinical breast examination. JAMA 2004 Sep 15; 292:1317-25.