Annual mammography may result in many false positives

After 10 years of annual mammography, more than half of women will receive at least one false-positive screen, and 7% to 9% will receive a false-positive biopsy recommendation, according to a study.

Results also indicated that biennial screening appears to reduce these risks but may be associated with a small and not statistically significant absolute increase in the probability of late-stage cancer diagnoses. Researchers conducted a prospective cohort study among 169,456 women who underwent first screening mammography at age 40 to 59 years between 1994 and 2006 and 4,492 women with incident invasive breast cancer diagnosed between 1996 and 2006. Results appeared in the Oct. 18 Annals of Internal Medicine.

The study included 386,799 mammograms from 169,456 women. Nearly half (47.7%) of the women had only one screening mammogram; 11.8% had 5 or more. In the cohort, 9,331 women (5.5%) had only one year of follow-up and 4,891 (2.9%) were observed for 10 or more years.

Most initial mammograms (78.9%) were for women aged 40 to 49. Among subsequent mammograms, 55.6% occurred at an approximately annual screening interval (within 9 to 18 months of a prior mammogram) and 27.6% occurred approximately biennially (within 18 to 30 months of a prior mammogram). The remainder of mammograms occurred at longer than biennial intervals.

The likelihood of being recalled due to a false positive was 16.3% at first and 9.6% at subsequent mammography, researchers reported. Probability of false-positive biopsy recommendation was 2.5% at first examinations and 1.0% at subsequent ones. The availability of comparison mammograms halved the odds of a false-positive recall (adjusted odds ratio, 0.50 [95% CI, 0.45 to 0.56]).

When screening began at age 40, the cumulative probability of a woman receiving at least one false-positive recall after 10 years was 61.3% (95% CI, 59.4% to 63.1%) with annual screening and 41.6% (95% CI, 40.6% to 42.5%) with biennial. Cumulative probability of false-positive biopsy recommendation was 7.0% (95% CI, 6.1% to 7.8%) with annual and 4.8% (95% CI, 4.4% to 5.2%) with biennial screening. Estimates were similar when screening began at age 50.

A non-statistically significant increase in the proportion of late-stage cancers was observed with biennial compared with annual screening (absolute increases, 3.3 percentage points [95% CI, −1.1 to 7.8 percentage points] for women ages 40 to 49 and 2.3 percentage points [95% CI, −1.0 to 5.7 percentage points] for women ages 50 to 59) among women with incident breast cancer.

The authors noted that although there was no statistically significant absolute difference in the overall proportion of late-stage cancer between biennial and annual screening, the findings could not exclude an increase in late-stage cancer as great as 7.8% among women in their 40s and 5.7% among women in their 50s. The study's small sample size of breast cancer resulted in broad confidence intervals, and a larger study is required to exclude the possibility of a clinically significant increase in late-stage cancer with less frequent screening.

The authors also noted that most mammograms in this analysis were film-screen examinations. Digital screening mammography is rapidly becoming the predominant screening method, with 76.2% of accredited facilities using full-field digital machines by May 2011.

A second study in the same issue of Annals found that digital and film-screen mammography offered comparable results in community practice, but that digital mammography was better at detecting estrogen receptor-negative tumors and cancer in extremely dense breasts. Researchers conducted a prospective cohort study of 329,261 women aged 40 to 79 years who underwent 869,286 mammograms (231,034 digital; 638,252 film-screen).

Cancer detection rates and tumor characteristics were similar for digital and film-screen mammography, but the sensitivity and specificity of each modality varied by age, tumor characteristics, breast density, and menopausal status. Compared with film-screen mammography, the sensitivity of digital mammography was significantly higher for women aged 60 to 69 years (89.9% vs. 83.0%; P=0.014) and those with estrogen receptor-negative cancer (78.5% vs. 65.8%; P=0.016). The digital test's sensitivity was borderline significantly higher for women aged 40 to 49 years (82.4% vs. 75.6%; P=0.071), those with extremely dense breasts (83.6% vs. 68.1%; P=0.051), and pre- or perimenopausal women (87.1% vs. 81.7%; P=0.057). It was slightly lower for women aged 50 to 59 years (80.5% vs. 85.1%; P=0.097). The specificity of digital and film-screen mammography was similar by decade of age, except for women aged 40 to 49 years (88.0% vs. 89.7%; P<0.001).

An editorial about both studies concluded that annual mammography is less efficient than biennial screening, and also that digital technology is probably no better than film-based mammography for preventing advanced disease. The editorialist wrote, “Given the absence of clear reductions in breast cancer deaths, the higher rates of false-positive results, and the greater detection of in situ tumors of unknown clinical significance, the generalization of digital mammography may reduce the efficiency of breast screening.”