Benefit from annual screening
However, these data conflict with other studies that demonstrate the benefit of annual compared with biennial screening mammography. One large retrospective review of prospectively collected data evaluated outcome differences based on mammography frequency.30 For those undergoing annual versus biennial screening, the median tumor size was 11 mm (versus 15 mm), the percentage of lymph node metastasis was 14% (versus 24%), and cancer stage II or higher was 17% (versus 29%). The study overall demonstrated that annual screening resulted in lower recall rates (P<.0001) and detection of smaller tumors that carried a more favorable prognosis (P<.04).30
Another observational study from 2004 that assessed data from 7 different mammography registries nationwide noted that, among those aged 40 to 49, patients who underwent biennial screening had an increased likelihood of late-stage disease compared with those with annual screening (28% vs 21%, respectively; odds ratio [OR], 1.35; 95% CI, 1.01–1.81), although this discrepancy was not observed in people aged 50 or older.31
A study that critiqued the previous 2012 version of the USPSTF guidelines used CISNET modeling, which demonstrated a 39.6% mortality reduction with annual screening for those aged 40 to 84 versus 23.2% for biennial screening for those aged 50 to 74.5
More recent data also reflect these findings. A retrospective cohort study that evaluated patients aged 40 to 84 diagnosed with breast cancer found that those who previously underwent annual versus biennial screening mammography had lower incidences of late-stage diagnoses (24.0% vs 43.8%, respectively; P=.02), fewer interval cancers (10.5% vs 37.5%; P<.001), and smaller mean (SD) tumor diameter (1.4 [1.2] cm vs 1.8 [1.6] cm; P=.04).21 Postmenopausal patients in this cohort also demonstrated similar findings when comparing mammogram frequency. Although not significant, biennial (or greater) frequency of screening mammography also resulted in an increased likelihood of axillary lymph node dissection and chemotherapy.
Similarly, authors of another large prospective cohort study concluded that breast cancers diagnosed in premenopausal patients were more likely to be larger with less favorable prognostic characteristics (tumor size >15 mm, relative risk [RR], 1.21 [95% CI, 1.07–1.37]; P=.002); any less favorable prognostic characteristics (RR, 1.11 [95% CI, 1.00–1.22]; P=.047), and higher stage (stage IIB or higher, RR, 1.28 [95% CI, 1.01–1.63]; P=.04) for those who underwent biennial screening compared with breast cancers diagnosed by annual screening.32 However, this trend was not observed in postmenopausal patients not taking hormone therapy.32
Some international studies also show more favorable outcomes with annual screening mammography. A Swedish study evaluated mammography screening intervals of 21 months compared with 18 or 12 months in patients aged 40 to 49.33 Data showed an improved effectiveness of 1.6% to 9.8% for interval cancers and 2.9% to 17.4% for both interval and screening-detected cancers by reducing the screening frequency to 12 months, with authors suggesting a further reduction in breast cancer–related mortality rates for this age group.33
Results from another descriptive study from Europe also showed increasing interval breast cancer rates with increasing screening intervals.34 After a negative screen, the interval cancer rates and regional ranges for 0 to less than 12 months, 12 to less than 24 months, and 24 to less than 36 months per 1,000 screened were 0.55 (0.43–0.76), 1.13 (0.92–1.47), and 1.22 (0.93–1.57), respectively.34
Finally, a study conducted in Canada evaluated interval breast cancers among people with dense breasts screened between 2008 and 2010.35 Those with screening programs with policies that offered annual screening reported fewer interval cancers (interval cancer rate, 0.89 per 1,000; 95% CI, 0.67–1.11) compared with those who had policies that used biennial screening (interval cancer rate, 1.45 per 1,000 [annualized]; 95% CI, 1.19–1.72), which was 63% higher (P=.002). For those for whom radiologists recommended screening, interval cancer was lower for annual (0.93 per 1,000; 95% CI, 0.71–1.16) versus biennial screening (1.70 per 1,000 [annualized]; 95% CI, 0.70–2.71) (P=.061).35
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