Obesity, mammography use and accuracy, and advanced breast cancer risk.

BACKGROUND: Being overweight or obese is associated with increased breast cancer risk and disease severity among postmenopausal women, but whether extent of mammography use and accuracy modify this association and further contribute to increases in disease severity at diagnosis among overweight and obese women is unclear.
METHODS: We prospectively collected data during 1996-2005 on 287,115 postmenopausal women not using hormone therapy (HT) who underwent 614,562 mammography examinations; 4,446 women were diagnosed with breast cancer within 12 months of a mammography examination. We calculated rates per 1,000 mammography examinations of large (>15 mm), advanced-stage (IIb, III, or IV), high-grade (3 or 4), estrogen receptor (ER)-positive and -negative, and screen-detected and non-screen-detected breast cancer across body mass index (BMI, kg/m(2)) groups defined as normal (18.5-24.9), overweight (25.0-29.9), obese class I (30.0-34.9), and obese class II/III (> or =35.0), adjusting for age, race/ethnicity, and mammography registry and use. All statistical tests were two-sided.
RESULTS: Adjusted rates per 1000 mammography examinations of overall breast cancer increased across BMI groups (6.6 normal, 7.4 overweight, 7.9 obese I, 8.5 obese II/III; P(trend) < .001), as did rates of advanced disease, including large invasive (2.3 normal, 2.6 overweight, 2.9 obese I, 3.2 obese II/III; P(trend) < .001), advanced-stage (0.8 normal, 0.9 overweight, 1.3 obese I, 1.5 obese II/III; P(trend) < .001), and high nuclear grade (1.5 normal, 1.7 overweight, 1.7 obese I, 1.9 obese II/III; P(trend) = .10) tumors. Rates of ER-positive tumors increased across BMI groups (P(trend) < .001); rates of ER-negative tumors did not. Rates of screen-detected cancers were higher among overweight and obese women than normal and underweight women, but rates of non-screen-detected (false-negative) cancers were similar. Rates of advanced breast cancer increased across BMI groups regardless of extent of mammography use.
CONCLUSIONS: Patterns of mammography use and mammography accuracy are not the primary reasons for higher rates of advanced breast cancer among overweight and obese postmenopausal women not using HT; thus, biologic differences in breast tumor development and/or progression may be important.