Increased mammography use and its impact on earlier breast cancer detection in Vermont, 1975–1999

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Abstract
BACKGROUND A trend toward earlier breast carcinoma detection in the United States has been attributed to screening mammography, although direct evidence linking this trend to the increased use of mammography in a general population is lacking. This study examined the effects of mammography on tumor size and axillary lymph node metastasis in Vermont over 25 years. METHODS Pathology and mammography data from 3499 Vermont women who were diagnosed with invasive breast carcinoma during 1975–1984, 1989–1990, and 1995–1999 were compared. Logistic regression analysis was used to estimate the effects of age, mammography use, and period on the odds of a tumor ≤ 2 cm and the odds of negative lymph nodes. RESULTS The proportion of breast tumors that were detected by screening mammography increased from 2% during 1974–1984 to 36% during 1995–1999 (P < 0.001), and these tumors were more likely to measure ≤ 2 cm than tumors that were detected by other methods. Among women age > 50 years, the odds ratio (OR) was 4.5, with a 95% confidence interval (95% CI) of 3.5–6.4. The effect was smaller in younger women (OR, 1.8; 95% CI, 1.1–3.0). Mammographic detection increased the odds of negative lymph nodes by a similar amount in both age groups, although women age > 50 years were more likely to have negative lymph nodes than younger women (OR, 1.3; 95% CI, 1.1–1.6). Tumor size and lymph node metastasis also were related to the number of mammograms and to the mammographic interval. CONCLUSIONS Most of the trend toward earlier detection in Vermont was due to mammography. Mammography had a lesser effect on tumor size among younger women, which may be related to less frequent screening, although its effect on lymph node metastasis was not age dependent. Women age < 50 years were more likely to have positive lymph nodes, independent of the method of detection or the frequency of mammography. Cancer 2002;94:2160–8. © 2002 American Cancer Society. DOI 10.1002/cncr.10459