Comparison of clinical-pathologic characteristics and outcomes of true interval and screen-detected invasive breast cancer among participants of a Canadian breast screening program: a nested case-control study.

BACKGROUND: Previous analyses of interval breast cancers have been limited because of a lack of control for screening interval length and patient age, failure to restrict the interval group to 'true' intervals, and incomplete descriptions of pathology, adjuvant therapies and clinical outcomes. PATIENTS AND METHODS: A nested case-control study within the population-based Nova Scotia Breast Screening Program was performed. All true interval cases between 1991 and 2004 were identified, matched 1:2 to screen-detected cases (age, screening interval, time period), and compared in terms of pathologic characteristics and adjuvant therapies via logistic regression. Disease-free and overall survival was estimated, controlling for pathology and adjuvant chemotherapy receipt.
RESULTS: A total of 241 true interval invasive cases were matched to 481 screen-detected cases. Interval cases were more likely to be > 1 cm (odds ratio [OR] = 1.76; 95% CI, 1.10-2.83), grade 3 (OR = 2.71; 95% CI, 1.49-4.92), and have lymphovascular invasion (OR = 3.06; 95% CI, 1.85-5.07). Interval cases received more adjuvant chemotherapy (OR = 4.37; 95% CI, 3.03-6.30) and radiation (OR = 1.43; 95% CI, 1.02-2.00). The 5-year Kaplan-Meier estimates of disease-free and overall survival rates for true intervals and screens were 0.830 (95% CI, 0.770-0.875) versus 0.926 (95% CI, 0.898-0.947) and 0.860 (95% CI, 0.804-0.901) versus 0.937 (95% CI, 0.910-0.956), respectively.
CONCLUSION: True interval breast cancers have more adverse prognostic factors compared with screen-detected cases and, despite receiving more adjuvant chemotherapy, are associated with significantly poorer survival outcomes.