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Some breast cancer tumors evade detection even among women who undergo regular screening mammography. To characterize screen-detected versus interval cancers (those diagnosed after negative mammograms and before the next recommended screening), investigators used data from the population-based Ontario Breast Screening Program to conduct a case-control analysis. In all, 450 screen-detected cancers, 288 true interval cancers (not mammographically detectable), and 87 missed interval cancers (retrospectively identifiable on review of last screening mammogram) were analyzed; the rate of true interval cancers was 11%.
Compared with screen-detected tumors, interval cancers — whether missed or true — were larger and more poorly differentiated. Misse…