A new study has reported that most of the difference in survival between screen-detected and symptomatic breast cancers is a result of differences in pathological features known to influence prognosis. The study was designed to investigate whether breast cancer detected by screening mammograms are biologically different from symptomatic tumors (which typically are found by the women themselves). Screen-detected breast cancers have been observed to have a better prognosis than symptomatic tumors, even after taking into account pathological tumor attributes, raising the question as to whether screen-detected tumors are substantially biologically different from symptomatic cancers.
The study included UK women aged 50-64 years in whom 21,282 breast cancers were diagnosed between 1988 and 2004. Data regarding tumor pathology and survival outcomes according to mode of detection was collected and analyzed.
Screen-detected tumors were found to be significantly smaller, better differentiated, and less likely to be lymph node positive than symptomatic cancers. Furthermore, a higher fraction of screen-detected cancers were hormone receptor positive, and a higher proportion were tubular carcinomas. Ten-year survival was found to be considerably better among women with screen-detected breast cancers. Survival was 86% for screen-detected cancers, 70% for interval cancers (tumors found by the women themselves between screening visits), and 58% for tumors in women not undergoing any screening. Age, tumor size, lymph node status, tumor grade, histological type, and year of diagnosis accounted for 64% of the survival differences for interval cancers and 68% for unscreened women. Overall survival improved over the study period as more of the population underwent screening mammograms; approximately half of the improvement was due to the increase in the proportion of tumors that were screen-detected. The authors conclude that the majority of the difference in survival between screen-detected and symptomatic breast cancers is a result of differences in routinely measured pathological features (size, type, grade, and lymph node status), leaving a small residual difference to be accounted for by other biological differences.