Comparison of nonbreast cancer incidence, survival and mortality between breast screening program participants and nonparticipants

Abstract

Comparisons of cancer mortality between users and nonusers of screening are potentially biased because of the effects of self-selection. Previous studies of breast screening have found that individuals likely to participate have lower breast cancer mortality than those unlikely to participate. This study compares the incidence, survival and mortality for all cancer types other than breast between participants and nonparticipants in a service screening mammography program. British Columbian females having their first mammogram between the ages of 40 and 79 and the years 1988 and 2004 were identified as a cohort of "participants". Person-years of follow-up of participants were aggregated by age and year. Nonparticipant person-years were obtained by subtraction from the total female population. Cancer diagnoses other than breast were identified for participants and nonparticipants. Age, calendar year, and income adjusted relative risks of cancer incidence were estimated from generalized additive models with Poisson errors. Hazard ratios were estimated by Cox regression. Observed cancer mortality in participants was compared with expected mortality generated from nonparticipant incidence and survival rates. Incidence rates of cancer showed a mixed relationship with some elevated, some decreased and others similar to nonparticipant rates. Cancer survival was higher among participants for most cancer types, with an overall hazard ratio of 0.76 (0.73-0.79). Observed mortality in participants was less than expected for most cancers, with an overall mortality ratio of 0.60 (0.58-0.62). The general cancer experience of screening program participants is different from that of the general population.