Cancer Research Lab

Women who participate in a screening mammography program, compared with non-participants, tend to have better survival rates for cancers other than breast cancer, according to study results published in the January issue of the International Journal of Cancer.

However, “comparisons of cancer mortality between users and nonusers of screening are potentially biased because of the effects of self-selection,” Drs. Norm Phillips and Andrew Coldman, from the British Columbia Cancer Agency, Vancouver, Canada, point out. “Previous studies of breast screening have found that individuals likely to participate have lower breast cancer mortality than those unlikely to participate.”

In the current study, the researchers compared incidence and mortality for all cancer types other than breast cancer between subjects who participated and those who did not participate in a screening mammography program.

They identified a cohort of participants among British Columbian women who had their first mammography between 1988 and 2004, and aggregated person-years of follow-up by age and year. The team obtained non-participant person-years by subtraction from the total female population.

There were 65,885 cancers diagnosed during the study period in women between the ages of 40 and 79 years. Of these, 21,685 cancers were diagnosed in women after becoming participants. Cancer incidence rates among the participants were mixed, with some showing elevated rates, some reduced rates, and others no difference from rates among non-participants.

Overall cancer mortality was lower among participants than non-participants, with a hazard ratio of 0.76 (p < 0.001). Cancer deaths among participants were also lower than expected for most cancers. The overall mortality ratio was 0.60.

“Because participants are self-selected, the observed differences in survival between participants and non-participants may be due to a self-selection bias,” Dr. Phillips told Reuters Health, “so one should be cautious in interpreting observed differences between participants and non-participants.”

“Our research underlines the importance of carrying out randomized control trials, rather than observational studies,” Dr. Phillips commented. “But as the benefits of a screening program are established, it becomes ethically harder to perform randomized control trials, making observational studies necessary,” he explained. “But researchers and consumers of research must remember the surprisingly powerful biasing effects of self-selection.”