Follow-up of controls participating in case-control studies for cancer risk factors

Case-control studies have been carried out in Alberta in the last seven years to investigate possible risk factors for cancer of the prostate, skin and ovary. For each study, controls were randomly selected from an age- and sex-matched population. The information obtained from each control that agreed to participate in each study (n = 1236) included name, sex, date of birth, address and date of participation as well as the specific questions for each study. A review of these controls was carried out in 1988 by checking the number of deaths through death certificate listings from Vital Statistics (n = 142) and cancer incidence through the population-based Alberta Cancer Registry (n = 134). Of the 619 controls interviewed for the prostate cancer case control study in 1982 and 1983, 25 have been diagnosed with cancer of the prostate. From the non-melanoma skin cancer study, 15 cases from the 409 controls interviewed in 1983-1984 have been diagnosed with non-melanoma skin cancer. As for the 208 controls for ovarian cancer interviewed in 1985-1986 no new cases have been diagnosed. This possible ascertainment bias should be taken under consideration when case-control study analyses are carried out.