INDUCED-ABORTION AND RISK FOR BREAST-CANCER - REPORTING (RECALL) BIASIN A DUTCH CASE-CONTROL STUDY

Background: In general, no association has been found between spontane ous abortion (naturally occurring termination of a pregnancy) and the risk for breast cancer. With respect to induced abortion (termination of a pregnancy by artificial means), the results have been more inconc lusive. A positive association was found in five studies, no associati on was found in six studies, and a negative association was found in t he only cohort study. It is thought that part of the inconsistency of the reported results may be attributable to reporting (recall) bias, s ince all but two studies on induced abortion used the case-control des ign and were based only on information obtained from study subjects. I n comparison with breast cancer case patients, healthy control subject s may be more reluctant to report on a controversial, emotionally char ged subject such as induced abortion. Thus, differential underreportin g may be a cause of spurious associations in case-control studies. Pur pose: Our goal was threefold: 1) to evaluate the relationship between a history of induced or spontaneous abortion and the risk for breast c ancer in a Dutch population-based, case-control study; 2) to examine r eporting bias by comparing risks between two geographic areas (i.e., w estern regions and southeastern regions in The Netherlands that differ in prevalence of and attitudes toward induced abortion): and 3) to co mpare reporting bias in data on induced abortion with reporting bias i n data on oral contraceptive use. Methods: Data analyzed in this study were obtained from 918 women (20-54 years of age at diagnosis) who we re diagnosed with invasive breast cancer during the period from 1986 t hrough 1989 and had been initially enrolled in a population-based, cas e-control study investigating oral contraceptive use and breast cancer risk. The women resided in one of four geographic areas that were cov ered by Regional Cancer Registries: two western regions (Amsterdam and West) and two southeastern regions (East and Eindhoven). Each case pa tient was pair-matched, on the basis of age (within 1 year) and region , with a control subject who was randomly selected from municipal regi stries that fully covered the Dutch population. Both thecase patients and the control subjects were interviewed at home by the same trained interviewer, who used a structured questionnaire. Reporting bias was e xamined indirectly by comparing risks between the western and the sout heastern regions of the country, which differ in the prevalence of and attitude toward induced abortion. Multivariate conditional logistic r egression methods for individually matched case-control studies were u sed to estimate relative risks (RRs). Reported P values are two-sided. Results and Conclusion: Among parous women, a history of induced abor tion was associated with a 90% increased risk for breast cancer (adjus ted RR = 1.9; 95% confidence interval [CI] = 1.1-3.2). Among nulliparo us women, no association between induced abortion and breast cancer wa s found. Neither among parous women nor among nulliparous women was a history of spontaneous abortion related to the risk for breast cancer. The association between induced abortion and breast cancer was strong er in the southeastern regions of the country, which have a predominan tly Roman Catholic population, than in the western regions (adjusted R R = 14.6 [95% CI = 1.8-120.0] versus adjusted RR = 1.3 [95% CI = 0.7-2 . 6], respectively; test of difference between regions, P =.017), sugg esting reporting bias. Support for reporting bias as an explanation fo r the regional differences was also found in data supplied by both stu dy subjects and their physicians on the use of oral contraceptives. In comparison with physicians, control subjects in the southeastern regi ons underreported the duration of their oral contraceptive use by 6.3 months more than control subjects in the western regions (P =.007). Im plication: Reporting bias is a real problem in case-control, studies o f induced abortion and breast cancer risk if these studies are based o n information from study subjects only. More quantitative assessment o f this bias in future studies is essential.