Re: Induced Abortion and Risk for Breast Cancer: Reporting (Recall) Bias in a Dutch Case-Control Study

Extract

Rookus and van Leeuwen (1) have recently reported, as have many others, a statistically significant, increased risk of breast cancer among women exposed to induced abortion, a finding they “ largely attribute” to “ reporting bias.” To support this interpretation, they cited a much greater relative risk (RR) of 14.6 among women from the Roman Catholic southeastern region of The Netherlands, compared with the more secular western region (RR = 1.3). However, this apparently huge difference was obtained by limiting the analysis to parous women only under 45 years old, a subset containing only 13 subjects exposed to induced abortion in the southeast. It is not prudent to make such a strong claim based on such a small sample, regardless of statistical significance.

To bolster the claim, Rookus and van Leeuwen also compared self-reports with prescribers's records of oral contraceptive use in the two regions. They found a slight but significant tendency for southeastern control subjects, compared with western control subjects, to underreport the duration of their oral contraceptive use. However, since the authors found no evidence of reporting bias between case patients and control subjects (who had been matched for region), reporting bias could not logically be held accountable for the observed positive association between induced abortion and breast cancer.