EDITOR—We agree with Wartenberg and Stapleton that dicophane (DDT) is one of the possible aetiological factors for breast cancer in female airline cabin attendants.1 Two other factors (exposure to cosmic radiation and recall bias) also merit attention.

We are not told how the cases and controls were selected or matched in their survey. The exposure data should be only data up to the date of diagnosis for flight attendants in whom breast cancer was diagnosed (cases) and data up to the date of pseudo-diagnosis for controls (the date on which the breast cancer was diagnosed in the cases). The cabin attendants with breast cancer would have stopped flying because of illness, and controls would have continued to fly; this would distort the measurement of exposure to cosmic radiation. Moreover, the cabin attendants with breast cancer would probably have had fewer flights in the year before the diagnosis owing to the undiagnosed breast cancer. Ideally, exposure should be measured until the year before diagnosis to avoid this bias. The breast is the organ most sensitive to radiation carcinogenesis in postpubertal women,2 and hence cosmic radiation cannot be ruled out as a potential factor in the aetiology of breast cancer. A cohort study of Canadian …