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. 1999 Jan 9;318(7176):125.

Risk of breast cancer among female airline cabin attendants

Findings may have been due to exposure to cosmic radiation or recall bias

P Badrinath 1, Sam Ramaiah 1
PMCID: PMC1114587  PMID: 9880298

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 air pilots showed an increased risk of acute myeloid leukaemia, the type of leukaemia induced by exposure to radiation (standardised incidence ratio 4.72 (90% confidence interval 2.05 to 9.31)).3

Recall bias needs to be considered in any study in which exposure is measured after a disease has been diagnosed or an event has occurred.4 In the case of exposure to DDT, recall bias could have led to the non-significant increase in risk that Wartenberg and Stapleton found. Because of the strong link between pesticides and cancers, flight attendants with breast cancer would have been more likely than controls to recall pesticide spray. It is always important to verify the records, if they are available, to measure their agreement with reported exposure in a proportion of cases and controls.

Although the point estimate for exposure to DDT that Wartenberg and Stapleton report is high (odds ratio 2.2), the wide confidence interval (0.4 to 10.9) suggests that recall bias could account for the observed odds ratio. We agree with the authors’ suggestion that large studies will be the way forward to determine the role of DDT in breast cancer among airline attendants. These studies should also explore the joint effect of DDT and cosmic radiation.

References

  • 1.Wartenberg D, Stapleton PC. Risk of breast cancer is also increased among retired US female airline cabin attendants. BMJ. 1998;316:1902. doi: 10.1136/bmj.316.7148.1902. . (20 June.) [DOI] [PMC free article] [PubMed] [Google Scholar]
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BMJ. 1999 Jan 9;318(7176):125.

Large European studies are now being carried out

Anssi Auvinen 1,2,3, Eero Pukkala 1,2,3, Maria Blettner 1,2,3

Editor—Wartenberg and Stapleton suggest1-1 that pesticides used in aircraft could contribute to the increased risk of breast cancer among flight attendants previously reported from Finland1-2 and Denmark.1-3 The frequency and magnitude of exposures to pesticide are unknown but probably limited. To our knowledge, pesticide spraying was practised only on international flights to a few destinations. Exposure of Finnish flight attendants has certainly been rare and is unlikely to have contributed to our findings. Furthermore, there is little evidence for organochlorine pesticides as a risk factor for breast cancer.1-4,1-5 Therefore there is little evidence to support the suggestion that pesticides could contribute substantially to the risk of breast cancer.

In our view, the strongest factors that have been shown empirically to affect risk of breast cancer among cabin crew are parity and age at first birth.1-1 More research is clearly needed to elucidate the reasons for increased risk of breast cancer among cabin crew. Two European studies (coordinated by EP and MB) are now being carried out. One is being done in 10 countries and among 30 000 pilots and 45 000 members of cabin crews, with mortality from cancer as the end point; the other, a Nordic study, is of cohorts from five countries, with smaller numbers of cases and incidence of cancer as the end point. These studies are expected to provide more information on the issue.

References

  • 1-1.Wartenberg D, Stapleton CP. Risk of breast cancer is also increased among retired US female airline cabin attendants. BMJ. 1998;316:1902. doi: 10.1136/bmj.316.7148.1902. . (20 June.) [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 1-2.Pukkala E, Auvinen A, Wahlberg G. Incidence of cancer among Finnish airline cabin crew. BMJ. 1995;311:649–652. doi: 10.1136/bmj.311.7006.649. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 1-3.Lynge E. Risk of breast cancer is also increased among Danish female airline cabin attendants. BMJ. 1996;312:253. doi: 10.1136/bmj.312.7025.253. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 1-4.Hunter DI, Hankinson SE, Laden F, Colditz GA, Manson JAE, Willett WC, et al. Plasma organochlorine levels and the risk of breast cancer. N Engl J Med. 1997;337:1253–1258. doi: 10.1056/NEJM199710303371801. [DOI] [PubMed] [Google Scholar]
  • 1-5.Van’t Veer P, Lobbezoo IE, Martin-Moreno JM, Guallar E, Gomez-Aracena J, Kardinaal F, et al. DDT (dicophane) and premenopausal breast cancer in Europe: case-control study. BMJ. 1997;315:81–85. doi: 10.1136/bmj.315.7100.81. [DOI] [PMC free article] [PubMed] [Google Scholar]
BMJ. 1999 Jan 9;318(7176):125.

Authors’ reply

Daniel Wartenberg 1, Cecile Pryor Stapleton 1

Editor—We did not address several issues in our letter because of space limitations. The study design that we used was a retrospective cohort postal survey of members of an organisation of retired flight attendants from one American airline. As Badrinath and Ramaiah suggest, selection bias, follow up bias, and recall bias could all have occurred. With regard to selection bias, we were more likely to have omitted flight attendants who developed breast cancer (or died) than healthy flight attendants owing to their lack of continued involvement in organisations for flight attendants. In terms of follow up bias, the suggestion that we should have adjusted flying times seems logical, but all of the women who developed breast cancer either received the diagnosis several years after stopping work or reported that they continued to work after diagnosis.

Therefore we do not believe that our method underestimated exposure to radiation differentially, although we also believe that radiation may have been a contributing factor to the breast cancers. Recall bias, as in all survey studies, is likely but difficult to evaluate since we know of no available records of pesticide use. Auvinen et al suggest that, although they do not know the magnitude and frequency of pesticide use in the past, it was probably limited. We based our suggestion on the World Health Organisation’s recommendation in 1961 for pesticide treatments on international flights.2-1 More data, from additional studies, are needed to resolve this.

We disagree that there is little evidence that organochlorine pesticides are a risk factor for breast cancer. Results in the literature are mixed, but several studies currently under way were designed to test the hypothesis of a link between DDT and breast cancer more specifically than those already conducted. We recognise that reproductive factors are likely to contribute to the risk in our population (parity, age at first birth, age at menarche, age at menopause), but our sample was too small to assess this effect directly.

Overall, our preliminary study was too small to rule out or confirm any risk factor for breast cancer among flight attendants. We simply proposed adding use of DDT pesticides to the list of risks to be considered in exploring the aetiology of breast cancer among flight attendants. We look forward to the results of the studies being carried out to provide further insight.

References

  • 2-1.Aircraft disinfection. Eleventh report of the expert committee on insecticides. WHO Tech Rep Ser 1961;206. [PubMed]

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