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We thank Dr Hocking for his comments on our paper and offer the following information in response to the points he has raised.We have already acknowledged in the Discussion section of our paper that a factor, which may have contributed to the negative findings in our study, was the use of proxies to complete the work history questionnaire for 44% of the cases, but for only 2% of the controls [1]. Proxies usually have incomplete knowledge about workplace exposures of subjects, but reliability has been shown to be greater for job titles than for specific exposures [2]. In our study, proxies were asked only to provide information about job titles. It is also important to highlight that the results of our study were similar when analyses were restricted to index subjects, as noted in our paper.
FINJEM has been updated since it was first published in 1998 [3]. For the different types of radiation, FINJEM provides 89% of exposure estimates in the five periods between 1960 and 2003 (1960–84, 1985–94, 1995–97, 1998–2000 and 2001–03). There was a small number of occupations listed under the different radiation types, which did not provide estimates for the period 1960–84; however, these had estimates which were consistent across the other four periods and are not expected to be much different prior to 1984. Therefore, for the small number of occupations for which there were some gaps in exposure data, we believe these can be estimated by extrapolation from other time periods. In addition, there is no reason to suppose that errors in estimating exposure in the period 1960–84 are different between cases and controls. This should mainly lead to non-differential misclassification and any bias in the risk estimates would be towards the null [4].
It is recognized that although the use of FINJEM can be a useful tool for assessing a variety of exposures at very low cost in studies outside Finland, the differences in work practices, processes and technology in the same industries in Australia may affect its transportability. The reliability of radiation estimates in FINJEM when applied in Australia has not been formally evaluated. For radiofrequency radiation, measurements that were conducted by Joyner and Bangay [5] in Australia showed that workers of radio antennas were exposed to levels lower than those specified in FINJEM. However, given that only 16 subjects (nine cases and six controls) were classified in our study as ‘telephone installation crew, linemen and cable jointers’, any exposure misclassification within this group is not likely to change the results greatly.
Finally, the weaknesses of the self-report and FINJEM exposure assessment methods are described in our paper. However, the assessment of exposure to radiofrequency and ionizing radiation in our study included expert review by an industrial hygienist with 15 years experience in assessing workplace radiation exposures in Australia. The expert assessment also did not show an association with glioma.
1 Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, Victoria 38800, Australia
2 Non-Ionising Radiation Branch, Australian Radiation Protection and Nuclear Safety Agency, Yallambie, Victoria 3085, Australia
3 Centre of Expertise for Good Practices and Competence, Finnish Institute of Occupational Health, Helsinki 00250, Finland
e-mail: ken.karipidis{at}arpansa.gov.au
References
- Karipidis KK, Benke G, Sim MR, Kauppinen T, Giles G. Occupational exposure to ionizing and non-ionizing radiation and risk of glioma. Occup Med (Lond) (2007) 57:518–524.[CrossRef][Medline]
- McGuire V, Nelson LM, Koepsell TD, Koepsell TD, Checkoway H, Longstreth WT Jr. Assessment of occupational exposures in community-based case-control studies. Annu rev Public Health (1998) 19:35–53.[CrossRef][Web of Science][Medline]
- Kauppinen T, Toikkanen J, Pukkala E. From cross-tabulations to multipurpose exposure information systems: a new job-exposure matrix. Am J Ind Med (1998) 33:409–417.[CrossRef][Web of Science][Medline]
- Copeland KT, Checkoway H, McMichael AJ, Holbrook RH. Bias due to misclassification in the estimation of relative risk. Am J Epidemiol (1977) 105:488–495.
[Abstract/Free Full Text] - Joyner KH, Bangay MJ. Exposure survey of civilian airport radar workers in Australia. J Microw Power Electromagn Energy (1986) 21:209–219.[Web of Science][Medline]
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