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Mesothelioma Incidence in NSW

There are 4 registers that have collated data on mesothelioma incidence in NSW on which to model future trends.  The Australian Mesothelioma Surveillance Program (AMSP), which ran from 01/01/1980 to 31/12/1985, the Australian Mesothelioma Register (AMR), which followed and has operated since 01/01/1986, the NSW Central Cancer Registry (CCR) and the Dust Diseases Board’s (DDB) records of applicants seeking compensation for mesothelioma.  The estimated annual incidence of mesothelioma in NSW published from these registers can vary considerably.  This has been particularly so in more recent years (see Fig. 1). 

The observed variations may be due to a number of factors such as differences in coding practices, policies regarding histopathological verification, methods for soliciting notifications, degree of underreporting and definition of a NSW case.  For example, notification of cancers such as mesothelioma to the CCR is a statutory requirement for all NSW public and private hospitals, radiotherapy departments, nursing homes, pathology laboratories, outpatient departments and day procedure centres.  By contrast, both the AMSP and the AMR have operated as voluntary notification schemes, with no statutory requirement for notification and cases are only brought to the attention of the DDB if there is reason to expect occupational exposure to asbestos has occurred.  In addition, the figures compiled by the CCR include mesothelioma cases for which no histopathological verification was obtained.  The percentage of cases with no histopathological verification has varied over the years.  In 2000, it was reported to be 20%.  In the case of the AMR only histologically confirmed cases of mesothelioma are accepted.

Prior to 1999, mesotheliomas were not classified by cancer data collection agencies as a separate cancer.  Instead they were coded according to site of origin under the International Classification for Diseases 9^th revision (ICD-9) classification system and included in cancers of the pleura (163) or cancers of the retroperitoneum and peritoneum (158).  Neither code was exclusive to mesotheliomas.  Re-classification to mesothelioma, C45 of the ICD-10 classification system, was conducted retrospectively by the different collection agencies.  This is potentially another reason for the variation in previously reported incidence numbers.

Despite the assumption of 100% enumeration by population-based registries underreporting is an inherent feature of any epidemiological surveillance system.  Furthermore, the degree of underreporting can be expected to vary from year to year and, if not corrected for, may distort reported trends over time. 

Mesothelioma incidence numbers for NSW adjusted for underreporting are not currently available.  Capture-recapture methods are a means of correcting incidence numbers for underreporting, and were originally developed to estimate the size of animal populations.  At one point in time as many animals as possible in a given area are captured, tagged and released, then the exercise is repeated at a second point in time.  The number of animals caught on each occasion and the number common to both are used to estimate the number missed enabling a more accurate indication of population size together with confidence intervals.  The number of registers that record incident mesothelioma cases in NSW in past years, makes this an ideal disease with which to explore the contribution capture-recapture methods can make to determining disease incidence.

Compiled by Sandra Ware, Researcher, DDB Research and Education Unit