The additional risk of malignant mesothelioma in former workers and residents of Wittenoom with benign pleural disease or asbestosis

Aims: To examine the hypothesis that people with benign pleural disease or asbestosis have an increased risk of malignant mesothelioma beyond that attributable to their degree of asbestos exposure. Methods: Former workers and residents of the crocidolite mining and milling town of Wittenoom are participating in a cancer prevention programme ( n = 1988). The first plain chest radiograph taken at the time of recruitment into the cancer prevention programme was read for evidence of benign pleural disease and asbestosis, using the UICC classification. Crocidolite exposure of former workers was derived from employment records and records of dust measurements performed during the operation of the asbestos mine and mill between 1943 and 1966. Based on fibre counts, exposure for former residents was determined using duration of residence and period of residence ( before and after a new mill was commissioned in 1957) and interpolation from periodic hygienic measures undertaken from personal monitors between 1966 and 1992. Cox proportional hazards modelling was used to relate benign pleural disease, asbestosis, asbestos exposure, and mesothelioma. Results: Between 1990 and 2002, there were 76 cases of mesothelioma ( 56 of the pleura and 20 of the peritoneum). Cases had more radiographic evidence of ( all) benign pleural disease, pleural thickening, blunt/obliterated costophrenic angle, and asbestosis than non-cases. Adjusting for time since first exposure ( log years), cumulative exposure ( log f/ml-years), and age at the start of the programme, pleural thickening ( OR = 3.1, 95% CI 1.2 to 7.6) and asbestosis ( OR = 3.3, 95% CI 1.3 to 8.6) were associated with an increased risk of peritoneal mesothelioma. There was no increased risk for pleural mesothelioma. Conclusion: The presence of benign pleural disease, in particular pleural thickening, and asbestosis appears to increase the risk of mesothelioma of the peritoneum, but not of the pleura beyond that attributable to indices of asbestos exposure in this cohort of subjects exposed to crocidolite.