A cluster of brain tumours in a New South Wales colliery: a problem in interpretation

Following the reporting of a cluster of cases of brain tumour in the workforce of an underground coal mine (Mine A) in the Newcastle coalfield, a study was carried out to determine whether this phenomenon was due to chance alone or whether an environmental cause could be postulated. The study design was a historical cohort study over 15 years comparing the incidence of brain tumour (ICD9 191 and 192) in the index mine with that in two control mines (Mines B and C) in the same area and with that in the general Australian population. We compared environmental exposures (ionising and nonionising radiation and chemical exposure) in the three mines. With Australian brain tumour incidence rates as reference, the standardised incidence ratio for brain tumour in Mine A was 5.3 (95 per cent confidence interval (CI) 1.08 to 14.04) and in Mines B and C combined was 1.23 (CI 0.02 to 3.80). On most environmental assessments the three mines were similar but Mine A used larger volumes of solvents than the other mines. This study poses two questions: was the increase in cases of brain tumour in Mine A 'real’ and if so, was it related to the use of solvents? Data, from an investigation of a cluster such as this, are unlikely to be conclusive. Nevertheless, such answers are demanded not only by those at risk but also by the mine management, which is responsible for a safe working environment. Some of the difficulties involved with this judgment are discussed.