Mortality from lung cancer among silicotic patients in Sardinia: an updatestudy with 10 more years of follow up

Objectives-To evaluate the association between silica, silicosis and lung c ancer, the mortality of 724 patients with silicosis, first diagnosed by sta ndard chest x ray film between 1964 and 1970, has been analysed by a cohort study extended to 31 December 1997. Methods-Smoking and detailed occupational histories were available for each member of the cohort as well as the estimated lifetime exposure to respira ble silica dust and radon daughters. Two independent readers blindly classi fied standard radiographs according to the 12 point International Labour Or ganisation (ILO) scale. Lung function tests meeting the American Thoracic S ociety's criteria were available for 665 patients. Standardised mortality r atios (SMRs) for selected causes of death were based on the age specific Sa rdinian regional death rates. Results - The mortality for all causes was significantly higher than expect ed (SMR 1.35, 95% confidence interval (95% CI) 1.24 to 1.46) mainly due to tuberculosis (SMR 22.0) and to non-malignant chronic respiratory diseases ( NMCRD) (SMR 6.03). All cancer deaths were within the expected numbers (SMR 0.93; 95% CI 0.76 to 1.14). The SMR for lung cancer was 1.37 (95% CI 0.98 t o 1.91, 34 observed), increasing to 1.65 (95% CI 0.98 to 2.77) allowing for 20 years of latency since the first diagnosis of silicosis. Although morta lity from NMCRD was strongly associated to the severity of radiological sil icosis and to the extent of the cumulative exposure to silica, SMR for lung cancer was weakly related to the ILO categories and to the cumulative expo sure to silica dust only after 20 years of lag interval. A significant exce ss of deaths from lung cancer (SMR 2.35) was found among silicotic patients previously employed in underground metal mines characterised by a relative ly high airborne concentration of radon daughters and among ever smokers wh o showed an airflow obstruction at the time of the first diagnosis of silic osis (SMR 3.29). Mortality for lung cancer related to exposure was evaluate d with both the Cox's proportional hazards modelling within the entire coho rt and a nested case-control study (34 cases of lung cancer and 136 matched controls). Both multivariate analyses did not show any significant associa tion with cumulative exposure to silica or severity of silicosis, but confi rmed the association between mortality for lung cancer and relatively high exposure to radon, smoking, and airflow obstruction as significant covariat es. Conclusions - The findings indicate that the slightly increased mortality f or lung cancer in this cohort of silicotic patients was significantly assoc iated with other risk factors uch as cigarette smoking, airflow obstruction , and estimated exposure to radon daughters in underground mines-rather tha n to the severity of radiological silicosis or to the cumulative exposure t o crystalline silica dust itself.