In reference to Asbestos exposure and laryngeal cancer mortality.

During the course of research into the association between asbestos exposure and laryngeal cancer, I read the recent meta-analysis by Peng and colleagues. It has come to my attention that the authors overlooked one of the largest cohort studies relevant to this topic, the study of Canadian asbestos miners published by Liddell and colleagues. It is the purpose of this letter to update the calculations of Peng and colleagues by including the results from the Canadian mining cohort. In 1997, Liddell and colleagues published their article on the mortality follow-up of a cohort of some 11,000 chrysotile miners. They reported 30 deaths from laryngeal cancer and an standardized mortality ratio (SMR) of 1.04. I added the results of this study to the data presented in Table I of Peng and colleagues and calculated a pooled SMR using a random effects model in the Stata statistical software program metan (StataCorp LP, College Station, TX). Because of its size, the Canadian study contributed 14.6% of the weighted data in the model, second only to the 24.9% contributed by the English study of Harding and colleagues. The original result of Peng and colleagues and the revised result are presented in Table I below. It can be seen that the addition of the large Canadian study had an 8% effect on the size of the pooled SMR, lowering it from 1.69 to 1.56. The pooled SMR continued to be significantly elevated, and there continued to be no significant heterogeneity among studies (as measured by Q and I). Because the Canadian study was the largest of the chrysotile-exposed cohorts, I used the Stata program metareg to compare the laryngeal cancer SMRs by asbestos fiber type. The result is shown in Table II. I conclude that inclusion of the Canadian study produces a more complete result, in keeping with the goals of meta-analysis, but that there were no substantial changes to the findings published by Peng and colleagues.