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We analyzed the time trends of lung cancer by histological subtype in Hong Kong during 1991-2005, and examined how the time trends were influenced by the effects of birth cohort and calendar period of diagnosis. Cancer incidence data were obtained from Hong Kong Cancer Registry and population data from Census and Statistics Department. Age-standardized incidence rates were computed by the direct method using WHO 1966 standard population as reference. Period and cohort effects were assessed by using two separate Poisson regression models adjusting for age. From 1991 to 2005, the incidence rates in Hong Kong Chinese males decreased steadily. The decline in overall lung cancer incidence rates was limited primarily to the decrease in squamous cell carcinoma, which could be explained by the decreasing trend of cigarette smoking. Adenocarcinoma had been the most predominant histological subtype all along. The relatively horizontal trend of adenocarcinoma and the lack of cohort effect implied the important roles of gene-environment interaction and/or the use of low-tar and filter tip cigarettes. Our study suggests that different histological subtypes may represent different disease entities with perhaps some distinct risk factors. The hypotheses generated from this ecological study will need confirmation by subsequent analytic studies.

The gender difference in epidemiology of lung cancer has been postulated to be due to the higher susceptibility of women to risk factors especially tobacco smoking. Alternatively, such difference may also be explained by some unknown gender-specific etiological factors, which can have been masked if both the female and male prevalence of smoking are high. Hong Kong has a low female smoking prevalence rate and therefore the trend of the female incidence of lung cancer is particularly interesting because it can reflect the effects of the non-smoking related risk factors more clearly. The present study examined the trends of incidence rates for the major histologic types and smoking prevalence from 1983 to 2000 in Hong Kong with respect to gender. The prevalence of daily smokers decreased from 39.7% in 1982 to 22% in 2000 in males and from 5.6 to 3.5% in females. The time trends of the lung cancer incidence (overall or with respect to age and histology) were similar for both genders. The overall incidence decreased progressively throughout the study period, attributable to the decrease in squamous cell, small cell and large cell carcinoma. The decline occurred in all age groups but to a greater extent in the younger age groups. The incidence of adenocarcinoma increased until 1988-1990 and then stabilized. The initial increase was restricted to the older age groups. These temporal patterns suggested that the same etiological factors affected both genders to a different extent but manifested as similar changes in the direction of incidence over time. To confirm this hypothesis, further studies were needed to clarify the nature of these etiological factors for the non-smoking related lung cancer cases.