Socio-economic status and lung cancer risk including histologic subtyping—A longitudinal study
Introduction
From being a rare disease at the beginning of the 20th century, lung cancer has currently become one of the most common forms of cancer worldwide, and has the highest mortality rate [1], [2]. Smoking is the primary cause of lung cancer in the majority of the cases [2]. Cigarette smoking is associated with all histologic types of lung cancer, but the strength of the association depends on the histologic subtype [1] Over the past two decades the distribution of histologic subtypes has changed in some western populations. The incidence of squamous cell carcinoma and small cell carcinoma has decreased while the incidence of adenocarcinoma has increased [1], [3]. Squamous cell carcinoma is the most common histologic subtype in men and adenocarcinoma the most common in women [3]. Among non-smokers, exposure to environmental smoke and residential exposure to radon have been associated with an increased risk of lung cancer [2]. Adenocarcinoma is the most common histologic subtype in never smokers and former smokers [4].
Low socio-economic status (SES) is associated with an increased risk for certain cancers. SES is regarded as a proxy for life-style related differences in behaviour, occupational and environmental exposures, diet, and reproductive factors that have been identified as possible risk factors for cancer [5]. Associations between low SES and an increased lung cancer incidence and mortality have been observed in previous studies [5], [6]. It is generally believed that these differences are caused by an increased prevalence of smoking in lower SES groups [7]. However, only a few studies have assessed the influence of smoking habits on differences in lung cancer risk between SES groups [5]. A case-cohort study from the Netherlands, a Danish study carried out in a male cohort of workers, and a Swedish population-based study have shown a higher risk of lung cancer associated with low SES in men, which persisted after adjustment for smoking habits [7], [8], [9]. Furthermore, a case-control study from Canada assessed the influence of SES on lung cancer risk in both genders and found similar associations between low education and income and increased risk of lung cancer in both men and women after adjustment for smoking habits [10].
To the best of our knowledge, no prospective, population-based study of lung cancer incidence incorporating data on SES smoking habits and histologic subtype has been conducted previously. Increased knowledge regarding SES differences in risk with respect to histologic subtype could generate new hypotheses concerning the SES-related mechanisms operating in lung cancer development.
Therefore, the aims of this study were firstly to assess whether low SES is an independent risk factor for lung cancer after adjustment for known risk factors, and secondly to investigate the relationship between SES and histologic subtype.
Section snippets
Study population
With a population of 250,000 inhabitants, Malmö is Sweden's third largest city. The Malmö Preventive Program (MPP), a preventive, case-finding program for cardiovascular risk factors and alcohol abuse, was created in 1974 at the Department of Preventive Medicine of Malmö University Hospital. The aim of this program was to screen large strata of the adult population, mostly middle-aged men and women born in prespecified years (age cohorts), in order to find high-risk individuals for preventive
Analysis
The relative risk (RR) of lung cancer in the low SES groups, with the high SES groups as the reference group, was calculated using the Cox proportional hazards regression model with adjustment for age as a continuous variable, and marital status as a categorical variable (married/not married). No differences in lung cancer risk were found between the categories of “not married” (not married, divorced or widowed) [17]. In smokers, the tobacco consumption variable (<20 g/≥20 g per day) and
Characteristics of the study population
The mean age at baseline was slightly higher in women (49 years) than in men (46 years). There was a inverse relationship between SES and smoking prevalence. In the low SES group of men the proportion of current smokers was highest (55%) and the proportion of never smokers was lowest (22%). Women were less likely to be current smokers in both the low (39%) and high (33%) SES groups.
The total number of lung cancer cases was 421 in men and 129 in women (Table 1).
Validation of smoking status with COHb levels
Median COHb levels increased with
Discussion
This population-based study showed an increased risk of lung cancer in individuals of low SES despite accounting for smoking habits. Low SES was associated with an increased risk of squamous cell carcinoma in smokers of both genders and an increased risk of mesothelioma in smoking men only.
Conclusion
Our results indicate that low SES confers an increased risk of lung cancer despite accounting for smoking habits. Furthermore, the risk of squamous cell carcinoma among smokers was increased in the low SES group. Squamous cell carcinoma is strongly related to smoking, it is therefore possible that some of the SES differences in squamous cell carcinoma risk at least partly may be explained by differences in smoking exposures that could not be obtained from self-reported smoking habits.
Acknowledgments
This study was supported by grants from the City of Malmö, The Swedish Medical Research Council, and the Swedish Heart and Lung Foundation.
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