In this case-control study on lung cancer among non-smokers, we observed the association between family history of lung cancer and susceptibility to lung cancer, and lung disease history (especially asthma) and susceptibility to lung cancer. We used two methods to develop an environmental exposure index according to 12 significant environmental factors of lung cancer. We then established restricted cubic spline model of EEI and lung cancer in non-smokers, which prompted us to conclude that there is no non-linear relationship between the two. The results from combined effects of lung disease history, family lung cancer history, and EEI score among non-smokers showed that EEI was associated with an increased risk of lung cancer and the positive association was apparently stronger in those with lung disease history or family lung cancer history. Furthermore, we found a statistically significant addictive interaction between EEI and lung disease history, as well as a possibly addictive interaction between EEI and family lung cancer history on lung cancer.
Recent studies have concluded that previous lung diseases may be linked to lung carcinogenesis in many different populations. Our research has reached a similar conclusion. Most previous studies focused on all lung cancer subjects but did not divide smokers and non-smokers. Smoking is considered a major risk factor for lung cancer and many other lung diseases. The aim of our study was restricted to non-smoker subjects to eliminate interference of lung disease as adjoint relation, which could provide a better way to identify their exact causal association with lung cancer. We also noted that previous asthma significantly increased the risk of lung cancer susceptibility in our research. A meta-analysis study [24] showed asthma was associated with the increased risk of lung cancer (OR = 1.44; 95% CI 1.31–1.59; P < 0.00001; I2 = 83%), but it was not associated with lung adenocarcinoma risk. Asthma is a disease of chronic airway inflammation involving multiple cells and cellular components [25]. Inflammatory mediators secreted by inflammatory cells can activate relevant protein kinase signaling pathways, regulate cell proliferation or apoptosis, induce tissue damage and repair, induce continuous cell renewal, and increase genetic mutations, thereby predisposing individuals to lung cancer. Probably, owing to the small number of subjects, we did not observe a connection between other lung disease history and lung cancer.
In the evaluation of genetic susceptibility, family history has been used as a surrogate for genetic risk [26]. Most studies have showed an increase of overall lung cancer risk for individuals with family history of lung cancer [27,28,29], which was inconsistent with our results. Risk assessment according to type of cancer (lung cancer and other cancer) and relatives (first-degree relatives and others) has rarely been performed in examination of non-smokers. Regarding lung cancer risk in relation to family history of cancer, the present study showed that the positive association tended to be evident for family history of lung cancer, family history of other cancer (except lung cancer), and family history of total cancer in first-degree relatives yet was not associated with the risk of lung cancer in non-smokers. Our conclusions regarding lung cancer susceptibility in relation to family history of other cancers were inconsistent with some previous studies. A study [27] from Japan indicated history of breast cancer in siblings was positively associated with the risk of overall lung cancer, small cell carcinoma, and adenocarcinoma among men. However, another Japanese study [30] reported that women with a family history of breast cancer in first-degree relatives are at higher risk of adenocarcinoma than men. Our study did not collect specific classification of other cancers in non-smokers, so we did not cover this issue in a more in depth. Further studies of the type of family cancer history and lung cancer are required.
Our results also implied the importance of shared exposure to environmental and residential factors in the development of lung cancer. Among the significant environmental risk factors in present study, intake of smoked food ≥ 3/wk showed the strongest effects on lung cancer, which was similar to our previous research result [31]. Food would likely be contaminated to a certain extent when it was smoked at high temperatures and the content of carcinogen (i.e., Benzo-a-pyrene [B(a)P]) in food would increase greatly. Extensive in vitro and in vivo investigations have demonstrated the capability of B(a)P to impose a variety of lethal factors capable of instigating and/or promoting the multi-step lung carcinogenesis process [32]. B(a)P can cause mutations in crucial genes by binding to DNA, which can contribute to the conversion of normal cells into tumor cells. B(a)P is also capable of promoting the outgrowth of transformed cells and contributing to the generation of visible tumor cell masses. A study [33] of barbecued meat, which also can accumulate B(a)P, demonstrated a positive association with risk of lung cancer. This emphasizes the importance of the cooking method of food in lung cancer.
To improve statistical efficiency, we integrated multiple environmental factors into a unitary environment exposure index to evaluate the overall influence of environmental exposure. These two methods were based on β and OR, respectively, and were used to calculate EEI score. Additionally, we applied restricted cubic spline that provided a useful and flexible basis for modeling relationships among continuous predictors to analyze the non-linear relationship between EEI and lung cancer in non-smokers. Since the calculation formula of OR is related to β, the present result revealed EEI was not non-linear-related with lung cancer in women and men, and the increase of environmental exposure factors appears to be associated with an increased risk of lung cancer using two different EEI scores. Therefore, an individual who was exposed to more environmental factors of high-risk rating would have a higher risk of lung cancer.
Although some previous studies have explored the influence of environmental factors in lung cancer, few illuminated the combined effects of lung disease history, environmental exposures, and family history of lung cancer among non-smokers. After adjusting general demographic factors, our environmental exposure index showed a positive gradient association with lung cancer whether for those “without family lung cancer history and lung disease history”, “only with lung disease history”, or “only with family lung cancer history”. Moreover, no matter what level of EEI was used, family history of lung cancer was a higher risk of developing lung cancer than lung disease history for non-smokers. However, non-smokers who had lung disease history were at higher risks of lung cancer than non-smokers who had family cancer history, especially for those who had intermediate-high levels of EEI. This is consistent with the above conclusion that family history of other cancers is not associated with the risk of lung cancer in non-smokers. Furthermore, there was an addictive interaction between EEI and lung disease history/family lung cancer history on lung cancer. Therefore, the present study indicated that environmental exposure had a combined effect with lung disease history or family history of lung cancer, which suggested that environmental factors and lung disease history or environmental factors and family history of lung cancer might interact with each other and synergistically increase the risk of developing lung cancer.
We do acknowledge that our study has several limitations. The first limitation was that the sample size may not have been sufficient, which resulted in no patients classified with family history of lung cancer and lung disease history. However, we are continuing to collect cases and controls to expand the sample size. Second, the present research was a hospital-based case-control study and the potential selection bias and recall bias could not be completely avoided. Third, we mainly focused on environment–disease history–family history and did not explore genetic susceptibility in this study. Family history of lung cancer is a characteristic of inherited susceptibility in lung cancer and cannot be completely used as a surrogate for genetic risk.