Lung cancer is one of the malignant tumors with the highest incidence and mortality in the world. In 2017, there were 2.2 million incident cases of Tracheal, bronchus, and lung (TBL) cancer and 1.9 million (95% UI, 1.8–1.9 million) deaths. It is the second most common cancer and the leading cause of cancer death among men worldwide (1). In China, lung cancer has the incidence and is the leading cause of death in malignant tumors. According to the latest statistics of the National Cancer Center, about 787,000 patients were diagnosed lung cancer and 631,000 patients died of lung cancer in 2015. Most of the patients were in advanced stage when they were diagnosed where their five-year survival rate was only 5%. But for stage I lung cancer patients whose 5-year survival rate can be as high as 70% after surgery. Therefore, early diagnosis of lung cancer is particularly important to improve the prognosis of these patients (2). For decades, there are lots of biomarkers found including protein, microRNAs, epigenetic changes and also molecules in exhaled breath for screening and early diagnosis of lung cancer, but they are limited in clinical application due to lower specificity and higher expenses (3, 4). Thus, it is of great significance to find a simple and efficient predictor to screening the high-risk lung cancer populations for an early detection and treatment (5, 6).
Bilirubin is one of the endogenous antioxidant and anti-inflammatory molecule, whose level is reported to negatively correlated with the incidence of cardiovascular diseases (CVD), stroke, metabolic syndrome and a variety of cancers, such as rectal cancer and breast cancer (7–13). A few cohort studies around the world and one of them in Korea showed an inverse association between bilirubin and lung cancer (14). Hence, in this work, we hypothesized this correlation is also exist in China.
Smoking is a major epidemiological cause of lung cancer, however, it’s not sufficient enough to identify the highest risk individuals when using it alone (15, 16). Therefore, novel biomarkers for lung cancer incidence and mortality are urgently needed for clinical guidance of screening high-risk groups of lung cancer, particularly among smokers. Numerous studies have found that smokers have lower bilirubin levels than nonsmokers. To figure out whether bilirubin level can be a predictor for lung cancer among smokers in China, we designed this study to compare the difference of serum bilirubin level between patients with lung cancer and those without lung cancer, and analyze the influence of smoking on bilirubin level.