Lung cancer kills 160,000 Americans every year. It is the leading cause of cancer death in the United States and worldwide. Lung cancer is significantly different from other types of cancers because it is associated with prominent modifiable risk factors such as exposure to tobacco smoke that may result in the onset of the disease.
However, not all lung cancer cases are linked to cigarette smoking. There are other risk factors as well such as exposure to haloethers, asbestos, arsenic, nickel, and polycyclic aromatic hydrocarbons. Researchers are also trying to identify the potential role of exposure to radon and environmental tobacco smoke, i.e. second hand smoke or passive smoking. The list of probable risk factors include genetic factors, dietary factors, and the existence of underlying benign types of parenchymal lung disease such as pulmonary fibrosis and chronic obstructive lung disease.
Does the prognosis of lung cancer cases differ based on the cause of the cancer?
In general, a “never smoker” is a person who has smoked no more than 100 cigarettes in his/her lifetime. Getting reliable data on the occurrence of lung cancer amongst non-smokers is difficult because most cancer registries (population-based), including the National Cancer Institute's Surveillance, Epidemiology and End Results (SEER) database, generally do not gather information related to the patient’s smoking habits. Data available with SEER has been associated with tobacco use information (population-based), but it can only be used to determine occurrence of lung cancer amongst never smokers within wide geographic areas.
Globally, the incidence of lung cancer amongst never smokers comprises around 15% of cases in men and about 53% in women. Major geographic differences exist, especially in Asia, where around 60 to 80 percent of female lung cancer patients are never smokers.
In an analysis carried out in the United States, it was estimated that around 19% of lung cancer cases in females occurs in never smokers, in comparison to around 9% in males. These results have been derived from an analysis of data pertaining to patients in five large cohort studies conducted in the United States. The age-adjusted occurrence rate of lung cancer in case of never smokers (within the 40 to 79 age bracket) varied between 11.2 and 13.7 for every 100,000 person-years for males and between 15.2 and 20.8 for every 100,000 person-years for females. These occurrence rates are similar to the occurrence rates of myeloma in males (13.2 per 100,000) or cervical cancer in females (15.4 per 100,000) in the United States. In comparison, the age-adjusted rates, relevant to the incidence of lung cancer amongst current smokers in the same cohorts were about 12 to 30 times higher.
During an analysis of lung cancer patients who were diagnosed between 1991 and 2005 in Southern California, a unique test-mining programming algorithm was used to assess smoking status based on data and information sourced from electronic medical record abstracts available with the regional cancer registry. It was determined during the study that approximately 8.9 percent of more than 25,000 patients classified under the known smoking status were never smokers. This included 6 percent of male patients and 14 percent of female patients.
However, in another analysis involving 13 cohorts and 22 cancer registries, there was no apparent difference in the occurrence rate of lung cancer amongst never smokers by sex overall, even though the incidence rate amongst never smokers was higher in case of women in comparison to men aged 40 to 59 years. It was demonstrated through this multinational analysis that the age-standardized occurrence rates for lung cancer (per 100,000 as per the 2000 world population age standard) in case of female patients were 15.0 for Asian women, 12.4 for women of European descent, and 19.4 for women of African American descent. This compared to incidence rates of 12.9, 11.2, and 12.3, respectively, for men of Asian, European, and African-American background.
With studies generating conflicting results, it is not clear whether there is a change in the incidence of lung cancer amongst never smokers:
As opposed to this, the large pooled study of 13 cohorts compared new observed rates with those in Connecticut during the 1930s when smoking amongst women was very rare and did not find any significant change in due course.
In order to carefully evaluate trends in the occurrence rate of lung cancer in never smokers, cancer registry data will be required to collect information about the smoking status of patients. It can be a challenge to obtain self-reported smoking status, but the effectiveness of this method has been established in several studies which have recorded only small smoker misclassification rates.
In spite of the higher occurrence rate of lung cancer amongst never smokers in women in comparison to men in several studies, the age-adjusted mortality rate of lung cancer amongst never smokers was higher in case of men in comparison to women in two American Cancer Society cancer prevention study cohorts. Both these cohorts combined included a significant percentage of patients of European descent in a larger pooled assessment of 13 cohort studies and several different cancer registries which recorded age-standardized lung cancer mortality rates of 12 for every 100,000 in case of men and 9.5 for every 100,000 in case of never smoking women of European descent. Amongst Asians, the rates were 26 and 16.1 per 100,000 respectively for men and women. Even though women with lung cancer have higher life expectancy in comparison to men, it is not known whether this is enough to explain the differences recorded between incidence and mortality.
In comparison, in a Japanese study, it was noticed that 33 percent of lung cancer related deaths in men and 85 percent of lung cancer related deaths in women were not associated with cigarette smoking. The percentage of never smoking women diagnosed with lung cancer is much higher in Japan.
The risk of developing smoking-related lung
cancer varies by race/ethnicity.
Precise data on the variation in rates of lung cancer (based on race or ethnicity) amongst never smokers is extremely limited.
More research is required to gain a better understanding of the racial/ethnic differences associated with this disease.
While it has been suggested that lung cancer,
in case of never smokers, occurs at a younger age, the same has not been
confirmed in a recent cohort study in Western populations. However, in
studies conducted in Asia, it has been noticed that lung cancer, in case
of never smokers, occurs mostly amongst younger people.