Worldwide, lung cancer is the leading cause of cancer-associated death. The main causes of lung cancer are smoking and air pollution, as well as genetic factors. In familial lung cancers, there are environmental factors and genetic factors involved.  A familial cancer diagnosis occurs when two or more first-degree relatives are diagnosed with the same type of cancer. Genetic and environmental factors are believed to contribute to familial cancer.
Most cancer patients have no family history of the disease. The genetic alterations that occur in cancer cells occur predominantly sporadic in the body cells, not in the cells responsible for reproduction. Cancers caused by defects in reproduction cells account for 1–5% of all cases.  Cancer that is passed down in families, in general, is physiologically and pathologically different from their sporadic counterparts.
In recent years, it has become more evident that tumours passed down in families exhibit distinctive bio-clinical characteristics, therefore, requiring tailored treatment strategies. Genetic testing is necessary to identify the genes responsible for familial cancers. Studies on familial cancers have the potential to identify individual genes and groups of genes responsible for cancer development and to identify new cancer-associated genes and biomarkers, as well as to estimate the risk of cancer among members of the general population. 
More Complex Forms
Clinical studies have frequently demonstrated a family history of lung cancer. Cancer in relatives who have it is typically more complex than that of relatives with other forms of cancer. Little is known about the role genetics play in lung cancer development. It is difficult to predict how lung cancer will develop in never-smokers. 
It has been shown that lung cancer can be passed from generation to generation through family members with lung cancer. A genetic inheritance accounts for approximately 8% of lung cancers. Those with a family history of lung cancer are more likely to develop the disease due to genetic recombination during reproduction. 
The risk of lung cancer increases significantly when you have a first-degree relative who has the disease and when you have multiple family members with the disease.