Nearly 40% of lung cancers in the US are adenocarcinoma, which usually originates in peripheral lung tissue.[3] Most cases of adenocarcinoma are associated with smoking; however, among people who have smoked fewer than 100 cigarettes in their lifetimes ("never-smokers"),[4] adenocarcinoma is the most common form of lung cancer.[5] Its incidence has been increasing in many developed Western nations in the past few decades, where it has become the most common major type of lung cancer in smokers (replacing squamous cell lung carcinoma) and in lifelong nonsmokers.[1] According to the Nurses' Health Study, the risk of adenocarcinoma of the lung increases substantially after a long duration of previous tobacco smoking, with a previous smoking duration of 30 to 40 years giving a relative risk of approximately 2.4 compared to never-smokers, and a duration of more than 40 years giving a relative risk of approximately 5.[6]