Adenocarcinoma is now the most common form of lung cancer and accounts for about 38% of all lung carcinomas, followed by squamous cell carcinoma (20%), small cell carcinoma (14%), large cell carcinoma (3%) and other types (25%). Adenocarcinoma is slightly more common in women than in men, and is more likely to occur in younger age group than other types of lung cancer. Mixed histologic patterns e.g. adenocarcinoma + squamous cell carcinoma OR squamous cell carcinoma + small cell carcinoma etc. occur in about 10% of cases (Source: Pathologic Basis of Disease, 9th Edition). Squamous cell carcinoma and small cell carcinoma arise in more proximal airways whereas adenocarcinomas arise in more peripheral airways. Smoking is linked to all histologic types of lung cancer, but the strongest association is with squamous cell carcinoma and small cell carcinoma. Adenocarcinoma is the commonest type found in never-smokers. However, due to changes in the design and composition of cigarettes since the 1950s, the risk of adenocarcinoma of the lung from cigarette smoking has also increased. Lung cancers in never-smokers are usually adenocarcinomas. They are likely to have EGFR mutations and almost never have KRAS mutations. The frequency of TP53 mutations are also less frequent than in smoking-related cancers.