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There are two major forms of lung cancer that can be differentiated on the basis of the structural (morphological) appearance of the cancer cells under a microscope. These two major types of lung cancer are:

• Non-small cell lung cancer
• Small cell lung cancer

Non-Small Cell Lung Cancer

Non-small cell lung cancer (NSCLC) is the most common type of lung cancer and accounts for about 80% of all lung cancer cases. There are several different subtypes of NSCLC:

• Squamous cell (epidermoid) carcinoma - accounts for about one-third of NSCLC cases and usually occurs in smokers.

• Adenocarcinoma - accounts for about 35% of NSCLC cases and can occur in both smokers and non-smokers.

• Large cell carcinoma - this is a very aggressive form of NSCLC that grows and spreads rapidly. It accounts for less than 10% of NSCLC cases.

Small Cell Lung Cancer

Small cell lung cancer (SCLC) was recognized as a distinct form of lung cancer as early as 1926. In the past, small cell lung cancer was also called "oat cell carcinoma" because, when examined under a microscope, the lung cancer cells resemble grains of oats. In 1988, the International Association for the Study of Lung Cancer recommended that the "oat cell carcinoma" designation be dropped in favor of the term small cell lung cancer. Historically, small cell lung cancer has accounted for about 20% of all lung cancer cases, however, recent epidemiological studies indicate that the proportion of patients with small cell lung cancer has decreased to about 14% of all lung cancer cases.

The association between small cell lung cancer and smoking has been well established and, in fact, SCLC occurs almost exclusively in people who smoke. Smoking cessation, however, significantly reduces a person's risk for developing small cell lung cancer as well as non-small cell lung cancer. The recent trend in the decline of the proportion of small cell lung cancer cases (from 20% to 14%) most likely reflects a reduction in the proportion of smokers in the general population.