context: Lung cancers are broadly classified into two types: small cell lung cancers ( SCLC) and non-small cell lung cancers ( NSCLC). This classification is based upon the microscopic appearance of the tumor cells. These two types of cancers grow, spread, and are treated in different ways, so making a distinction between these two types is important. SCLC comprises about 10%-15% of lung cancers. This type of lung cancer is the most aggressive and rapidly growing of all the types. SCLC is strongly related to cigarette smoking. SCLCs metastasize rapidly to many sites within the body and are most often discovered after they have spread extensively. NSCLC is the most common lung cancer, accounting for about 85% of all cases. NSCLC has three main types designated by the type of cells found in the tumor. They are: Adenocarcinomas are the most common type of NSCLC in the U. S. and comprise up to 40% of lung cancer cases. While adenocarcinomas are associated with smoking like other lung cancers, this type is also seen in non-smokers -- especially women -- who develop lung cancer. Most adenocarcinomas arise in the outer, or peripheral, areas of the lungs. They also have a tendency to spread to the lymph nodes and beyond. Adenocarcinoma in situ (previously called bronchioloalveolar carcinoma) is a subtype of adenocarcinoma that frequently develops at multiple sites in the lungs and spreads along the preexisting alveolar walls. It may also look like pneumonia on a chest X-ray. It is increasing in frequency and is more common in women. People with this type of lung cancer tend to have a better prognosis than those with other types of lung cancer. Squamous cell carcinomas were formerly more common than adenocarcinomas; today, they account for about 25% to 30% of all lung cancer cases. Also known as epidermoid carcinomas, squamous cell cancers arise most frequently in the central chest area in the bronchi. This type of lung cancer most often stays within the lung, spreads to lymph nodes, and grows quite large, forming a cavity. Large cell carcinomas, sometimes referred to as undifferentiated carcinomas, are the least common type of NSCLC, accounting for 10%-15% of all lung cancers. This type of cancer has a high tendency to spread to the lymph nodes and distant sites. Other types of cancers can arise in the lung; these types are much less common than NSCLC and SCLC and together comprise only 5%-10% of lung cancers: Bronchial carcinoids account for up to 5% of lung cancers. These tumors are generally small (3-4 cm or less) when diagnosed and occur most commonly in persons under age 40. Unrelated to cigarette smoking, carcinoid tumors can metastasize, and a small proportion of these tumors secrete hormone-like substances. Carcinoids generally grow and spread more slowly than bronchogenic cancers, and many are detected early enough to be surgically removed. Cancers of supporting lung tissue such as smooth muscle, blood vessels, or cells involved in the immune response are rare in the lung. As discussed previously, metastatic cancers from other primary tumors in the body are often found in the lung. Tumors from anywhere in the body may spread to the lungs either through the bloodstream, through the lymphatic system, or directly from nearby organs. Metastatic tumors are most often multiple, scattered throughout the lung and concentrated in the outer areas rather than central areas of the organ.