Lung cancers, also known as bronchogenic carcinomas ("carcinoma" is another term for cancer), 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 a distinction between these two types is important.
SCLC comprises about 20% 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 with only 1% of these tumors occurring in non-smokers. SCLC metastasize rapidly to many sites within the body and are most often discovered after they have spread extensively.
Histologically, these tumors are composed of fibrous or epithelial elements or both. The epithelial form occasionally causes confusion with peripheral anaplastic lung carcinomas or metastatic carcinomas. Attempts at diagnosis by cytology or needle biopsy of the pleura are often unsuccessful. It can be especially difficult to differentiate mesothelioma from adenocarcinoma on small tissue specimens. Thoracoscopy can be valuable in obtaining adequate tissue specimens for diagnostic purposes. Examination...
NSCLC is the most common lung cancers, accounting for about 80% of all cases. NSCLC has three main types that are named based upon 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 30% to 40% of NSCLC. While adenocarcinomas are associated with smoking like other lung cancers, this type is especially seen as well 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. 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 very common in non-smoking women and in the Asian population.
Squamous cell carcinomas were formerly more common than adenocarcinomas; at present they account for about 30% of NSCLC 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.
Mixtures of different types of NSCLC are also seen.
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%-10% of lung cancers. These tumors are generally small (3-4 cm or less) when diagnosed and occur most commonly in persons under 40 years of age. 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 can rarely occur 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.