Lung Cancer

Lung cancer is classified into two major types: nonsmall-cell lung cancer (NSCLC) and small-cell lung cancer (SCLC). They are distinguished from one another by several characteristics:

• The appearance of the cells under microscopy
• The location of the tumor in the lung
• The relative tendency for the tumor to spread within or outside of the lung

As a result, the approaches to the treatment of NSCLC and SCLC are different.

NSCLC is the most common type of lung cancer, accounting for 85% of lung cancers, and is composed of three main subtypes:

• NSCLC - 85%
• Squamous-cell carcinoma (about 25-30%)
• Adenocarcinoma (40%)
• Large-cell undifferentiated carcinoma (10-15%)

Other types of NSCLC tumors include carcinoid tumors and mesothelioma (approximately 15%).

• SCLC accounts for the remaining 15% of lung cancers.

 

NSCLC Subtypes

Squamous-cell carcinoma is the most common type of lung cancer in Europe and the second most common type of cancer in the United States. This type of tumor tends to originate in the bronchus, where it arises from metaplastic epithelium. It may block the bronchus as it enlarges. Squamous-cell carcinoma shows a strong dose-response relationship with smoking history (ie, it is seen most frequently in heavy smokers) and is more common in men than in women. This form of lung cancer is slow-growing, and in its early stages tends to be confined to the lung.

Adenocarcinoma is an increasingly common form of lung cancer, particularly in women. This form of lung cancer usually arises in the peripheral portions of the lung (ie, in the smaller bronchioles and alveoli). Adenocarcinoma lacks a doseresponse relationship with smoking and occurs at a similar rate in light and heavy smokers. Adenocarcinoma may occur in a single site or in multiple sites and often metastasizes at an early stage, before cancer is diagnosed. As a result, adenocarcinoma has a poorer prognosis than squamous-cell carcinoma.

Large-cell carcinoma tends to form large, bulky tumors. The cells are anaplastic; that is, they show no evidence of differentiation as lung tissue. This form of lung cancer resembles adenocarcinoma in its peripheral location and tendency to metastasize, and is associated with a similarly poor prognosis.

 

SCLC

SCLC accounts for 15 to 20% of all lung tumors and almost all cases are associated with a history of smoking. SCLC is a rapidly growing tumor and has a tendency to metastasize to the bone, brain, liver, and adrenal gland. The cells are small and round, and have oval-shaped nuclei. Compared with NSCLC, SCLC has a greater tendency to metastasize and a greater sensitivity to chemotherapy. For these reasons, treatment strategies for NSCLC and SCLC are very different.

 

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