Types of Lung Cancer
There are several different types of lung cancer. Common types of lung cancer classification (e.g., based on histopathologic [diseased tissue] factors) include the following:
- Small cell carcinoma (also called oat cell carcinoma; lung cancer composed of anaplastic [unspecialized, undifferentiated] small cells)
- Squamous cell carcinoma (cancer of the layered, squamous epithelium [surface cells] of the lungs or bronchi)
- Adenocarcinoma (cancer of the glandular tissue, or cancer in which the tumor cells form recognizable glandular patterns)
- Large cell carcinoma (lung cancer composed of large-sized cells that are anaplastic in nature and often arise in the bronchi)
- Broncho-alveolar carcinoma
- Mixed and undifferentiated pulmonary carcinomas
Because of treatment concerns, most experts separate lung cancers into two groups: small cell lung carcinoma (SCLC) and non-small cell lung carcinoma (NSCLC). Small cell lung carcinoma often is widespread by the time of diagnosis; therefore, treatment usually is limited to chemotherapy and/or radiation therapy. By contrast, non-small cell carcinoma may not have spread at the time of diagnosis, so that surgical resection (cutting away) of the tumor may be possible.
Non-Small Cell Lung Carcinoma (NSCLC)
Non-small cell lung carcinoma (NSCLC) includes squamous cell carcinoma, adenocarcinoma, and large cell carcinoma. Large cell carcinoma and adenocarcinoma usually are found on the periphery (outer edges) of the lungs and may occur as solitary nodules, masses, or scar cancer. Squamous cell carcinoma and small cell carcinoma often are centrally located and may appear to be pneumonia (inflammation of the lungs), atelectasis (collapsed lung), or pit-like masses. Squamous cell cancers frequently are slow growing and can take several years to progress from a confined tumor into invasive cancer. Adenocarcinoma tends to have a worse prognosis than squamous cell cancer in all stages. The prognosis of large cell cancer, an uncommon NSCLC, is similar to that of adenocarcinoma.
Small Cell Lung Carcinoma (SCLC)
Small cell lung carcinoma (SCLC) accounts for approximately 20% of all primary lung cancers, or about 30,000–35,000 cases per year. The histologic distinction between non-small cell lung cancer and small cell lung cancer is extremely important. There are substantial differences between the two groups in both treatment and prognosis. In general, small cell lung cancer tends to be more aggressive and spreads sooner to distant sites. Some studies suggest that 60–70% of patients with small cell lung cancer have evidence of distant spread at the time of initial diagnosis. Yet, small cell lung cancer also is inclined to be more responsive to chemotherapy and chest radiotherapy (radiation).
Treatment for small cell lung cancer is not based upon surgery; rather, most physicians employ a systemic approach that includes chemotherapy and local control with radiotherapy. Smoking is the major risk factor for the development of small cell lung carcinoma.
In smokers, lung cancer usually is associated with three principal histologic types–small cell carcinoma, squamous cell carcinoma, and adenocarcinoma.
Adenocarcinoma accounts for about 30% of all lung cancers in male smokers and about 40% of all lung cancers in female smokers. These figures approach 60–80% of lung cancers in male and female nonsmokers. In people who have not smoked for more than 25, the adenocarcinoma percentage increases nearly to that of nonsmokers, and the squamous cell carcinoma percentage also decreases. Small cell cancers are reduced in linear proportion to the number of years since quitting.
Among certain racial groups (e.g., Asians) the distribution of adenocarcinoma in nonsmokers is similar to that in smokers, although the rate of lung adenocarcinoma is also much higher in Asian than in Western populations. In Hong Kong, there is a very high rate of adenocarcinoma among Chinese women. In addition, the rate of adenocarcinoma in Koreans is increasing in both sexes, especially among nonsmokers. According to the National Cancer Institute (NCI), adenocarcinoma now is more common than squamous cell carcinoma in both Caucasian and African American women. In addition, the rate of adenocarcinoma has increased more than squamous cell carcinoma, particularly in men.
The causes for race- and sex-related differences in lung cancer histopathology are unknown. Some experts suggest that adenocarcinoma in women may be increased by hormonal (endocrine) factors. In particular, researchers believe that estrogen replacement therapy (ERT) increases the risk for adenocarcinoma. Studies also have indicated that women may have a greater genetic susceptibility to specific types of lung cancer due to female, X-linked inheritance (inheritance of genes located on the X chromosome).
Occupational exposure also is associated with certain histopathologic types of lung cancer. For example, recent findings suggest that long-term (20+ years) exposure to mineral and metal dust is related to the growth of both small cell and squamous cell carcinomas. In addition, asbestos exposure may be associated with the development of adenocarcinoma and, to a lesser degree, with squamous cell or anaplastic carcinoma (cancer in which the cells have lost specialized characteristics, including physical placement). Some investigators theorize that household exposure to low-dose radon may increase the risk of small cell lung carcinoma over long periods of time.