What is lung cancer?

Causes and Symptoms

Most forms of lung cancer fall within one of four categories: squamous cell (or epidermoid) carcinomas and adenocarcinomas, small or oat cell carcinomas (accounting for about 15 percent of lung cancers), and large cell carcinomas. Each of these forms can be further categorized on the basis of cell differentiation within the tumor: either well differentiated (resembling the original cell type) or moderately or poorly differentiated. Upon biopsy, stage groupings are also determined on the basis of size, invasiveness, and possible extent of metastasis.

Oat or small cell carcinomas usually consist of small, tightly packed, spindle-shaped cells, with a high nucleus-to-cytoplasm ratio within the cell. Oat cell carcinomas tend to metastasize early and widely, often to the bone marrow or brain. As a result, by the time that symptoms become apparent, the disease is generally widely disseminated within the body. Coupled with a resistance to most common forms of radiation and chemotherapy, oat cell carcinomas present a particularly poor prognosis. In general, patients diagnosed with this form of cancer have a survival period measured, at most, in months.

Adenocarcinomas are tumors of glandlike structure, presenting as nodules within peripheral tissue such as the bronchioles. Often these forms of tumors may arise from previously damaged or scarred tissue, such as occurs in smokers. The development of adenocarcinoma of the lung is not as dependent on smoke inhalation, however, as are other forms of lung cancer.

Squamous cell, also called epidermoid, carcinomas tend to be slower-growing malignancies that form among the flat epithelial cells on the surface of a variety of tissues, including the bladder, cervix, or skin, in addition to the lung. The cells are often polygonal in shape, with keratin nodes on the surface of lesions. Squamous cell carcinomas tend to metastasize less frequently than other forms of lung cancer, allowing for a more optimistic prognosis.

Large cell carcinomas are actually a more general form of cancer in which the cells are relatively large in size, with the cell nucleus being particularly enlarged. Often these carcinomas arise as either squamous cell carcinomas or adenocarcinomas. Metastasis, when it occurs, is frequently within the gastrointestinal tract.

There is no question that the single leading cause or factor resulting in lung cancer is smoking. Persons who do not smoke, and indeed even smokers who smoke fewer than five cigarettes per day, are at relatively low risk of developing any form of lung cancer. Those who smoke more than five cigarettes per day run an increased risk of developing lung cancer at rates approaching two hundred times that of a nonsmoker. This risk is greatest for oat cell carcinomas and least for adenocarcinomas (but still approximately a tenfold risk over that of nonsmokers). The relative risk is related to the number of cigarettes smoked: the more cigarettes, the greater the risk. In addition, though other environmental hazards can be related to the development of lung cancers, the risks associated with those hazards are without exception amplified by cigarette smoke.

Exposure to other specific environmental factors has also been associated with the formation of certain forms of pulmonary cancers. Individuals chronically exposed to materials such as asbestos, hydrocarbon products (coal tars or roofing materials), nickel, vinyl chloride, or radiochemicals (uranium and pitchblende) are at increased risk. Chronically damaged lungs, for whatever reason, are at significantly increased risk for development of cancer.

The symptoms of lung cancer may represent the damage caused by the primary tumor or may be the result of metastasis to other organs. The most common symptom is a persistent cough, sometimes accompanied by blood in the sputum or difficulty breathing. Chest pain may be present, especially upon inhalation. There may also be repeated attacks of bronchitis or pneumonia that tend to persist for abnormal periods of time.

Treatment and Therapy

Diagnosis of a tumor in the lung generally includes a chest x-ray, along with use of a variety of diagnostic tests: bronchography (x-ray observation of the bronchioles following application of an opaque material), tomography (cross-sectional observation of tissue), and cytologic examination of sputum or bronchiole washings. Recent evidence indicates that low-dose computed tomography (CT) scans can be effective in early diagnosis of lung cancer, detecting it earlier than x-rays are able to. Confirmation of the diagnosis, in addition to determination of the specific type of tumor and its clinical stage, generally requires a needle biopsy of material from the lung.

The treatment of the tumor is dependent on the form of the disease and the extent of its spread. Surgery remains the preferred method of treatment, but because of the nature of the disease, fewer than half of cases are operable at the time of diagnosis. Of these, a large proportion are beyond the point at which the surgical removal of the cancer and resection of remaining tissue are possible. A variety of chemotherapeutic measures are available and, along with the use of radiation therapy, can be used to produce a small number of remissions, or at least temporary alleviation of symptoms. Nevertheless, only a small proportion of lung cancers, perhaps 10 percent, respond with a permanent remission.

Lung cancer is the leading cause of cancer deaths among American men and women. In 2014, there were an estimated 224,210 new cases of lung cancer in the United States, and lung cancer accounted for 27.2 percent of all cancer deaths. Between 2004 and 2010, the five-year survival rate was only 16.8 percent. The prognosis for most forms of lung cancer remains poor.

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