Lung cancer

Lung cancer is a malignant tumor originating in the lung or, less commonly, in the lining of the airways. About 85% of cases are classified as non-small-cell lung cancer (NSCLC), a group that includes adenocarcinoma, squamous-cell carcinoma, and large-cell carcinoma; the remainder are usually small-cell lung cancer (SCLC).¹

NSCLC and SCLC are distinguished by microscopic appearance and molecular behavior. SCLC tends to grow and spread rapidly, often involving distant sites early, whereas NSCLC contains several biologically distinct subtypes that may be treated according to specific mutations or proteins. Tumors can obstruct an airway, invade nearby tissue, or enter blood and lymphatic vessels. A cancer found in the lung but originating elsewhere is a metastasis, not primary lung cancer.

Tobacco smoke is the dominant established cause of lung cancer, but risk reflects multiple exposures and susceptibility. Cigarettes, cigars, pipes, and secondhand smoke contain carcinogens that damage DNA in airway cells; smoking cessation lowers risk, although former smokers remain at higher risk than people who never smoked.²

Radon, a naturally occurring radioactive gas that can accumulate indoors, is the second-leading cause in the United States and the leading cause among people who do not smoke.³ Occupational exposure to asbestos, arsenic, chromium, nickel, silica, diesel exhaust, and some combustion products can add risk. Outdoor air pollution, prior chest irradiation, chronic lung disease, family history, and inherited or tumor-acquired genetic changes also contribute. Most cases result from a mixture of factors rather than one identifiable event.

Persistent or progressive respiratory symptoms prompt evaluation, but early lung cancer may produce no symptoms. Possible signs include a lasting cough, coughing blood, chest pain, shortness of breath, recurrent pneumonia, hoarseness, unexplained weight loss, or fatigue. Diagnosis generally combines imaging, often chest computed tomography, with examination of cells or tissue obtained by bronchoscopy, needle biopsy, or surgery. Staging determines whether disease is confined to the lung, in nearby structures or lymph nodes, or metastatic.

For people at sufficiently high risk, annual low-dose computed tomography can reduce mortality; it is not a general test for all adults and can produce false-positive findings.⁴ Treatment may include surgery, radiation therapy, chemotherapy, targeted therapy, and immunotherapy. Pathology and molecular testing can identify alterations such as EGFR, ALK, ROS1, BRAF, KRAS, or MET that make targeted treatment appropriate in some NSCLCs.⁵

Lung cancer is a collection of diseases whose outlook varies greatly by stage, subtype, molecular profile, and general health. A small peripheral tumor may be found incidentally on imaging, while a central tumor can cause symptoms by narrowing an airway. Some tumors produce hormone-like substances, causing paraneoplastic syndromes such as abnormal blood sodium or muscle weakness; these effects may precede recognition of the primary tumor.

Never-smokers with lung cancer are not a single biological group: adenocarcinoma is common among them, and actionable driver alterations are relatively important in treatment decisions. Screening is therefore different from diagnostic testing: screening targets people without symptoms who meet age and smoking-exposure criteria, while symptoms require clinical assessment regardless of eligibility. Survival statistics describe populations, not an individual prognosis; U.S. registry data show substantially better survival for localized disease than for cancer diagnosed after distant spread.⁶

Adenocarcinoma

A gland-forming cancer and the most common histological subtype of non-small-cell lung cancer.

Molecular testing

Laboratory analysis of tumor genes, proteins, or other biomarkers to help classify cancer and select treatment.

Paraneoplastic syndrome

A group of symptoms caused by substances produced by a tumor or by an immune response to it, rather than by direct invasion.

Staging

Assessment of tumor extent, including spread to lymph nodes or distant organs, used to guide treatment and estimate prognosis.