Lung cancer classification


 * Associate Editor(s)-In-Chief: Kim-Son H. Nguyen, M.D., M.P.A., Beth Israel Deaconess Medical Center, Harvard Medical School, Boston MA,

Overview
The vast majority of lung cancers are carcinomas—malignancies that arise from epithelial cells. There are two main types of lung carcinoma, categorized by the size and appearance of the malignant cells seen by a histopathologist under a microscope: non-small cell (80.4%) and small-cell (16.8%) lung carcinoma. This classification, based on simple histological criteria, has important implications for clinical management and prognosis of the disease.

Non-small cell lung carcinoma (NSCLC)
The non-small cell lung carcinomas are grouped together because their prognosis and management are similar. There are three main sub-types: squamous cell lung carcinoma, adenocarcinoma and large cell lung carcinoma.

Accounting for 31.1% of lung cancers, squamous cell lung carcinoma usually starts near a central bronchus. Cavitation and necrosis within the center of the cancer is a common finding. Well-differentiated squamous cell lung cancers often grow more slowly than other cancer types.

Adenocarcinoma accounts for 29.4% of lung cancers. It usually originates in peripheral lung tissue. Most cases of adenocarcinoma are associated with smoking. However, among people who have never smoked ("never-smokers"), adenocarcinoma is the most common form of lung cancer. A subtype of adenocarcinoma, the bronchioloalveolar carcinoma, is more common in female never-smokers, and may have different responses to treatment.

Accounting for 10.7% of lung cancers, large cell lung carcinoma is a fast-growing form that develops near the surface of the lung. It is often poorly differentiated and tends to metastasize early.

Small cell lung carcinoma (SCLC)
Small cell lung carcinoma (SCLC, also called "oat cell carcinoma") is less common. It tends to arise in the larger breathing tubes and grows rapidly, becoming quite large.

The "oat" cell contains dense neurosecretory granules (vesicles containing neuroendocrine hormones) which give this an endocrine/paraneoplastic syndrome association.

While initially more sensitive to chemotherapy, it ultimately carries a worse prognosis and is often metastatic at presentation. This type of lung cancer is strongly associated with smoking.

Metastatic cancers
The lung is a common place for metastasis from tumors in other parts of the body. These cancers are identified by the site of origin, thus a breast cancer metastasis to the lung is still known as breast cancer. They often have a characteristic round appearance on chest x-ray.

Primary lung cancers themselves most commonly metastasize to the adrenal glands, liver, brain, and bone.