Lung Cancer. Light micrograph of carcinoma cancer cells in an alveolus of the lung. An alveolus is one of the small air sacs that diffuse gases between the air and the body. This alveolus is full of cancerous cells with large semicircular nuclei. Cancer cells divide continuously, growing chaotically to form clumps of cells called tumors. A carcinoma is a cancer that develops in the epithelium cells that line body organs. Lung cancers are fatal unless caught early, and are often caused by tobacco smoking.