Carcinoma

A carcinoma is a malignant tumor that arises from epithelium, the sheets of tightly packed cells that form the skin and line the airways, gut, glands, ducts and other organs. Carcinoma is the most common type of cancer: most cancers of the breast, lung, colon and prostate are carcinomas. Cancers of supporting tissues such as bone, cartilage or muscle are called sarcomas instead.

Carcinomas are subdivided by the kind of epithelial cell involved. Adenocarcinomas come from gland cells that secrete mucus or other fluids. Squamous cell carcinomas arise from flat surface cells in the skin, mouth, esophagus and other linings. Basal cell carcinoma starts in the deepest layer of the skin's outer epidermis, and transitional cell, or urothelial, carcinoma begins in the stretchy lining of the bladder.

When abnormal cells are still confined to the layer where they formed, without breaking into deeper tissue, the condition is called carcinoma in situ, or stage 0. It may progress to invasive carcinoma, which can spread to lymph nodes and distant organs, so it is often removed or closely watched.