Any of various types of malignant neoplasm derived from epithelial cells, chiefly glandular (adenocarcinoma) or squamous (squamous cell carcinoma); the most commonly occurring kind of cancer.
Like other malignant neoplasms, carcinomas display uncontrolled cellular proliferation, anaplasia (regression of cells and tissues to more primitive or undifferentiated states), and a tendency to invade adjacent tissues and to spread to distant sites by metastasis. A carcinoma arises from a single cell with a genome that either contains an inherited aberration (oncogene) or has acquired one as a consequence of spontaneous mutation or damage by a chemical toxin (carcinogen), radiation, viral infection, chronic inflammation, or other external assault. Probably a complex sequence of biochemical and genetic injuries must take place for a carcinoma to develop. Some carcinomas (e.g., prostate, breast) depend partly on the presence of hormones (androgen, estrogen) for their proliferation. Carcinomas are graded histologically according to evidence of invasiveness and changes that indicate anaplasia (loss of polarity of nuclei, loss of orderly maturation of cells particularly in squamous cell types, variation in the size and shape of cells, hyperchromatism of nuclei with clumping of chromatin, and increase in the nuclear-cytoplasmic ratio). Carcinomas may be undifferentiated, or the neoplastic tissue may resemble to varying degrees one of the types of normal epithelium. Carcinomas can secrete a variety of hormonelike factors capable of inducing systemic (paraneoplastic) effects (e.g., hypercalcemia, thrombophlebitis). The most common site of origin of carcinoma in both sexes is the skin; the second most common site in men is the prostate and in women the breast. However, the most frequently lethal carcinoma in both sexes is bronchogenic carcinoma.
Reference: Stedman's Medical Dictionary