[1] Specifically, a carcinoma is a cancer that begins in a tissue that lines the inner or outer surfaces of the body, and that arises from cells originating in the endodermal, mesodermal[2] or ectodermal germ layer during embryogenesis.

Very rarely, tumors may contain individual components resembling both carcinoma and true sarcoma, including carcinosarcoma and pulmonary blastoma.

This may include the production of one or more forms of cytokeratin or other intermediate filaments, intercellular bridge structures, keratin pearls, and/or tissue architectural motifs such as stratification or pseudo-stratification.

Eventually, the tumor burden increasingly interferes with normal biochemical functions carried out by the host's organs, and death ultimately ensues.

[13] Metastatic carcinoma is cancer that is able to grow at sites distant from the primary site of origin; thus, dissemination to the skin may occur with any malignant neoplasm, and these infiltrates may result from direct invasion of the skin from underlying tumors, may extend by lymphatic or hematogenous spread, or may be introduced by therapeutic procedures.

[citation needed] For example, in the case of lung cancer, DNA damage is caused by agents in exogenous genotoxic tobacco smoke (e.g. acrolein, formaldehyde, acrylonitrile, 1,3-butadiene, acetaldehyde, ethylene oxide and isoprene).

[17] Endogenous (metabolically caused) DNA damage is also very frequent, occurring on average more than 60,000 times a day in the genomes of human cells.

The high frequency of mutations in the total genome within carcinomas suggests that, often, an early carcinogenic alteration may be a deficiency in DNA repair.

In addition, faulty repair of these accumulated DNA damages may give rise to epigenetic alterations or epimutations.

[20] Microscopic examination by a pathologist is then necessary to identify molecular, cellular, or tissue architectural characteristics of epithelial cells.

Staging of carcinoma refers to the process of combining physical/clinical examination, pathological review of cells and tissues, surgical techniques, laboratory tests, and imaging studies in a logical fashion to obtain information about the size of the neoplasm and the extent of its invasion and metastasis.

In more recent staging systems, substages (a, b, c) are becoming more commonly used to better define groups of patients with similar prognosis or treatment options.