Carcinoma is a malignancy that develops from epithelial cells. 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 or ectodermal germ layer during embryogenesis.
Carcinomas occur when the DNA of a cell is damaged or altered and the cell begins to grow uncontrollably and become malignant. It is from the (itself derived from meaning crab).<ref>Oxford English Dictionary, 3rd edition, s.v.</ref>
Classification
As of 2004, no simple and comprehensive classification system has been devised and accepted within the scientific community. Traditionally, however, malignancies have generally been classified into various types using a combination of criteria, including:
The cell type from which they start; specifically: Epithelial cells ⇨ carcinoma Non-hematopoietic mesenchymal cells ⇨ sarcoma Hematopoietic cells Bone marrow-derived cells that normally mature in the bloodstream ⇨ leukemia Bone marrow-derived cells that normally mature in the lymphatic system ⇨ lymphoma Germ cells ⇨ germinoma
Other criteria that play a role include: The degree to which the malignant cells resemble their normal, untransformed counterparts The appearance of the local tissue and stromal architecture The anatomical location from which tumors arise Genetic, epigenetic, and molecular features
Histological types
Adenocarcinoma (adeno = gland) Refers to a carcinoma featuring microscopic glandular-related tissue cytology, tissue architecture, and/or gland-related molecular products, e.g., mucin. Squamous cell carcinoma Refers to a carcinoma with observable features and characteristics indicative of squamous differentiation (intercellular bridges, keratinization, squamous pearls).
Adenosquamous carcinoma Refers to a mixed tumor containing both adenocarcinoma and squamous cell carcinoma, wherein each of these cell types comprise at least 10% of the tumor volume. Anaplastic carcinoma Refers to a heterogeneous group of high-grade carcinomas that feature cells lacking distinct histological or cytological evidence of any of the more specifically differentiated neoplasms. These tumors are referred to as anaplastic or undifferentiated carcinomas. Large cell carcinoma Composed of large, monotonous rounded or overtly polygonal-shaped cells with abundant cytoplasm. Small cell carcinoma Cells are usually round and are less than approximately 3 times the diameter of a resting lymphocyte and with little evident cytoplasm. Occasionally, small cell malignancies may themselves have significant components of slightly polygonal and/or spindle-shaped cells.
There are a large number of rare subtypes of anaplastic, undifferentiated carcinoma. Some of the more well known include the lesions containing pseudo-sarcomatous components: spindle cell carcinoma (containing elongated cells resembling connective tissue cancers), giant cell carcinoma (containing huge, bizarre, multinucleated cells), and sarcomatoid carcinoma (mixtures of spindle and giant cell carcinoma). Pleomorphic carcinoma contains spindle cell and/or giant cell components, plus at least a 10% component of cells characteristic of more highly differentiated types (i.e. adenocarcinoma and/or squamous cell carcinoma). Very rarely, tumors may contain individual components resembling both carcinoma and true sarcoma, including carcinosarcoma and pulmonary blastoma. A history of cigarette smoking is the most common cause of large cell carcinoma.
Carcinoma of unknown primary site
The term carcinoma has also come to encompass malignant tumors composed of transformed cells whose origin or developmental lineage is unknown (see cancer of unknown primary origin; CUP), but that possess certain specific molecular, cellular, and histological characteristics typical of epithelial cells. 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.