PRINCIPLES OF TOXICOLOGY - Biology East Borneo

13.1 THE TERMINOLOGY OF CANCER 267Histogenesis: Tissue OriginNearly all tumors arise from either epithelial (ectodermal or endodermal) or mesenchymal (mesodermal)tissues. Epithelial tissues include lining epithelium (e.g., skin epidermis and the epithelium of thegastrointestinal tract and urinary system) and glands (e.g., pancreas, liver, mammary, prostate, sweat,and sebaceous glands). Mesenchymal or connective tissues include cartilage, bone, muscle, lymphoid,and hematopoietic cells.Behavior of NeoplasmThe distinction between benign and malignant tumors is extremely important because the malignancyof a tumor is typically what defines human cancer as a disease state. As noted previously, the abilityto metastasize is the definitive characteristic of a malignant neoplasm. Not all malignant tumorsmetastasize (e.g., CNS tumors, intraocular tumors), but no benign tumors do. Metastasis is the majorcause of morbidity and mortality associated with cancer. Treatment is much less likely to be successfulonce metastasis has occurred. In contrast to malignant tumors, benign neoplasms do not grow beyondtheir boundaries. However, benign tumors can inflict damage by localized obstruction, compression,interference with metabolism, and even secretion of unneeded hormones. Once a benign tumor isremoved, so is its harmful influence. Table 13.1 provides further distinction between benign andmalignant neoplasms.Combined ClassificationBenign neoplasms, whether arising from epithelial or mesenchymal tissues, are named by adding thesuffix oma to the tissue type. A benign epithelial neoplasm is an adenoma. This can be made morespecific by the addition of modifiers that refer to the tissue of origin (e.g., mammary adenoma). Thereare many exceptions that are too rooted in common usage to be eliminated. For example, melanomausually refers to a malignancy of melanocytes; the preferred term is malignant melanoma. Lymphomausually refers to a malignancy of lymphoid tissue; the preferred term is lymphosarcoma.Malignant neoplasms arising from epithelial tissues are named by adding carcinoma to the tissueor cell of origin (e.g., thyroid carcinoma). The prefix -adeno is added if the tumor is gland forming(e.g., thyroid adenocarcinoma). Malignant neoplasms arising from mesenchymal tissues are named byadding sarcoma to the tissue or cell of origin (e.g., fibrosarcoma). Table 13.2 provides several examplesof tumors classified using this scheme.TABLE 13.1 Distinctions between Benign and Malignant NeoplasmsBenignWell differentiated; resembles a cell of originGrows by expansionWell circumscribed, often encapsulated by aperipheral rim of fibrous tissueGrows at a normal rateFew mitotic figuresGrowth may be limitedDoes not metastasize, seldom dangerousAdequate blood supplyMalignantPoorly differentiated or anaplasticGrows by expansion and infiltrationPoorly circumscribed and invades stromaand vesselsGrowth rate usually increasedFrequent mitotic figuresProgressive growthMetastasis usual; can be fatalOften outgrows blood supply, becomes necrotic