Neoplasia is a disease in which genetically altered cells escape from normal growth regulation. Important concepts in the definition of neoplasia include: (i) the presence of an abnormal mass with growth that is uncoordinated with normal tissues; and (ii) persistence of excessive growth after cessation of the stimulus evoking the lesion (Willis, 1967). The abnormal growth is to some extent structurally and functionally independent of the host because neoplastic cells are partially free of the controls that act to regulate and limit growth of normal cells (Sirica, 1989). Persistence of growth after removal of the factor evoking the neoplasm indicates that the neoplastic trait is a change in structure or expression of DNA that is inherited by succeeding generations of cells.

Several morphological features distinguish neoplasms from normal tissues and from other types of lesions. Neoplastic growth is not controlled by the same mechanisms controlling normal tissues. This results in a persistent, expanding or infiltrating growth without the architecture of normal tissue. Neoplasms commonly form grossly visible masses, but this is not an essential part of the concept of neoplasia; for example, some types of lymphomas consist of invasive cells that do not form macroscopically visible tumours (Kieser et al., 1991). Neoplasms have varying degrees of abnormality in cellular appearance and growth rates, and functional differences are usually apparent between neoplastic tissue and related normal tissue. However, histological criteria for neoplasia are not always sufficient for a definitive diagnosis.

Hyperplasia can be difficult to distinguish from neoplasia. Hyperplastic growth sometimes results in a mass of tissue, but cessation of the stimulus causing the lesion results in regression of the growth. Usually, the cellular appearance and tissue architecture of hyperplastic masses more closely resemble normal tissue than neoplasms. Examples of lesions that resemble neoplasia are presented later in this chapter under the heading of pseudoneoplasms. The term 'hyperplasia' has been used by some authors to include proliferation of cells in neoplasia, but in this chapter hyperplasia will be used only for lesions that are not neoplastic.

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