CLASSIFICATION OF NEOPLASMS

24 May

Behavioristic (Biological) Classification

Since our definition of neoplasia is presently based on the biological behavior of neoplasms, it is proper to make a classification on the basis of such behavior. However, in making this classifica- tion, it should be noted that all neoplasms that we are considering conform to the definition of Ewing. As we shall see, the distinction between benign and malignant neoplasms in the behav- ioristic classification has considerable usefulness in determining the prognosis in a specific pa- tient but is of little use to the scientist who is studying  the mechanisms  of neoplasia  at the molecular level.

The principal  behavioristic  characteristics  of benign and malignant  neoplasms  are as follows:

The majority of the differences between benign and malignant neoplasms are relative. The critical difference between the two types is point no. 7, in that benign neoplasms by definition do not exhibit metastatic growth, whereas all malignant neoplasms have the potential for successful metastatic growth. A metastasis is defined as the secondary growth of a neoplasm, originating from a primary neoplasm and growing within the host organism in a location distant from the initial or primary site of neoplastic growth. As is shown in Chapter 7, there are various routes and mechanisms of metastases for malignant neoplasms.

Although  there is little doubt from the literature that most pathologists  and students of oncology define a malignant neoplasm by its ability to metastasize (Ackerman and de Regato,1962; Bland-Sutton, 1911; Cappell, 1958; Hopps, 1964; Montgomery, 1965), the artificiality of this distinction from the viewpoint of the natural history of neoplasia will soon become evident. It is well known that a number of benign neoplasms may at some time during their natural his- tory take on the behavior of a malignant neoplasm. This phenomenon, which is discussed later under the heading  “Progression  of Neoplasia”  (Chapters  7, 9, and 10), was emphasized  by Foulds (1965), who considered the behavioristic distinction between benign and malignant neo- plasms to be essentially nonexistent. In the United States (Shubik et al., 1977; Huff et al., 1989) as well as internationally (Faccini et al., 1992), both benign and malignant neoplasms have been considered important in the determination of the carcinogenicity (Chapter 3) of a specific chem- ical agent. However, students of oncology have made the distinction between benign and malig- nant neoplasms presented here.

Histogenetic Classification

Although the behavioristic classification is one of the most commonly accepted segments of the nomenclature  of neoplasms, the most important principle in the classification  of neoplasms is their grouping according to the type of tissue from which the neoplasm has arisen (Table 2.2). Ritchie (1970) distinguished  groups of neoplasms  on the basis of their histogenetic  origin as follows:

1.     Epithelium

2.     Connective tissue

3.     Hemopoietic and immune systems

4.     Nervous system

5.     Multiple histogenetic cellular origin

6.     Miscellaneous

This classification has considerable usefulness in itself, especially when considered with other aspects of the lesions, from both the diagnostic and the biological viewpoints. For exam- ple, it is very important to determine the region, origin, or tissue from which the neoplasm arose. In addition,  other descriptive  terms are often utilized in classifying  or diagnosing  a specific problem. Such descriptive terms as papillary, cystic, follicular, and others may relate to various histological characteristics of neoplasms of epithelial origin. In addition, some neoplasms have been named according to the individual first describing the lesion; examples are Ewing tumor of bone, Hodgkin disease of lymph tissue, and Wilms’ tumor of the kidney.

The behavioristic  and histogenetic  classifications  of neoplasms  are presently  the most widely used by both physicians and scientists alike. However, it is becoming increasingly evi- dent, as our knowledge of histogenesis and differentiation  increases, that a reevaluation,  espe- cially of the histogenetic classification, would be appropriate (Gould, 1986). Later in the text are discussions of various molecular markers of neoplasms that relate both to the definition of neo- plasia and to the histogenesis  of tissues from which such neoplasms  arose. As such markers come to be more widely known and the ultrastructural  techniques for cytological histogenesis become more widely used, the classification of neoplasia may evolve into the use of more mo- lecular terminology.

Table 2.2 Examples of Neoplasms According to Histogenetic Classifications

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