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.
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:
2. Connective tissue
3. Hemopoietic and immune systems
4. Nervous system
5. Multiple histogenetic cellular origin
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