The Problem Of Tumor Classification

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All the difficulties encountered in the classification of human thyroid tumors have to be faced in an attempt to classify thyroid tumors in animals, particularly in the rat. In early experiments, the well-known absence of clear-cut histological criteria for distinguishing reactive hyperplasia from neoplasia or benign from malignant tumorous growth has been reflected in publications describing corresponding lesions in the rat. In most cases, the authors have tried to apply the nomenclature of human pathology to the lesions observed in the rat thyroid. This approach has not only some easily recognizable advantages, but also some disadvantages. On the one hand, the use of similar terms would provide an opportunity for conducting a comparative analysis of

Thyroid Adenoma Histology Classification

Figure 1. Experimental thyroid tumors in rats induced by x-ray irradiation: Low power view of a follicular adenoma with surrounding capsule (a), high power view of papillary carcinoma (b), follicular carcinoma (c) and squamous cell (epidermoid) carcinoma.

Figure 1. Experimental thyroid tumors in rats induced by x-ray irradiation: Low power view of a follicular adenoma with surrounding capsule (a), high power view of papillary carcinoma (b), follicular carcinoma (c) and squamous cell (epidermoid) carcinoma.

experimental and clinical observations. On the other hand, the same terms might be, and have already been, applied to morphological lesions that are superficially similar, but whose biological behavior is different. The broadest definition applicable to both benign and malignant neoplasms was formulated by Axelrad and Leblond (1955). It fits in with the criteria applied to rat tumors by most experimentalists and can be paraphrased as follows: a pathological change in the rat thyroid can be regarded as a neoplasm when it is focal; it is distinct from the rest of the gland cytologically and architecturally and shows evidence of progressive growth. As to the definition of such terms as "adenoma" and "carcinoma", the wording suggested here is based on the above definition of a neoplasm and on personal experience gained in studying the peculiarities of behavior of rat thyroid tumors.

An encapsulated epithelial neoplasm without evidence of invasive growth or distant metastasis can be considered an adenoma (Figure 1a). A carcinoma in the rat thyroid is an epithelial neoplasm of any histological structure that shows destructive invasive growth, which leads to metastasis to the body (Figure 1b-d). Apart from clear proof of local invasive growth, there is no convincing evidence of malignant growth except the demonstration of metastases. The tendency to overestimate the malignancy of rat thyroid neoplasms and to diagnose them as carcinomas simply on the grounds of their "malignant" appearance is discernable in many publications. The classification given below, was drawn up under consideration of certain data on embryogenesis of this gland, heterogeneity of its epithelial cell population, and the already mentioned peculiarities of normal growth of the thyroid epithelium. However, to make this classification more practicable and comparable with that of WHO for human tumors microscopic morphology rather than histogenesis has been selected as a basis:

Benign tumors:

Follicular adenoma (including microfollicular, polymorphofollicular and trabecular adenoma), papillary adenoma, simple solid adenoma, light-cell solid adenoma, and squamous cell (epidermoid) cystadenoma.

Malignant tumors:

Follicular carcinoma (including microfollicular and polymorphofollicular carcinoma), papillary carcinoma, solid carcinoma (including small cell, polymorphous solid and light-cell solid carcinoma), squamous cell carcinoma, sarcomas and mixed tumors (carcinosarcoma).

This classification does not include such entities as leiomyoma, hemangioma, lymphoma, teratoma, neurogenous tumors, and some other neoplasms that have been observed in humans and several animal species other than the rat. An attempt has also been made to avoid the use of proper names that have already led to some diagnostic confusion (for instance Hurtle cell tumor or Lindsay tumor). Of the characteristic and predominant histological patterns observed in neoplastic epithelial nodules, the three most common ones (follicular, solid and epidermoid) were selected to designate the main categories of tumors. As mentioned earlier, the proliferation of rat thyroid epithelial cells in solid aggregations must be considered a normal feature. The listed tumors are rarely found in their pure morphological form. Most experimental tumors in animals represent virtually different transitional variations in between these artificially separated entities. The vast majority of follicular neoplasms contain solitary or numerous foci of solid cell nests, and it is the rare solid tumor that does not show areas of follicular structure. An introduction of all the subdivisions covering even the most frequent transitional forms of tumors would make this classification useless. An attempt to classify the endless variety of histological pictures produced by physiological shifts in this correlation is hardlyjustifiable.

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