| To distinguish benign from malignant tumors, the variation in certain characteristics is assessed within the cell population. As a general rule, the more malignant the cell, the less differentiated it becomes and the more variation there is within the cell population. Benign tumors have cells that are often uniform in size, with a uniform nucleus:cytoplasm ratio, and strongly resemble the cell of origin. Nuclear criteria are the major indicators of malignancy. Criteria for
malignancy include variation in cell size and shape, increased cell exfoliation, increased nuclear size, increased nucleus:cytoplasm ratio, variation in nuclear size and an increase in multinucleated cells, increased mitosis with abnormal mitotic figures, a coarse and often clumped chromatin pattern, altered shape of the nucleus due to close approximation of a nucleus from an adjacent cell (nuclear molding), and large (often multiple) nucleoli of irregular and abnormal shape. There
are several exceptions, however. Thyroid carcinomas usually have fairly uniform, well differentiated cells. A diagnosis of thyroid carcinoma is usually made on size and capsular and soft tissue invasion, features that are only identifiable histologically. Other tissues in which it may be impossible to differentiate benign from malignant cells include apocrine gland carcinomas, basal cell tumors, melanomas, and proliferative hepatic lesions. Some normal structures, such as hepatoid
glands, have more than one cell type (ie, reserve and terminal cells) and can exhibit variation in size or morphology even when benign. Many mammary tumors show marked variation in cell morphology but histologically are classed as benign. These exceptions often make cytologic interpretation unreliable, particularly for the tissues listed. |
| Lymphoma is most commonly characterized by a uniform population of larger than normal lymphoid cells. Variation in morphology, therefore, is not necessary for a diagnosis of malignancy with this tissue type. Most spindle cell tumors do not metastasize but are locally aggressive and often difficult to remove. The criteria for malignancy are often less important with respect to the behavior of these tumors than for epithelial tumors. |
