Adequate samples are essential for optimal cytologic evaluation (see Chapter 6 ). The first step in slide evaluation is to locate an area with adequate cellularity and good quality of cells. The next step is to determine if the specimen is comprised primarily of inflammatory cells, non-inflammatory cells, or a mixture of both. Inflammatory cell populations are further classified according to the types of cells present, since this helps to narrow the list of etiologic possibilities. Non-inflammatory cell populations are categorized as one of the following: (1) normal cells for the given anatomic site, (2) hyperplasia or benign neoplasia (these often cannot be cytologically differentiated from one another), or (3) malignant neoplasia. Mixed cell responses are quite common and present as inflammatory cells mixed with normal, hyperplastic, or neoplastic cells. When cells are suspected of being neoplastic, the next step is to determine the general cellular category: epithelial cells, mesenchymal cells, or one of the discrete round cell neoplasms. Epithelial cells are round to polygonal and have a tendency to arrange in cohesive clusters. Mesenchymal cells often have one or more cytoplasmic tails. Those with one cytoplasmic tail may be described as flame shaped, those with two as spindle shaped, and those with more than two as stellate shaped. In some mesenchymal neoplasms, such as the lipoma, the cells may be round to oval. Mesenchymal cells tend to appear as single forms, but aggregates of cells may be present. The discrete round cell neoplasms, which are technically of mesenchymal origin, are usually placed into a separate category because they are made up of round cells that do not form cohesive clusters. Melanomas are of neuroectodermal origin and do not fit well into the above morphologic classification system, since their cells may exhibit round, epithelial and mesenchymal characteristics. The key to diagnosing a melanoma is to find melanin granules in the cytoplasm of the tumor cells.

Cytologic criteria of malignancy are primarily used to determine if an epithelial or mesenchymal population is abnormal enough to be considered malignant ( Box 7-1 ). These criteria often relate to the variability in the cell population and are less useful for monomorphic round cell neoplasms. General criteria of malignancy include anisocytosis (variable cell size in a population that would normally consist of cells of similar size), higher cellularity than expected, and presence of cells in an abnormal location. Nuclear criteria of malignancy include variable nuclear size (anisokaryosis), increased nuclear to cytoplasmic volume ratios (N:C ratios), variable N:C ratios in a cell population not normally expected to exhibit this feature, multiple nucleoli, variable size of nucleoli, presence of abnormally shaped nucleoli, coarse and irregular chromatin clumping, thickening of the nuclear margin, large numbers of mitotic figures, mitotic figures of abnormal configuration, presence of multiple nuclei, and nuclear molding. Nuclear molding is characterized by the wrapping of a nucleus from one cell around an adjacent cell, or the wrapping of one nucleus around another in a multi-nucleated cell. Cytoplasmic criteria of malignancy are the least useful and include increased cytoplasmic basophilia and vacuolation. As a rule, at least four general or nuclear criteria of malignancy should be observed before assigning the term malignant , although there are exceptions to this rule. One can be most confident in assessing malignancy when many criteria are present, or when one or more very strong criteria are present. The presence of cells in an abnormal location, such as sheets of epithelial cells within a lymph node aspirate, is such a strong criterion of malignancy that it can be used to make the assessment of malignancy even when few or no other criteria are present.