Nodules aspirated through the skin may be non-neoplastic, neoplastic but benign, or neoplastic and malignant. The following paragraphs describe the cytologic features of some of the most commonly encountered dermal lesions.

Benign Cysts

Inclusion Cysts

Classic dermal inclusion cysts are benign cysts lined by several layers of squamous epithelium and filled by keratinous debris. When aspirated these cysts yield little more than fully keratinized squames.

Occasionally, inclusion cysts rupture, eliciting a marked foreign body inflammatory response in the dermis. These lesions are characterized by a mixed inflammatory response, keratinized squames, and cholesterol crystals cytologically. If overlying ulceration occurs, secondary bacterial infection may also be present.

Sialocele

Sialoceles are cysts formed by obstruction of a salivary duct or gland. They are seen as swellings on the neck or face and may cause secondary abnormalities such as protrusion of the eyeball. Cytologic findings are usually quite consistent and therefore relatively diagnostic. Aspirates are relatively low cellularity and contain a uniform population of large foamy macrophages, many of which contain black pigment granules. Often in the background purple-staining aggregates of inspissated mucus may be visualized.

Benign Epithelial Neoplasms

Sebaceous Adenoma

Cytologically, sebaceous adenomas are composed of small aggregates of large round cells with uniform eccentric round to oval nuclei. Cells are clearly cohesive. Cell cytoplasm is distinctively foamy as a result of the presence of abundant tiny secretory droplets. Sebaceous adenomas may be found anywhere on the body but often are located on the head.

Hepatoid Cell Tumors

These neoplasms are composed of ovoid cells with abundant granular pink cytoplasm and eccentric nuclei. The tumor gets its name because the cells resemble hepatocytes. In truth the tumor arises from modified sebaceous glands. This neoplasm may occur anywhere on the body but most commonly is found at the base of the tail. In this location it is known as the perianal adenoma.

Perianal adenocarcinomas also rarely occur. These are differentiated from the adenomas in that in addition to hepatoid cells there are large numbers of small epithelial cells with scant cytoplasm and a very high nuclear/ cytoplasmic ratio (reserve cells). In addition malignant hepatoid cells may exhibit nuclear criteria of malignancy.

Basal Cell Tumors

Basal cell tumors are borderline malignancies that arise from the innermost layer of the epidermis. They may be locally recurrent and invasive but rarely metastasize. They are generally more aggressive in cats than in dogs.

Cytologically these neoplasms are quite distinctive. They are comprised of cohesive cuboidal to low columnar cells arranged in cords or well-defined rows. Nuclei exhibit few if any criteria of malignancy. Nuclear/cytoplasmic ratio is quite high.

Benign Connective Tissue Tumors

Lipoma

Lipomas are benign fat tumors. When fatty tumors are aspirated, fat droplets may be readily seen on prepared slides. This is true both for benign lipomas and malignant liposarcomas; therefore, all aspirated fatty masses should be evaluated cytologically.

Microscopically, benign lipomas are composed of a honeycomb of large round cells filled by unstained secretory product (fat). Cell membranes are delicate and nuclei are compressed peripherally and may be difficult to recognize. The cells are quite delicate and often rupture during aspiration or slide preparation. As a result slides of lipoma are often low in cellularity.

Fibroma

Fibromas are benign connective tissue tumors. They are difficult to aspirate in vivo because the cells are embedded in a collagenous matrix and difficult to extract. As a result, aspirates of fibromas may be acellular or hypocellular at best. In those cases where cells cannot be aspirated, cytologic collection can be accomplished by scraping a cut surface of the tumor following excision.

Cytologically, fibroma cells are elongated spindle cells with oval nuclei. Cell size, nuclear size, and nuclear/ cytoplasmic ratios are constant, confirming the benign nature of the lesion.

Malignant Epithelial Neoplasms

Squamous Cell Carcinoma

Squamous cell carcinomas are malignant neoplasms arising from the superficial epithelium of the skin. They are often superficially ulcerated which results in secondary superficial bacterial contamination and inflammation. It is therefore important that samples are collected from the center of the mass as well as the surface.

The cells of squamous cell carcinomas are large with angular cytoplasmic margins. The cytoplasm often has a 'glassy' or smooth appearance due to the presence of keratin. Cytoplasmic keratohyalin droplets may also be present. Nuclei are usually round and centrally located and there is often marked chromatin clumping, a feature of malignancy. Other features of malignancy include marked variation in cell size, nuclear size, and nuclear/cytoplasmic ratios. Often nuclei appear too immature for the degree of cytoplasmic keratinization.

Adenocarcinomas

Adenocarcinomas of the skin include apocrine gland adenocarcinomas (sweat gland adenocarcinomas) and mammary adenocarcinomas. Most skin adenocarcinomas are specifically classified as to type primarily on the basis of location.

Adenocarcinomas are formed of rafts and clusters of round cells with basophilic cytoplasm and round nuclei. At least some of the cells contain numerous or single cytoplasmic vacuoles which represent a secretory product. Some cells are so filled with secretion that the nuclei are pushed peripherally; these are termed signet ring cells. Adenocarcinoma cells usually fulfill numerous malignant criteria including variable nuclear size, variable nuclear/cytoplasmic ratios, multiple nucleoli, large irregular nucleoli, and abnormally clumped chromatin.

Malignant Connective Tissue Neoplasms

Hemangiopericytoma

Hemangiopericytomas are spindle cell tumors of low-grade malignancy that appear to arise from the pericytes surrounding small vessels. They tend to be locally recurrent and invasive but do not metastasize until late.

Cytologically, aspirates from hemangiopericytomas are quite cellular for a spindle cell tumor. Tumor cells are plump with wispy cytoplasm and often indistinct cell margins. Nuclei are centrally located and round to oval. Nuclear atypia is generally minimal to mild unless the tumor is particularly aggressive. Because of their close association with blood vessels, distinct capillaries may be prominent in aspirates.

Fibrosarcoma

Fibrosarcomas are generally more cellular than their benign counterpart, the fibroma. Cells are usually larger, more basophilic, and plump. Nuclei are round to oval and exhibit significant malignant criteria.

Liposarcoma

Liposarcoma is a relatively rare spindle cell tumor of the skin. It is a typical spindle cell neoplasm in general morphology and is distinguished primarily by the presence of numerous large unstained vacuoles in the cytoplasm of a number of cells. These vacuoles contain fat.

Melanosarcoma

Melanomas of the skin may be either benign or malignant. Cytologic morphology is less important in determining malignancy than is location of the tumor. Melanomas of the mouth and distal extremities have greater potential for malignancy than those of the trunk.

Cytologically, melanomas are characterized as having a mixture of round cells and spindle cells. The distinguishing feature of these cells is the presence of black cytoplasmic granules. The degree of pigmentation varies from tumor to tumor and even among cells within the same tumor. Degree of pigmentation does not necessarily correlate with degree of malignancy. In most cases of malignant melanoma, nuclei will fulfill criteria of malignancy; however, if the cells are heavily pigmented these criteria may be obscured.

Discrete Cell Tumors

Histiocytoma

Histiocytomas typically occur in young dogs (less than three years old) and are benign. These tumors typically ulcerate and spontaneously regress. Histiocytomas in older dogs should be regarded as potentially malignant as they may progress to a disseminated form (malignant histiocytosis).

The cytologic appearance of histiocytomas in young dogs is variable depending upon their stage. Prior to ulceration, aspirates are relatively hypocellular and contain a uniform round to oval cells with eccentric nuclei and fairly abundant pink to bluish cytoplasm. Nuclei have the appearance of typical macrophage nuclei and generally do not fulfill malignant criteria. Following ulceration, aspirates are more cellular and include a significant inflammatory infiltrate. At this stage the morphology closely resembles that of chronic inflammation only and one must rely on the combination of history, cytology, and gross appearance to make the proper diagnosis.

Cutaneous Lymphosarcoma

Cutaneous lymphosarcoma may be seen as a single nodular lesion (often at the mucocutaneous junctions) or it may present as multiple nodules throughout the skin. With time, the tumor generally disseminates widely throughout the body.

Morphology is that of a typical lymphosarcoma arising in any other site. Aspirates are generally quite cellular and are comprised of a uniform population of round cells with scant cytoplasm and vesiculate nuclei often containing prominent and irregularly shaped nucleolar whorls. Mitoses may be present but are not common in the author's experience.

Transmissible Venereal Tumor (TVT)

TVTs generally occur on the genitalia or around the nose and mouth. These tumors may be either self-limiting and regressive or malignant with significant metastases. The eye is a relatively common site of metastatic disease.

Histologically TVTs are often confused with lymphosarcoma but cytologically they are usually quite distinctive. TVTs are comprised of individual large round cells, much larger than the cells of lymphosarcoma. Nuclei are usually very round and centrally located. Cytoplasm is gray to pale blue and often contains numerous distinctive small vacuoles at the cell periphery. Tumor cells generally exhibit significant variation in nuclear size and nuclear/cytoplasmic ratios. Mitotic index is high and abnormal mitoses may be seen. The tumor aspirates often contain significant numbers of infiltrating lymphocytes, plasma cells, and macrophages.

Plasmacytomas

Cutaneous plasmacytomas are generally benign tumors and may be located either in the skin or mouth. In the mouth they are most common in the gum adjacent to teeth. Aspirates collected from these tumors are usually quite cellular and are comprised of a relatively uniform population of discrete oval to round cells with eccentric round nuclei, pale perinuclear zones and abundant gray-glue to deep blue cytoplasm. Some multinucleated cells may be present. Tumor cells are remarkably reminiscent of normal plasma cells and are usually easily distinguished from the other round cell tumor types.

Mast Cell Tumor

Cutaneous mast cell tumors may arise as single or multiple nodules. Single nodules may be benign but if they recur or additional nodules appear later, they should be regarded as malignant. Multiple nodules are always regarded as potentially malignant. As in the case of melanoma, cytomorphology does not predict potential for malignancy in these tumors.

Aspirates taken from mast cell tumors are generally quite cellular. The predominant cell is the tumor cell but eosinophils may also be abundant and fibroblasts may be present in low numbers. Tumor cells are round to oval with round central nuclei. The diagnostic feature is the presence of variable numbers of deep purple cytoplasmic granules which may be so numerous as to obscure nuclear detail. When quick stains are used, cytoplasmic granules may not be seen. If mast cell tumor is suspected but no granules are present, it may therefore be useful to re-aspirate the mass and stain either with a standard Wright's stain or new methylene blue which will also highlight the granules.