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

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 N/C 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.