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.
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.
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
Benign epithelial neoplasms
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
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.
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
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.