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 carcinoma 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 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
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.
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 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.
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
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.