Most feline cutaneous mast cell tumors (CMCT) are behaviorally benign; however, there is a subset of these tumors with, marked pleomorphism (previously termed poorly differentiated) that have been reported to be more aggressive. In this study, pleomorphic CMCT from 15 cats were identified from surgical biopsy submissions, and follow-up clinical data were obtained for 14 of these cats. Pleomorphic CMCT were discrete dermal nodules composed of sheets of pleomorphic round cells. Tumors from all 15 cats contained markedly cytomegalic and karyomegalic cells; 9/15 tumors (60%) contained multinucleated tumor giant cells. Typical mast cell granules were easily identified in sections stained with hematoxylin and eosin and with metachromatic stains and based on ultrastructural evaluation in cytomegalic as well as smaller tumor cells, indicating that the tumors were not poorly differentiated. The mitotic rate was very low (<1 mitosis per 10 high-power fields [hpf]) in 14 of 15 tumors (93%). Affected cats were 6-19 years old (mean age = 11.5 years), and there was no breed or sex predilection. Two cats had local recurrence. The only cat that had a pleomorphic CMCT with a high mitotic rate (1-2 mitoses/hpf) subsequently developed numerous other dermal neoplasms and was euthanatized. In this study, the large majority of feline pleomorphic CMCT were behaviorally benign. Mitotic rate is likely an important prognostic indicator of CMCT behavior.

Abstract. Teratomas are histologically complex neoplasms that are composed of structures derived from multiple germ cell layers (ectoderm, mesoderm, and endoderm). These neoplasms are uncommon in domestic animals and are usually found in the gonads. This paper describes teratomas of the adrenal gland in four domestic ferrets (Mustela putorius furo). Three of four of the neoplasms contained tissues from ectodermal, mesodermal, and endodermal germ cell layers; two of four contained rudimentary teeth. In one case, malignant epithelial cells had metastasized to local lymph nodes. Teratomas, although uncommon, should be included in the differential diagnosis for adrenal neoplasms in domestic ferrets.