Histopathology and Biologic Behavior of Pleomorphic Cutaneous Mast Cell Tumors in Fifteen Cats

Abstract

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.

Prognostic Value of Histologic and Immunohistochemical Features in Feline Cutaneous Mast Cell Tumors

“…The 20% of MCTs with malignant behavior in this series was similar to that observed in previous studies. 3,4,15,18,19,26,34,36 In contrast to studies in which male cats had higher prevalence of MCTs, 3,22 the male:female ratio in this study favored females, but sex was not correlated with outcome. The reported tendency of young Siamese cats to develop benign atypical cutaneous MCTs 36 was not evident; the only Siamese cat in this study was 15 years old and died with a disseminated mastocytic pleomorphic MCT.…”

“…Some authors reported high mitotic activity in pleomorphic MCTs, 10,11 whereas the mitotic index (MI) was variable but usually low in another study. 15 Pleomorphic MCTs are often more heavily infiltrated by eosinophils. [10][11][12] The atypical feline cutaneous MCT is rare and reported mainly in juvenile to middle-aged Siamese cats, in which it may spontaneously regress.…”

“…3,11 The rate of visceral involvement is usually low, but variable (0 to 22%). 3,4,18,34,36 In most reports, only a small proportion of tumors (3 of 42, 36 4 of 14, 3 0 of 32, 26 1 of 14, 15 2 of 30, 18 6 of 30 19 ) behaved aggressively or led to death or euthanasia of the cat. Disseminated nodules may be a component of generalized (cutaneous and visceral) MCT and have been associated with a worse prognosis.…”

“…19,29 However, cats with multiple cutaneous MCTs do not always develop generalized disease, and cats with solitary tumors may subsequently develop multiple cutaneous tumors without evidence of visceral involvement. 29 The morphologic features typically associated with malignancy, such as pleomorphism and infiltrative growth, have not correlated with clinical behavior in feline cutaneous MCTs, 3,11,15,26 with the exception of mitotic/proliferative activity. 1,15,18 The histologic grading system proposed by Patnaik et al in 1984 30 for canine MCT is not considered prognostically useful in cats.…”

“…29 The morphologic features typically associated with malignancy, such as pleomorphism and infiltrative growth, have not correlated with clinical behavior in feline cutaneous MCTs, 3,11,15,26 with the exception of mitotic/proliferative activity. 1,15,18 The histologic grading system proposed by Patnaik et al in 1984 30 for canine MCT is not considered prognostically useful in cats. 3,26 In dogs, MCTs with aberrant cytoplasmic localization of the KIT receptor tyrosine kinase had a worse prognosis than that of MCTs with a membranous pattern of KIT localization.…”