Atypical poorly granulated mast cell tumor in a cat
Meegan Larson
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2025;29(3):22.
A fourteen-year-old female spayed DSH cat presented for a firm freely movable cutaneous swelling on the face that had been growing gradually over several months. Incisional punch biopsies were taken and microscopic examination with routine hematoxylin and eosin staining revealed a neoplasm of uncertain cell type. Based on the tumor morphology, the primary differential diagnosis was atypical poorly granulated mast cell tumor; other differentials included histiocytic sarcoma and progressive histiocytosis (Fig. 1). Toluidine blue, a special/histochemical stain was done on the biopsy sections and highlighted variable numbers of cytoplasmic granules in a significant proportion of neoplastic cells confirming the diagnosis of atypical poorly granulated mast cell tumor (Fig. 2). The low mitotic rate of 2 mitotic figures in 2.37 mm2 was predictive of benign behavior and good response to treatment with surgical excision.
Approximately 20% of cutaneous neoplasms in cats are cutaneous mast cell tumors. The head, neck and trunk are common sites. Three histologic subtypes are recognized: well-differentiated, pleomorphic and atypical poorly granulated. Unlike canine cutaneous mast cell tumors, a well-tested and widely accepted grading scheme does not exist for prognostication. Most feline cutaneous mast cell tumors of all histologic subtypes are benign and can be cured by complete surgical excision; however, a small proportion can spread to local lymph nodes or viscera, or may be associated with disseminated cutaneous disease. Mitotic rate is the best histologic criteria for differentiation between benign and potentially aggressive feline cutaneous mast cell tumors. Atypical poorly granulated mast cell tumors are the least common subtype, the most difficult to diagnose histologically due to the sparse cytoplasmic granularity of the neoplastic cells, and have been reported to regress spontaneously in some cases.
Definitive diagnosis of neoplasms can sometimes be difficult with hematoxylin and eosin staining alone, even with provision of relevant clinical histories and good gross descriptions. Toluidine blue is a dye with an affinity for the cytoplasmic granules in mast cells and is frequently used in cases of suspected mast cell neoplasia in all species to confirm or rule out the diagnosis. If toluidine blue staining results are inconclusive, immunohistochemical staining using an antibody to cKIT (CD117), a protein expressed by mast cells, can be helpful for diagnosis. In this case, toluidine blue staining results were diagnostic.
Figure 1. This cutaneous neoplasm is composed of sheets of round to spindle-shaped cells. Cytoplasmic granules are not visible in the neoplastic cells with hematoxylin and eosin staining. H&E stain.
Figure 2. Few to many red-purple cytoplasmic granules are appreciated in most neoplastic cells with toluidine blue staining. TB stain.
References
1 Bettini G, Sabattini S. Prognostic value of histologic and immunohistochemical features in feline cutaneous mast cell tumors. Vet Pathol 2010;47(4):643-653.
2. Herrera C, Henry C. Mast cell tumors in cats: Clinical update and possible new treatment avenues. Journal of Feline Med and Surg 2013;15:41-47.