Mast Cell Neoplasms
C. Guillermo Couto
Alan S. Hammer
Mast cells are considered to be derived from the myelomonocytic precursors in the bone marrow. Therefore, neoplasms affecting these cells are discussed in this section.
Mast cell neoplasms are uncommon in the cat. These neoplasms represent 2 to 3 percent of all cutaneous tumors in this species, while they comprise 20 to 25 percent of all cutaneous tumors in the dog. Unfortunately, there are no epidemiologic data concerning overall prevalence or incidence of this feline neoplasm. Most cats with mast cell tumors are middle-aged or older (median 10 years old; range 2 to 18 years old), there is apparently no sex predilection, and, according to one report, most are FeLV negative.
As opposed to the dog, in which most mast cell tumors are cutaneous/subcutaneous, feline mast cell tumors occur in two main forms: visceral and cutaneous. It is controversial as to whether cutaneous forms are more common than visceral forms and whether both forms coexist in the same patient. Different publications report that the cutaneous form is more common, while other investigators report that visceral forms predominate. A review of 58 published cases by Garner and Lingemann revealed that hemolymphatic organs were involved in 71 percent of cats, and that simultaneous cutaneous and systemic involvement were detected in 44 percent. Nielsen et al. also observed that 50 percent of the cats evaluated had coexistence of the cutaneous and systemic forms, while in Wilcock's series of 85 cases, there is no documentation of coexistence of both anatomic forms. A single case report documented the progression from cutaneous to visceral form, despite corticosteroid therapy. This difference in biologic behavior may reflect geographic variations, such as those observed in cats with lymphoma.
Visceral mast cell neoplasms are characterized by either hemolymphatic or intestinal involvement. Cats with hemolymphatic involvement are classified as having systemic mast cell disease (or mast cell leukemia), since bone marrow, spleen, liver, and blood are commonly involved. Most cats present because of nonspecific signs, such as anorexia and vomiting; however, abdominal distention due to splenomegaly is a consistent feature. Hematologic abnormalities in cats with systemic mast cell disease are extremely variable and include cytopenias and presence of circulating mast cells or basophilic; however, a high percentage of cats may have normal CBCs. Cats with the intestinal form usually present for evaluation of GI signs, such as anorexia, vomiting, or diarrhea. In a series of 24 cats with intestinal mast cell neoplasms, abdominal masses were palpated in approximately 50 percent of cases. Most tumors (88 percent) involved the small intestine, and 67 percent of cats had metastatic disease affecting the mesenteric lymph nodes, liver, spleen, and lungs on first presentation. None of the cats evaluated had circulating mast cells. Gastrointestinal ulceration, a feature of canine and human patients with mast cell tumors, bas also been documented in the cat.
Cutaneous mast cell neoplasms usually present as multiple cutaneous masses, affecting primarily the head and neck . Wilcock et al reviewed the clinical and pathologic findings in 85 cats with cutaneous mast cell neoplasms. According to the clinical, epidemiologic, and histologic features, cats were classified in either of two groups: mast celltype mastocytoma and histiocytic-type mastocytoma. Cats with the mast cell type were usually over 4 years of age and had solitary dermal masses; there was no apparent breed predilection in this group, which comprised 77 percent of the cases. Cats with the histiocytic type were primarily Siamese cats under 4 years of age, with multiple (miliary) subcutaneous masses characterized by a benign biologic behavior; this histologic subtype represented 21 percent of cases; 4 of the 18 tumors regressed spontaneously.
Therapy for cats with mast cell neoplasms is controversial. Most cats reported in the literature have not been treated. In a series of seven cats with systemic mast cell disease treated by splenectomy, the median survival was 11 months (range 0 to 34 months postoperatively). Two cats with systemic mast cell disease treated by splenectomy alone survived for 2.5 and 7 months after surgery, respectively while another cat treated with prednisone alone survived approximately 3 months. A cat with multiple cutaneous mast cell neoplasms treated with prednisone and cimetidine, an Hz antihistamine supposed to prevent GI ulceration and act as a nonspecific immunomodulator, died of systemic mast cell disease 2 months following initial diagnosis.
Despite inconsistent results in cats with mast cell neoplasms, we recommend the following approaches: (1) cats with solitary mast cell neoplasms should be treated by surgical excision; if the excision is incomplete, or if the mass is in an area in which surgery would result in significant complications (e.g., eyelid), radiotherapy (30 to 50 Gy) should follow or replace surgical excision; (2) cats with multiple cutaneous mast cell neoplasms should be treated by surgical excision of the masses (if feasible), followed by systemic administration of prednisone (40 mg/m2 PO g24h for 1 week, then 20 mg/m2 PO q48h); and (3) cats with systemic mast cell disease and splenic involvement should be splenectomized and treated with adjuvant prednisone as above.
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