In this review, the authors discuss the clinical challenges in the diagnosis and treatment of feline mast cell disease and provide an overview of current treatment options.

Overview
Mast cell tumors (MCTs) are the most common splenic tumor in cats and the second most common skin tumor. About 15% to 26% of splenic disease in cats may be attributable to mast cell disease. Cutaneous MCTs are histologically classified into either mastocytic or atypical and often develop on the head, neck, and trunk. The atypical form accounts for 10% to 20% of feline MCTs and typically affects cats more than 4 years of age. These lesions will spontaneously regress over time, usually within 24 months. The mastocytic form of the disease is more common and is further classified into well-differentiated and poorly differentiated forms.

Diagnosis
Cytologic evaluation of fine-needle aspirates is usually all that is needed to diagnose MCT. For aspirates of visceral MCT (spleen or intestine), pretreatment with antihistamines is recommended because of concerns about mast cell degranulation and anaphylaxis.

Once MCT is diagnosed, staging should be performed in cats with either visceral MCT or cutaneous MCT. Staging includes a complete blood count (CBC) with buffy coat evaluation, serum chemistry profile, coagulation profile, and evaluation of a bone marrow aspirate. Abdominal and thoracic imaging may be helpful in identifying pleural effusion, organomegaly, lymphadenopathy, or the presence of concurrent disease. The authors note that cats with a single cutaneous nodule and no evidence of lymph node involvement may not require full staging, but they still recommend a CBC with buffy coat evaluation at a minimum with further staging if warranted.

Treatment
Histamine (H₁ and H₂) blockers and antihistamines such as diphenhydramine can be started at the time of cytologic confirmation. Surgical excision should be attempted whenever possible. Unlike in dogs, however, complete surgical excision of cutaneous MCT in cats does not decrease the risk of recurrence. Resection of intestinal forms of MCT has a poor postsurgical prognosis.

Specimens should be submitted for histopathologic evaluation and assessment of surgical margins. But keep in mind that, unlike the grading scheme used in dogs, there is no currently accepted grading scheme for feline MCTs that is reliably prognostic.

Irradiation with strontium 90 may be considered as an alternative to surgery in cats with single or multiple cutaneous MCTs and no evidence of metastasis. Chemotherapy with lomustine has shown some promise but requires close monitoring for bone marrow or pulmonary toxicity.

Novel agents, such as receptor tyrosine kinase inhibitors, have recently begun to emerge as potential new therapies for MCTs. Tyrosine kinases are growth factors that can result in abnormal cell proliferation when mutated. While further studies regarding their use in cats are warranted, preliminary data show favorable response.

Prognosis
Cats with solitary cutaneous MCTs have a good prognosis with low recurrence and metastatic rates. Conversely, cats with mastocytic poorly differentiated MCTs with a high mitotic rate are at greater risk of metastasis, and wider surgical margins should be considered. The authors caution that cats with multiple cutaneous tumors may actually have metastases from a visceral tumor, so careful evaluation is required. For cats with visceral MCT, the prognosis depends on location—cats with splenic MCT have a good prognosis with splenectomy while cats with intestinal involvement have a poor prognosis.

Henry C, Herrera C. Mast cell tumors in cats: Clinical update and possible new treatment avenues. J Feline Med Surg 2013;15(1):41-47.

Link to abstract: http://jfm.sagepub.com/content/15/1/41.abstract