Canine mast cell tumors: margins, markers & prognostic factors

General information

Mast cell tumors (MCT’s) are the most common tumor in the dog and the second most common tumor in the cat. MCT’s are primarily a disease of older dogs and cats, however, extremely young dogs and cats have been reported to have MCT’s. Canine breeds reported to be at increased risk for MCT’s are boxers, Boston terriers, Labrador retrievers, terriers and beagles. The only feline breed that has been reported to be at increased risk for MCT’s are Siamese. Most reports show no significant gender predilection for MCT’s in dogs or cats. The etiology of MCT’s is presently unknown. Many have suspected a viral etiology due to MCT transplantability to susceptible laboratory dogs (extremely young or immunocompromised) with tumor cells and cell-free extracts. Recent evidence shows that a significant percentage of dogs with higher-grade MCT’s have genetic mutations in c-kit (stem cell factor receptor) which may be responsible for the genesis and/or progression of MCT’s in dogs. Not all dogs with MCT’s have c-kit mutations, suggesting that they are not the only mechanisms for the development and/ or progression of MCT’s.

Eighty-five to ninety percent of dogs and cats with MCT’s have solitary lesions. It is important to note that not all dogs or cats with multiple MCT’s have metastatic or systemic mastocytosis. Studies suggest that well-differentiated MCT’s are slow-growing, usually < 3-4 cm in diameter, without ulceration of overlying skin, variably alopecic and commonly are present for more than 6 months. In contrast, poorly differentiated MCT’s are rapidly growing, variably sized (but generally large), with ulceration of the underlying skin and inflammation/edema of surrounding tissues and lastly rarely are present for more than 2-3 months before presentation. Since most MCT’s are of moderate-differentiation, signs may be somewhere between these two extremes.

History & Clinical signs

The history and clinical signs of dogs and cats with MCT’s can be extremely variable. Most do not show any clinical signs referable to their MCT, however, some may have signs referable to the release of heparin, histamine and/or other vasoactive amines. 
Mechanical manipulation or extreme changes in temperature can lead to degranulation of MCT’s and subsequent erythema/wheal formation (Darier’s sign) and gastrointestinal ulceration (anorexia, vomiting, melena, etc.).

Diagnosis & Staging

Fine needle aspiration and cytology (FNAC) is the mainstay for diagnosis of MCT prior to surgical removal. Mast cells of MCT’s have a characteristic discrete cell cytological appearance with eccentrically placed nuclei and abundant red to purple (ie metachromatic) cytoplasmic granules. Occasional MCT’s, predominately undifferentiated MCT’s, do not have the classic metachromatic cytoplasmic granules and must be diagnosed via other means (histopathology, special stains, etc.). Once a diagnosis is obtained, staging (looking for disease elsewhere) is routinely recommended, however, the completeness of staging is presently extremely controversial. After an FNAC diagnosis of MCT has been made, this author recommends routine staging diagnostics (full physical examination, bloodwork/urinalysis, FNAC of any local lymph nodes and abdominal ultrasound) but studies show ultrasound to be a low yield diagnostic test. Additional diagnostics such as thoracic radiography and bone marrow aspiration/cytology may be employed, especially in dogs with prior MCT’s and/ or a strong clinical suspicion for metastasis.

The use of buffy coat cytology and liver/spleen FNAC is presently controversial in the routine staging of dogs with MCT and this author does not routinely employ these diagnostics for staging of MCT’s in dogs. Some oncologists have begun to either not routinely utilize bone marrow aspiration & cytology (BMAC) for MCT staging, or have begun to utilize results of CBC/plt to delineate whether or not to perform a BMAC. This is incredibly controversial and results of a recent publication concerning incidence and risk factors of bone marrow infiltration for canine MCT will be presented at the lecture.

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