Canine Mast Cell Tumor

Mast cell tumors (MCT) are the most common dermal malignancy in dogs. You will commonly encounter them in small animal clinical practice and should be prepared to answer a few questions about this common tumor.

Key Points

• Predisposed breeds: boxers, pugs, Boston terriers, other brachycephalic breeds
  • These breeds frequently develop multiple (occasionally many) MCTs over their lifetime, but are usually associated with lower grade tumors that are less aggressive in metastasizing
• Mast cell granules release histamine, heparin, proteases, and cytokines when they degranulate
  • May cause GI ulcers, bleeding, poor wound healing, anaphylactoid reactions (vasodilation, hypotension, collapse, etc)
• Most common grading system for MCTs used in the US is the Patnaik system
  • Evaluates cell differentiation, mitotic figures, and invasiveness in surrounding tissues
  • Grade predicts likelihood for recurrence and metastasis where a grade 3 is most aggressive, 2 is intermediate, and 1 is least aggressive
  • Only MCTs arising from skin are graded
• Considered the “great pretender” as they can look and feel like anything from a skin plaque, nodule, rash, or lipoma

Diagnostics

• Fine needle aspirate cytology

  • Usually effective and the least invasive way of attaining a diagnosis
  • Grade can NOT be determined by cytology
  • Eosinophils are commonly abundant in samples
    

    

• Biopsy

  • Occasionally required for diagnosis if FNA is non diagnostic
  • Required for grading

• Fine needle aspirate or biopsy of regional draining lymph node

• Abdominal ultrasound and thoracic radiographs

  • If clinically indicated for aggressive MCTs with high risk for metastasis
  • To evaluate abdominal lymph nodes, liver, spleen, and thoracic lymph nodes, respectively

• Buffy coat analysis or bone marrow aspirate

  • Infrequently performed with low yield

Treatment

Surgery

• Excision with 2-3 cm lateral margins and 1 fascial plane deep
• Treatment of choice if metastasis not already present

Radiation therapy

• Adjuvant treatment to surgery if complete margins cannot be obtained

Chemotherapy

• Vinblastine, CCNU or other alkylating agents
• Indicated if metastasis present, at high risk for developing metastasis or recurrence, or if surgery or radiation are not options

Supportive care

• Glucocorticoids (prednisone)
  • Cytotoxic to mast cells
  • Stabilizes mast cell membranes
  • Reduces inflammation associated with tumor
• H1 blocker – diphenhydramine
• H2 blocker – famotidine, ranitidine, etc

Tyrosine kinase inhibitor

• Toceranib (Palladia)
  • First FDA-approved drug for canine cancer in the United States
  • Inhibits aberrant cell signaling pathways found in MCTs (KIT, VEGFR 2, PDGFR beta)
• Others to come to market soon

Prognosis           

Prognostic factors

• Grade
  • Grade 1: least likely to metastasize or recur (<10%)
  • Grade 2: up to 80% of all MCTs will be grade 2 (<25% will metastasize)
  • Grade 3: most aggressive with metastasis rates between 50-90%, and survival times between 6 months to 3 years
• Clinical stage
  • Evidence of lymph node or distant metastasis associated with worse prognosis
• Mitotic index
  • >5 mitoses/ 10 high power fields associated with shorter survival
• Other proliferation markers
  • Ki-67, PCNA, AgNOR
• Tumor location
  • Viscera – poor prognosis
  • Mucosa or mucocutaneous junctions – more aggressive with higher risk of metastasis
• Tumor size
• Rate of growth
• Clinical signs
  • Systemic signs such as anorexia, vomiting, diarrhea due to the tumor associated with a poor prognosis
• Local recurrence after resection
• Breed
  • Boxers, pugs, and possibly other brachycephalic breeds associated with low to intermediate grade tumors
• KIT mutation
  • Presence of mutation associated with worse prognosis