Mast Cell Tumors in Dogs: Grading, Staging, and Treatment Decision-Making

MCTs typically affect middle-aged to older dogs (mean age 8-9 years), though they can occur at any age. Brachycephalic breeds are overrepresented, particularly Boxers, Boston Terriers, Pugs, and Bulldogs. Interestingly, Boxers tend to develop lower-grade MCTs with better prognosis. Labrador Retrievers, Golden Retrievers, Shar-Peis, and Weimaraners are also predisposed, with Shar-Peis developing poorly differentiated MCTs at younger ages.

MCTs arise from transformed mast cells in the dermis and subcutis. Mutations in the c-KIT proto-oncogene (encoding the receptor tyrosine kinase KIT) are found in approximately 30% of canine MCTs and are associated with higher-grade tumors and aberrant KIT localization patterns. This has therapeutic implications for tyrosine kinase inhibitor therapy.

Two histologic grading systems are in common use. Understanding both is essential, as pathology reports may use either system.

The Kiupel system was developed to address the poor interobserver agreement for Patnaik grade II tumors. High grade is assigned based on any of: ≥7 mitotic figures per 10 HPF, ≥3 multinucleated cells per 10 HPF, ≥3 bizarre nuclei per 10 HPF, or karyomegaly. Most pathology labs now report both systems.

Staging determines the extent of disease and informs treatment decisions. The recommended staging workup for MCTs includes:

Regional lymph node aspirate: Even if the lymph node appears normal, fine-needle aspiration is recommended for all MCTs. Metastatic mast cells can be present in normal-sized nodes. Cytologic criteria for metastasis include: mast cells in clusters or aggregates, mast cells comprising >3% of nucleated cells, and poorly granulated mast cells.

Buffy coat smear: Circulating mast cells suggest disseminated disease, though low numbers of mast cells may be found in blood from dogs without MCTs.

Abdominal ultrasound: Evaluate liver, spleen, and abdominal lymph nodes. Hypoechoic nodules in liver and spleen warrant aspiration.

Hepatic and splenic aspirates: Recommended for high-grade tumors, large tumors, tumors in high-risk locations, or when ultrasound changes are present.

Surgery is the primary treatment for localized MCTs. The current recommendation is 3 cm lateral margins and one fascial plane deep for all MCTs where anatomically feasible. Studies demonstrate that 2 cm lateral margins are adequate for Kiupel low-grade tumors with complete histopathologic excision rates exceeding 90%.

Histopathologic margin assessment is critical. Dirty (incomplete) margins indicate tumor cells at the surgical border and carry a high risk of local recurrence. Options include: revision surgery (preferred if feasible), radiation therapy (excellent local control for microscopic disease), or close monitoring with adjuvant chemotherapy for high-grade tumors.

Tumor location matters for prognosis. MCTs in certain locations (inguinal, preputial, perineal, mucocutaneous junctions, nail bed, and oral mucosa) are associated with more aggressive behavior regardless of histologic grade.

Radiation therapy: Highly effective for microscopic residual disease. Local control rates exceed 90% for incompletely excised low-to-intermediate grade MCTs treated with a full-course radiation protocol (15-19 fractions).

Chemotherapy: Vinblastine/prednisone is the most commonly used protocol for MCTs with regional or distant metastasis, high-grade histology, or as adjuvant therapy after incomplete excision. A typical protocol consists of vinblastine (2 mg/m² IV) every 2 weeks for 4-6 treatments with concurrent prednisone (2 mg/kg PO daily, tapering).

Tyrosine kinase inhibitors: Toceranib phosphate (Palladia) and masitinib (Masivet) target the KIT receptor. Toceranib achieves response rates of approximately 40% in MCTs and is particularly useful for unresectable, recurrent, or metastatic disease. c-KIT mutation status predicts response but is not required for therapy.