Significance of Surgical Margins For Low Grade Mast Cell Tumors…New Insights

By Chanran K. Ganta, BVSc, PhD, DACVP, Kansas State Veterinary Diagnostic Laboratory

Canine cutaneous mast cell tumors (MCTs) are one of the most commonly diagnosed cutaneous malignant neoplasms in dogs. Approximately 90% of all canine MCTs are low grade according to the recent 2-tier grading system. Surgical excision remains the treatment of choice for low grade MCTs. Acquiring tumor free surgical margins can be challenging due to the location of the mass, the extent of local invasiveness, and the difficulty in distinguishing between inflammatory and neoplastic mast cells histologically. In addition, subcutaneous MCTs often extend to the surgical margins.Graph

The mitotic activity of MCTs is frequently measured by microscopic examination; however, the true proliferative nature of the tumor is better determined by measuring the proliferative index. Proliferative index is the product of individual scores of Ki67 and AgNOR tests (proliferative index = Ki67 x AgNOR). Ki67 is a nuclear staining proliferation marker and AgNOR is a growth rate indicator. Recent studies have shown that 60% of dogs with proliferative index scores of greater than 54 died due to MCT associated disease within 12 months of the first tumor detection. Preliminary analysis of MCTs diagnosed at KSVDL showed that 100% of high grade and approximately 20% of low grade MCTs had proliferative index scores of 54 or above.

Two recent studies showed that marginally excised low grade MCTs that had proliferative index scores of less than 54 had recurrence rates similar to that of low grade MCTs that were completely excised with tumor free margins. Thus, the proliferative index analysis can provide valuable information on low grade MCTs to improve case management.

As shown in the figure, the proliferative index score could help determine if additional treatment should be considered in a marginally excised low grade mast cell tumor. Even if surgical margins are not tumor free, a low proliferative index score (<54) indicates that no additional treatment is required. If the proliferative index score is high (>54), additional treatment is required.

KSVDL offers both the Ki67 ($29.00) and the AgNOR ($25.75) tests.

References available. Contact author.

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