Treatment of High-Risk Mast Cell Tumors in Dogs

In the Literature

Miller RL, Van Lelyveld S, Warland J, Dobson JM, Foale RD. A retrospective review of treatment and response of high-risk mast cell tumours in dogs. Vet Comp Oncol. 2016;14(4):361-370.

The Research …

Mast cell tumors (MCTs) are among the most commonly treated malignancies in practice.^1,2 Histopathologic Patnaik grading is the most reliable prognostic indicator.³ In this system, Grade I tumors are indolent with low metastatic potential, whereas Grade III tumors are highly metastatic and locally aggressive.^4,5 Grade II tumors, which represent the majority, have presented a challenge because of their variable behavior and a large interobserver disparity among pathologists.^6,7 Subsequently, a 2-tier grading system and proliferation indices (Ki67 and mitotic index) are being incorporated individually or as part of a prognostic panel to facilitate prognostication and treatment planning.^7,8 

Few studies have evaluated prognostic factors in dogs with stage IV disease (distant metastasis) or at high risk for metastasis, and there is no randomized study evaluating systemic treatments in these groups. This study sought to better define these groups for prognostic factors, outcome with common systemic treatments, and survival advantage in dogs treated with surgery and systemic therapy vs systemic therapy alone. 

Dogs undergoing systemic therapy without gross disease had significantly better survival time as compared with those with gross disease (MST, 462 vs 150 days). Of note, dogs with metastatic disease that underwent surgical removal of only the primary tumor along with systemic therapy had a significant survival advantage vs those that did not undergo surgery (MST, 278 vs 91 days), regardless of completeness of margins and clinical stage. Dogs with Grade II nonmetastatic disease showed a survival advantage when receiving vinblastine and prednisone as compared with those receiving masitinib (MST, 1946 vs 369 days).

A more recent study supported these findings.⁹ Multivariate analysis of dogs with Stage IV disease found a measurable primary tumor at time of diagnosis to be negatively associated with progression-free interval (median, 21 vs 125 days) and overall survival (MST, 93 vs 180 days); dogs receiving local (surgery and/or radiation therapy) and systemic treatment had better outcomes.⁹ These results suggest that surgical resection of the primary MCT followed by systemic therapy offers a significant survival advantage, regardless of metastasis, as compared with dogs receiving only systemic therapy.

… The Takeaways

Key pearls to put into practice:

• Histologic reports, especially of Grade II MCTs, must be critically evaluated.

• If clinical presentation is not in agreement with the assigned grade, histologic proliferation indices (eg, Ki67 and mitotic index, MCT prognostic panel) should be requested.

• In addition to systemic therapy, primary tumor control should be carefully considered even in dogs presenting with advanced Stage IV disease, as some will benefit if both are pursued.