Canine mast cell tumors

Mast cell tumors (MCTs) are the most common malignant skin cancer in dogs, and significant variability exists in their biological behavior. Most MCTs are cured with appropriatelocal therapy, but a subset shows malignant behavior with the potential to spread to lymph nodes, liver, spleen, and other areas and to thus become a systemic cancer. Because of this variable behavior, it is difficult to predict how any individual tumor is going to behave. The variability thus creates uncertainty in deciding what a particular dog’s prognosis is, whether staging tests to assess for metastasis are needed, and even what treatments will be necessary for best outcome. In addition to controversies over the potential for development of systemic disease, or diffuse metastasis, controversies also exist over what treatment is needed to best attain local control of these tumors. This article will briefly discuss the diagnosis of MCTs in dogs and will summarize the literature in regards to the controversial topics surrounding the more aggressive form of this disease, with recommendations made based on published studies.
Keywords: mitotic index, mastocytosis, tyrosine kinase inhibitor, histologic grade

Prognostic factors relating to history and physical examination

Some factors that can be obtained from a history and physical examination that are generally accepted to carry a more guarded prognosis in dogs with MCTs include recent, rapid tumor growth and fixed, ulcerated tumors.6–8 Although publications regarding these features are limited, one early study reported doubling of the survival percentage at 30 weeks post-MCT excision for dogs with slow-growing tumors versus (vs) those with more rapidly growing masses.

Biologically, both the ability to grow quickly and to become fixed to deeper tissues are physical manifestations of more aggressive behavior. Tumor location on the body can also be associated with biologic behavior; this topic is more controversial and the pertinent locations and published papers are highlighted as follows. Mucocutaneous location
In limited published cases, eyelid margin MCTs appeared to have relatively benign behavior and were effectively treated with local therapy, although one dog was reported to have regional lymph node (LN) metastasis.9–11 MCT of the conjunctiva may be of concern only locally, without reported metastasis in three dogs.12,13 In a paper evaluating chemotherapy for high-risk MCT patients, eleven dogs with mucous membrane MCTs (vulva, prepuce, conjunctiva, oral cavity) had significantly shorter median survival times (MST) than 50 dogs with MCTs of haired skin.14 However, a recent paper of 32 dogs with 33 conjunctival MCTs treated with surgery alone showed prolonged survival times, with only two dogs having local recurrence despite incomplete margins in 25 cases, and no dogs dying of mast cell-related disease. Early case reports described aggressive behavior and local metastatic disease at diagnosis in two dogs with MCT of the lip; survival times were 6 months or less.16,17 Of five dogs with MCT of the tongue, two presented with LN and/or systemic metastasis, and two of the remaining three had postoperative local recurrence leading to euthanasia. Larger, more recent studies confirmed that MCTs on the muzzle, perioral mucocutaneous junction, or oral mucosa have a more aggressive biologic behavior, with increased risk of locoregional LN metastasis.19–21 The rate of documented metastasis to local (mandibular) LNs was 55%–59%, compared with a ,10% rate for other cutaneous sites.

Despite a high rate of metastasis, the MSTs of the dogs were prolonged at 30 months, 52 months, and median not reached. Treatments varied in these cases, with many dogs receiving surgery, radiation, and chemotherapy. Dogs with LN metastasis had significantly shorter MSTs than dogs without nodal metastasis, with medians of 14 months, less than 20 months, and 9 months.