Page 1357 - Small Animal Internal Medicine, 6th Edition

CHAPTER 81 Selected Neoplasms in Dogs and Cats 1329

Given the fact that mast cells produce a variety of bioac- More recently, a group of pathologists proposed establish-
tive (mainly vasoactive) substances, dogs with MCTs may be ing a two-tier grading system for canine MCTs, using a low-
VetBooks.ir evaluated because of diffuse swelling (i.e., edema and inflam- grade and high-grade designation (Kiupel et al., 2011).
A group of 28 pathologists from 16 institutions evaluated
mation around a primary tumor or its metastatic lesion),
erythema, or bruising of the affected area. These episodes
Interestingly, when grading the tumors using the original
may be acute, and they may occur during or shortly after 95 MCTs from dogs treated by surgical resection alone.
exercise or exposure to cold weather. Percutaneous FNA of three-tier system, concordance among pathologists was
an unexplained subcutaneous swelling in dogs should always approximately 60% to 65% for grades 1 and 2 tumors and
be performed as part of the workup. 75% for grade 3 tumors. Using the newer two-tier grading
A “typical” MCT is a dermoepidermal, dome-shaped, alo- system, there was a 98.6% consistency among six patholo-
pecic, and erythematous lesion (Fig. 81.7). However, as dis- gists. Along with this, the MST for dogs with low-grade
cussed in previous paragraphs, MCTs rarely have a typical tumors was 23 months, compared with approximately 4
appearance. A clinical feature that may aid in the diagnosis months for dogs with high-grade tumors. This newer grading
of a MCT is the Darier sign, which is the erythema and wheal scheme, overall, has been shown to decrease inter-pathologist
that form after the tumor is slightly traumatized (i.e., scraped, variation as well as provide strong correlations with overall
compressed, aspirated, etc.). survival, MCT-associated mortality, and risk of metastasis.
As discussed in Chapter 74, MCTs are easily diagnosed However, other authors have questioned whether this
on cytology (see Fig. 74.8). Most dogs with MCTs have a grading system correlates well with survival times (Stefanello
normal, although eosinophilia (sometimes marked), baso- et al., 2015).
philia, mastocythemia, neutrophilia, thrombocytosis, or In some cases, special stains may be required to identify
anemia (or a combination of these) may be present. Serum the typical intracytoplasmic granules in poorly differentiated
biochemistry abnormalities are uncommon. neoplasms. The mitotic index is of prognostic relevance in
From a histopathologic standpoint, MCTs were tradition- dogs with MCTs, so it should be provided by the pathologist
ally classified using the Patnaik grading system into three (Romansik et al., 2007). In addition to the grading of the
categories: well differentiated (grade 1), moderately differen- tumor, the pathologist should provide the clinician with
tiated (grade 2), and poorly differentiated (grade 3). Several information regarding the completeness of the excision. A
studies have shown that dogs with grade 1 tumors treated dog with an incompletely excised MCT is rarely cured by the
with surgery alone have longer survival times than identi- initial surgical procedure and requires either a second
cally treated dogs with grade 3 tumors, mainly because well- surgery, irradiation of the affected area, or chemotherapy.
differentiated neoplasms are easier to resect and have a lower Markers of proliferation, such as AgNOR (argyrophilic
metastatic potential (i.e., most tumors in dogs with systemic nucleolar organizing region) and Ki-67, have been used
mast cell disease are grade 3). prognostically in some studies, and are now offered by
several commercial laboratories. In these studies, high
AgNOR and Ki-67 scores were associated with a shorter time
to relapse and MSTs. We do not routinely use these stains to
plan treatment of dogs with MCTs.
From a molecular standpoint, approximately 20% to 30%
of canine MCTs have internal tandem duplications in exons
11 and 12 of c-kit; c-kit is the stem cell growth factor recep-
tor, and its mutation results in immortalized clones that do
not undergo apoptosis (Jones et al., 2004). Mutations in c-kit
are more common in high-grade tumors and have been asso-
ciated with decreased MST and disease-free interval (DFI),
and a higher risk of local recurrence, metastasis, and MCT-
related mortality.

Biologic Behavior
The biologic behavior of canine MCTs can be summed up in
one word: unpredictable. Even though several criteria may
help in establishing the biologic behavior of these neoplasms,
they rarely apply to an individual dog (i.e., they may be
meaningful from the statistical viewpoint; however, they are
not always representative in a specific patient).
In general, solitary cutaneous MCTs that are termed low-
FIG 81.7 grade (Kiupel) or grade 1 (Patnaik) have a low metastatic
Dermoepidermal, dome-shaped lesion in the pinna of a potential and low potential for systemic dissemination.
Boxer. The cytologic diagnosis was mast cell tumor. However, the clinician may encounter a dog with several