Page 413 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 21 Mast Cell Tumors 391

Anatomic site amenable to wide surgical excision?

VetBooks.ir Yes No

Options:
Ideally a combination of:
1. Cytoreductive surgery
2. Adjuvant radiation therapy* regional lymph node
Excise with wide surgical margins 3. Adjuvant medical therapy
Assess margins and confirm grade
Alternative choices:
1. Cytoreduction followed by medical therapy
2. Medical therapy alone
3. Radiation therapy* and medical therapy

Options:
No
Complete surgical margins? 1. Reexcision with wider margins
2. Adjuvant radiation therapy to
primary site* regional node
Yes
Adjuvant medical therapy
regional node irradiation*

Routine follow-up: *Note: Coarsely fractionated radiation
• 1, 3, 6, 9, 12, 15, 18 months therapy protocols may be considered
• q 6 months thereafter in place of more traditional protocols
• Physical exam and lymph node exam, due to high likelihood of dissemination
abdominal ultrasound. with high-grade tumors.
• Fig. 21.8 Suggested treatment approach for high-grade, biologically aggressive canine mast cell
tumors.

recurrence and therefore should not be used. 176–181 It is important corticosteroids can inhibit canine MCT proliferation and induce
to note that none of these alternative local therapies are as thor- tumor cell apoptosis in vitro, 187 they may also contribute to
oughly investigated, clinically effective, or practical as surgery, RT, apparent antitumor response by decreasing peritumoral edema
or a combination of the two. Lastly, despite its common use, there and inflammation. The Veterinary Cooperative Oncology Group
are no published studies to suggest that adjuvant corticosteroid (VCOG) studied the efficacy of single-agent systemic prednisone
or antihistamine therapy is of benefit in cases of individual inter- therapy for intermediate- and high-grade canine MCTs. 186 Of
mediate-grade MCTs that have been either excised completely or 21 dogs receiving 1 mg/kg daily PO, only one complete response
treated with postoperative RT. (CR) and four partial responses (PRs) were noted, and these were
The treatment of anaplastic or undifferentiated MCTs remains short lived, lasting only a few weeks in the majority of cases. More
frustrating. This designation includes dogs with intermediate- recent studies have reported 70% to 75% response rates; how-
grade tumors with regional or distant metastasis, high-grade ever, tumors were excised or irradiated after short-term prednisone
tumors, MCTs with high proliferative activity as assessed by spe- treatment and thus duration of response was not evaluable. 185,188
cial stains, and mucous membrane or mucocutaneous junction One report found that response of MCTs to corticosteroids was
MCTs. There is some evidence to suggest that intermediate-grade dependent on expression of the glucocorticoid receptor; those dogs
tumors with only regional LN involvement have a better progno- with tumors that expressed low levels of receptor had MCTs that
sis than high-grade tumors. 113,114 In the authors’ opinion, until were resistant to prednisolone therapy. 189 These data suggest that a
convincing evidence exists, such tumors should still be treated as if subset of MCTs may indeed benefit from corticosteroid therapy if
they have a high potential for metastasis. Fig. 21.8 summarizes the there is adequate expression of the glucocorticoid receptor.
treatment recommendations for high-grade MCTs. The long-term Recently a number of studies have evaluated the response rates
prognosis for such dogs is less favorable, as regional and distant of measurable canine MCTs to various cytotoxic chemotherapy
metastasis is more likely. drugs and protocols (see Table 21.6). 32,120,125,190–196 Objective
Poorly differentiated and metastatic MCT will, in most response rates as high as 64% have been reported, and accumu-
instances, progress to kill the dog in the absence of effective post- lating evidence suggests that multiagent protocols may confer a
surgical intervention. Systemic adjuvant therapy should be offered higher response rate than single-agent therapy. 32,120,125,193,195,197
in such cases in an attempt to decrease the likelihood of systemic It is important to note that, in most instances, the response of
involvement, or at least potentially improve DFIs. Corticosteroids, bulky MCT to any chemotherapy protocol tends to be short lived,
such as prednisone, have been reported for many years in preclini- stressing the need for local control of disease before the institution
cal or anecdotal settings to be of some benefit. 182–186 Although of adjuvant therapy.