Page 411 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 21 Mast Cell Tumors 389

observed as a result of MCT metastasis. Knowledge of the extent said that most MCTs can be adequately treated with surgery alone
of MCT margins before surgery, usually accomplished by digital provided the site is amenable to adequate surgical resection. All
surgical margins should be evaluated histologically for complete-
palpation, can be enhanced with the use of diagnostic US or com-
VetBooks.ir puted tomography (CT). In dogs with cutaneous MCT or soft ness of excision. For tumors in which wide surgical excision is
not possible because of size, geographic constraints, or owner con-
tissue sarcomas the extent of local tumor margins was upgraded
in 19% and 65% of cases when imaged by US and CT, respec- cerns, a biopsy to determine histologic grade maybe helpful before
tively. 142 Such information allows more appropriate planning of definitive therapy to determine whether smaller lateral margins
definitive surgery or RT. The cost-effectiveness of such a study may be sufficient for complete histologic excision (i.e., 1 cm lat-
depends on the location of the tumor and whether wide excision eral margins for a low-grade MCTs).
is technically simple or difficult. The management of MCTs in locations where primary clo-
With respect to evaluation of buffy coat smears for evidence sure after wide excision is difficult to impossible, such as the
of systemic MC disease, peripheral mastocytosis (1–90 MCs/μL) distal extremities, can be challenging. Options include tailoring
is reported in dogs with acute inflammatory disease (in particu- the surgical approach according to preoperative determination
lar parvoviral infections), inflammatory skin disease, regenera- of histologic grade, wide excision followed by reconstruction of
tive anemias, neoplasia other than MCT, and trauma. 143–145 One the subsequent defect, limb amputation, and primary closure of a
study revealed that peripheral mastocytosis is actually more likely marginally excised MCTs followed by either RT or chemotherapy.
to occur and may be more dramatic in dogs with diseases other Histologic grade can be accurately diagnosed based on preopera-
than MCT. 144 Therefore this test is no longer routinely performed tive biopsies, 155 and histologic grade can be used to determine the
in the staging of MCT patients. In a report evaluating 157 dogs lateral margins required for complete histologic excision of low-
with MCT, the incidence of bone marrow infiltration at initial to intermediate-grade MCTs; 1-cm and 2-cm lateral margins are
staging was only 2.8%. 146 Although the presence of bone mar- usually sufficient for grade I and grade II MCTs, respectively. 149 If
row involvement is indicative of systemic MC disease, it is usually primary closure is not possible regardless of the width of the lateral
easier to find evidence of systemic involvement in other organs margins, then the two basic options include wide excision, with or
(liver, spleen). This is in contrast to dogs that present with vis- without chemotherapy depending on histologic grading, or mar-
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ceral MCT, in which 37% of buffy coat smears are positive for ginal excision followed by adjuvant therapy. Wide excision with
MC and 56% of bone marrow aspirates reveal MC dissemina- 2-cm lateral margins, 149,150 or proportional lateral margins 151 is
tion 128 ; however, these constitute a small minority of all MCT still feasible despite an inability to close the resultant defect pri-
cases. Therefore with the exception of the extremely rare case of marily. Options for management of these defects include closure
primary mastocytic leukemia, 147,148 involvement of bone marrow with random or axial pattern flaps or free-meshed skin grafts, or
or peripheral blood in the absence of disease in regional LN or healing by second intention. In one study of 31 dogs managed
abdominal organs is unlikely and the routine performance of bone with second-intention healing after excision of soft tissue sarco-
marrow aspirates for clinical staging has fallen out of favor. 146 mas with 2-cm lateral margins, 93.5% of resulting wounds healed
completely after a median time of 53 days. 156 Limb amputation
Treatment is the most aggressive option; however, although wide margins are
guaranteed, it results in the least functional outcome and is gen-
Treatment decisions are predicated on the presence or absence of erally not recommended given the availability of other effective
negative prognostic factors and on the clinical stage of disease. therapies.
In tumors localized to the skin in areas amenable to wide exci- Marginal excision of low- to intermediate-grade MCTs is
sion, surgery is the treatment of choice. Historically, surgical exci- also an acceptable approach if followed by treatment with RT.
sion to include a 3-cm margin of surrounding normal tissue has In this situation, the MCT must be able to be excised with no
been recommended. However, this recommendation was largely evidence of gross residual disease and the wound closed primar-
anecdotal. More recently, surgical margins have been evaluated ily. If this is possible, then adjuvant RT is associated with long-
for the excision of low- and intermediate-grade MCTs less than 5 term tumor control. Two-year control rates of 85% to 95% can
cm diameter, and two different approaches have been described. be expected for stage 0 tumors of low- or intermediate-grade
The metric approach uses a prescribed metric distance, with lateral MCTs. 110,123,157–159 If RT is either not available or declined, then
margins of 1 cm and 2 cm for low- and intermediate-grade MCTs, two studies have demonstrated a low rate of recurrence in dogs
respectively. 149,150 The proportional approach uses lateral margins with incompletely excised MCTs that receive some form of post-
proportional to the maximum dimension of the MCT. 151 For operative chemotherapy 160,161 ; however, both studies were single-
both approaches, deep margins include removal of one uninvolved arm retrospective studies with low case numbers and hence these
fascial plane in continuity with the tumor. If necessary, muscle results should be regarded with caution. There is also information
layers may also be removed deep to the tumor. In 100 dogs with to suggest that some low- or intermediate-grade MCTs will not
115 resectable MCT (primarily low- and intermediate-grade), no recur even if no adjuvant therapy is employed. 162 Although this
local recurrence or metastasis was noted for greater than 2 years approach is not considered optimal, it can be used in cases in
after excision with lateral histologic margins 10 mm or greater and which RT is unavailable or unaffordable. Regardless of the local
deep histologic margins 4 mm or greater, although the “quality” therapy chosen, dogs with low- and intermediate-grade tumors
83
of the margin (e.g., fascia vs loose connective tissue vs fat) needs should be reevaluated regularly (so-called active surveillance) for
to be considered for deep margin evaluation. It should be noted local recurrence and regional and distant metastasis. Local site
that these microscopic, formalin-fixed margin parameters may not and regional LN evaluation, complete physical examination, and
accurately reflect margin size at surgery; tissue shrinkage (up to FNA of any new cutaneous masses or enlarged LNs are performed
30% for cutaneous tissues) can occur subsequent to formalin fixa- at these intervals. More complete staging, including abdominal
tion. 152–154 Considering that the majority of naïve dermal MCTs US, should be included if the dog has an MCT with negative
encountered in practice are of low or intermediate grade, it can be prognostic factors.