Page 1266 - Cote clinical veterinary advisor dogs and cats 4th

Mast Cell Tumors, Dog 635

PHYSICAL EXAM FINDINGS DIAGNOSIS mast cells in a lymph node draining an MCT
suggest metastasis.
• MCTs have a highly variable appearance and Diagnostic Overview • Thoracic radiographs are of limited usefulness
VetBooks.ir insect bites. Skin and subcutaneous masses The diagnosis of MCT is generally straight- for dogs with MCT. Uncommonly, malignant Diseases and Disorders
may be mistaken for lipomas, skin tags, or
should always be aspirated and examined
pleural effusion containing mast cells occurs
forward. Microscopic exam of cytologic
in cases with disseminated disease.
cytologically to obtain a diagnosis.
• Some dogs have multiple cutaneous MCTs, preparations or tissue samples reveals round • Complete staging for systemic mast cell
cells with characteristic intracytoplasmic,
and a thorough exam of the entire skin metachromatic granules. Regional lymph nodes disease is indicated before surgery if lymph
surface is indicated. should be aspirated whenever possible before node metastasis, peritumoral edema, or
• Because of the histamine in mast cell granules, surgical removal of the primary tumor (most bruising is present or tumors are recurrent.
MCTs might shrink and swell intermittently often the treatment of choice), and additional Consider staging for tumors located on the
as degranulation (release of granules from the staging tests should be considered for lymph prepuce, scrotum, muzzle, digit, pinna or ear
mast cell cytoplasm) occurs. Degranulation node metastasis or in patients with clinical canal, or oral mucosa (locations potentially
can result from manipulation of the tumor signs of aggressive tumor behavior, including associated with a higher rate of metastasis).
or can be spontaneous. peritumoral edema, swelling, or bruising, • Complete systemic staging to examine
• Because of the presence of heparin in mast and for recurrent tumors. A diagnostic and patients for evidence of MCT metastasis
cell granules, MCTs may bleed excessively treatment approach is presented on p. 1436. includes the tests listed above and abdominal
when aspirated, but this is rarely clinically ultrasound possibly to include liver and
significant. Differential Diagnosis spleen aspirates, bone marrow aspiration for
• Peritumoral bruising and edema (i.e., Darier’s Gross appearance and palpation: cytologic evaluation (p. 1068), and biopsy
sign) and hyperemia (p. 494) are uncommon • Other tumor types: plasma cell tumor, of the primary tumor.
but are associated with aggressive MCTs. histiocytoma, transmissible venereal tumor, • Abdominal ultrasound
• Dogs with primary visceral or metastatic lymphoma, amelanotic melanoma, squamous ○ Affected lymph nodes may be enlarged,
MCTs may have abdominal effusion contain- cell carcinoma, hemangiosarcoma or hem- hypoechoic or hyperechoic, or irregular.
ing mast cells, organomegaly (spleen/liver), angioma, lipoma ○ Splenic/hepatic infiltration: hypoechoic
a palpable abdominal mass, or abdominal • Phlebectasia lesions throughout the parenchyma, rarely
pain on palpation. • Granuloma a solitary mass
• Abscess • The liver, spleen, and bone marrow normally
Etiology and Pathophysiology Cytologically, other round cell tumors may have contain a few mast cells, but increased
• Causes are further discussed on p. 632. a similar appearance (p. 893). numbers or clusters of mast cells suggest
• The likelihood of MCT metastasis depends MCT metastasis.
on tumor grade, mitotic index, location, and Initial Database • The buffy coat smear to evaluate circulating
other factors, and it varies from 10%-50%. • MCTs are readily diagnosed cytologically by mast cells has a high rate of false-positive
In dogs, MCTs most commonly metastasize their characteristic intracytoplasmic granules. results for dogs and is not recommended.
to regional lymph nodes, followed by spleen, In poorly differentiated MCTs, these granules
liver, mesenteric lymph nodes, other cutane- may not be visible, especially with Diff-Quik Advanced or Confirmatory Testing
ous sites, and bone marrow. stain (in-clinic stain). Wright stain (used in • Historically, MCTs were categorized by
• Chronic dermatitis or mutations in a proto- most university and commercial clinical grades based on their histologic appear-
oncogene (KIT) may predispose dogs to mast pathology laboratories) is more sensitive ance (Patnaik grading system): grade I
cell neoplasia. KIT encodes the tyrosine for detecting granules. = well-differentiated tumors, grade II =
kinase receptor KIT, which promotes mast • Before surgical excision, a CBC, serum intermediately differentiated tumors, grade
cell growth and differentiation. Mutations chemistry panel, urinalysis, and regional III = poorly differentiated tumors. Due to
in KIT (≈20%-30% of canine MCTs) allow lymph node aspiration (if accessible; regard- inconsistencies among pathologists (especially
abnormal continuous activation of the less of size of the lymph node) should be for classification of Patnaik grade II tumors),
receptor and predispose dogs to aggressive obtained. a two-tier histologic grading system (Kiupel,
MCTs that are more likely to recur and ○ CBC abnormalities associated with MCTs et al.) is also currently in use. This system
metastasize. Inhibition of receptor tyrosine can include eosinophilia, basophilia, and evaluates cellular criteria such as mitotic
kinases through targeted therapies (see Treat- regenerative or nonregenerative anemia. figures and nuclear characteristics. Dogs with
ment below) has a role in the treatment of • Normal lymph nodes may contain scattered high-grade tumors have a higher metastatic
advanced local or metastatic MCTs. mast cells; increased numbers or clusters of rate and shorter survival time than dogs with
low-grade tumors.
• Mitotic index (MI) is an important,
independent predictor of prognosis. The
MI (numbers of mitotic figures per 10
high-power fields) correlates with grade and
prognosis. Dogs having cutaneous MCTs
with an MI ≤ 5 had a median survival time
of 70 months, compared with 5 months for
an MI > 5.
• Mast cell granules may not be present or
visible in highly anaplastic MCTs; CD117
(KIT) immunohistochemistry or toluidine
blue staining can be used to confirm a
diagnosis of MCT.
• A mast cell tumor prognostic panel is avail-
able at the Diagnostic Center for Population
MAST CELL TUMORS, DOG Interdigital mast cell tumor in a dog. Note the ulceration, superficial infection, and Animal Health at Michigan State
and displacement of the digits associated with this tumor. University (animalhealth.msu.edu); tumors

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