Page 1358 - Small Animal Internal Medicine, 6th Edition

1330 PART XII Oncology

dozen cutaneous MCTs, which on histopathologic evalua- Diagnosis
tion are all considered low grade. The evaluation of a dog with a suspected MCT should
VetBooks.ir and a higher potential for systemic dissemination than grade nose cytologically. They consist of a monomorphic popula-
include FNA of the mass. MCTs are typically easy to diag-
Grade 2 and 3 tumors have a higher metastatic potential
tion of round cells with prominent intracytoplasmic purple
1 MCTs. The newer Kiupel grading scheme helps clinicians
determine how concerned to be about a specific MCT, which granules; eosinophils are frequently present in the smear (see
is especially helpful for MCTs with a grade 2 designation Fig. 74.8). In approximately one fourth to one third of MCTs,
(under the Patnaik system), as some of these are categorized the granules do not stain with Diff-Quik; hence if agranular
as low grade and some high grade. Grade 2, high-grade round cells are found in a dermal or subcutaneous mass
MCTs have a higher potential for metastasis and systemic resembling an MCT, the clinician should stain the slide with
dissemination than grade 2, low-grade MCTs. Metastases to Giemsa or Wright stain to reveal the characteristic purple
the regional lymph nodes commonly occur (particularly in granules (see Fig. 74.13). A cytologic diagnosis of MCT
dogs with high-grade tumors), although occasionally a allows the clinician to discuss treatment options with the
tumor “skips” the draining lymph node and metastasizes to owner and to plan therapeutic strategies (see the section on
the second or third regional node (e.g., a digital MCT in the treatment and prognosis).
rear limb metastasizing to the iliac or sublumbar node). Although clinical pathologists frequently state the degree
Because nodal metastases can be present in normal-size of differentiation of the cells in a cytologic specimen of an
lymph nodes, every lymph node in the region of a MCT should MCT, that scheme does not necessarily correlate with the
be aspirated before an aggressive surgery, regardless of histopathologic grading system. In other words, a cytologic
whether it is enlarged or not. Pulmonary metastases are diagnosis of a well-differentiated MCT does not necessarily
extremely rare. It also appears that MCTs in certain anatomic imply that it will be a low-grade tumor when evaluated his-
locations are more aggressive than tumors in other areas. For topathologically (i.e., cytologic grading may not have the
example, distal limb (e.g., toe), perineal, preputial, scrotal, same prognostic implications as histopathologic grading).
inguinal, muzzle, and extracutaneous (e.g., oropharyngeal, More recently, a variation of the Kiupel grading scheme was
intranasal) MCTs appear to have a higher metastatic poten- successfully applied to 152 cytologic specimens. Tumors
tial than similarly graded tumors in other regions (e.g., were classified as high grade if the cells were poorly granu-
trunk, neck). lated or had at least two of four findings: mitotic figures,
Another biologic characteristic of canine MCTs is that binucleated or multinucleated cells, nuclear pleomorphism,
they may become systemic, behaving like a hematopoietic or >50% anisokaryosis. This cytologic grading scheme had
malignancy (i.e., a lymphoma or leukemia). These dogs 88% sensitivity and 94% specificity relative to histologic
usually have a history of a high-grade or grade 3 cutaneous grading (Camus et al., 2016).
MCT that was excised, or a MCT in one of the locations The clinical evaluation of a dog with a cytologically con-
described earlier. Most dogs with systemic mast cell disease firmed MCT should include careful palpation of the affected
(SMCD) are evaluated because of lethargy, anorexia, vomit- area and its draining lymph nodes; abdominal palpation,
ing, and weight loss in association with splenomegaly, hepa- radiography, or ultrasonography to search for hepatospleno-
tomegaly, pallor, and (occasionally) detectable cutaneous megaly; a CBC, serum biochemistry profile, and urinalysis;
masses. The CBC in affected dogs commonly reveals cytope- and thoracic radiography if the neoplasm is in the cranial half
nias, with or without circulating mast cells. of the body (i.e., to detect intrathoracic lymphadenopathy).
MCTs can release bioactive substances that may cause Abdominal ultrasonography should be strongly considered
edema, erythema, or bruising of the affected area. Gastro- in dogs with clinical negative prognostic factors, including
intestinal tract ulceration may also occur as a result of hyper- an enlarged regional lymph node on physical examination, a
histaminemia (≈80% of dogs euthanized because of advanced MCT in any “high risk” location described earlier, an ulcer-
MCTs have gastroduodenal ulceration). Therefore any dog ated MCT, history of rapid growth, known recurrence of a
with an MCT should be considered to have an increased risk previous MCT, systemic signs that may indicate dissemina-
of gastroduodenal ulceration. Profuse intraoperative and tion, or certain breeds. For example, in a recent study Shih
postoperative bleeding and delayed wound healing occur in Tzus and Rottweilers were reported to have a higher preva-
some dogs as a consequence of the bioactive substances lence of high-grade MCTs (Mochizuki et al., 2017)
released from mast cells. If lymphadenopathy, hepatomegaly, or splenomegaly is
A clinical presentation with a very distinctive biological present, FNA of the enlarged lymph node or organ should
behavior is the subcutaneous MCT (Thompson et al., 2011). be performed to detect mast cells (i.e., local neoplasm versus
Most dogs with subcutaneous MCT have a “lipoma”-feeling metastatic tumor versus SMCD); as discussed earlier, regional
mass, usually in the leg (although other locations are possi- nodes should be aspirated, even if normal in size, before
ble). FNAs of these masses usually yield well-granulated performing an aggressive surgery. Additionally, dogs with
mast cells, with no anisocytosis or mitotic figures. This clini- ultrasonographically normal liver and spleen can still have
cal entity is very different than a “deep” dermal MCT, because cytologic evidence of MCT metastasis, so aspirating these
most dogs with subcutaneous MCTs are cured by surgical organs in dogs that have negative prognostic indicators (see
excision alone, even when the tumor is incompletely excised. previous discussion) is recommended.