Anatomic pathology perspective
Jorden Manasse, BVMS
Dr. Jorden Manasse
When submitting a potential mast cell tumor for histopathologic diagnosis, it is important to define surgical margins as medial/lateral,
proximal/distal, dorsal/palmar, or cranial/caudal. This can be done by using sutures placed along a selected margin. Histologic
margins are correlated with outcome: a recent study examining Grade I and II mast cell tumor histologic margins found that
with lateral margins of ≥ 10 mm and deep margins of ≥ 4 mm, no tumor recurrence or metastatic disease was observed.16
Histopathologic examination usually provides a definitive diagnosis. Mast cell tumors are generally well demarcated, nonencapsulated
dermal masses containing round cells arranged in cords and sheets admixed with variable numbers of eosinophils. Extension
into the subcutis is common, but extension to the dermal-epidermal junction is rare.17 Mast cell granules may be highlighted in poorly differentiated tumors by using a metachromatic toluidine blue special stain,
by staining for chymase activity, or by performing immunohistochemistry for the presence of tryptase antigen.18 Of the three, a toluidine blue stain is the easiest and fastest to perform.
3. The histologic appearance of the tumor in this case showing a pleomorphic population of round cells within the dermis.
The neoplastic mast cells are poorly differentiated, with multiple binucleated cells and occasional multinucleated cells present.
Eosinophils are seen scattered among the neoplastic population (hematoxylin and eosin stain, 400X; bar = 50 μm).
Recently, a new two-tier grading system (low grade, high grade) was proposed over the Patnaik three-tier grading system.19 This study found that classifying mast cell tumors as low or high grade provided the best prognostic information for survival
and that dogs with high-grade MCTs had a median survival time of < 4 months and dogs with low-grade MCTs had a median survival
time of > 2 years.20
In the case study discussed, a Grade III mast cell tumor was diagnosed, which would be considered high grade under the new
grading scheme. The deep dermis contained a well-demarcated unencapsulated mass of round cells showing marked cellular pleomorphism
with numerous binucleated and few multinucleated cells (Figure 3). The mitotic rate was 12/10 hpf (400X), and eosinophils were scattered throughout the mass. A toluidine blue stain showed
variable positive staining of granules within the neoplastic population (Figure 4).
4. The histologic appearance of the tumor in this case showing cytoplasmic granules of a binucleated cell (arrow) and many
surrounding neoplastic cells that stain magenta on a blue background with the toluidine blue stain (400X; bar = 50 μm).