•Help the pathologist help you
All excised tissue should be submitted to the pathologist for evaluation. Marking margins of resected tissue (i.e., deep and
lateral margins) with India ink or with the multicolor Davidson marking system (Figure 4) (
http://www.bradleyproducts.com/) can assist the pathologist in visualizing the margins and determining if they are free of neoplastic cells. It is best to
contact your pathologist to determine his or her preferences regarding margin evaluation.
Figure 4: Histologic appearance of a mast-cell tumor. Note that the tumor extends to the (blue) inked margin. The ink was
applied prior to placing tissue in formalin, and should aid the pathologist in determining the completeness of the excision.
•Interpreting the biopsy report
Mast-cell tumor grading is based on a histologic classification scheme, and cannot be determined cytologically. The Patnaik
grading system, which is the most commonly used grading scheme for canine mast-cell tumors, divides MCT into three categories
(grades) based on their histologic appearance and invasiveness (Table 1). After these tumors were categorized based on histologic
appearance, clinical follow-up information was obtained to determine if grade affects prognosis. The percentage of dogs surviving
1,500 days after diagnosis were 83 percent, 44 percent and 6 percent for grade I, II and III tumors, respectively (Patnaik,
1984). Tumor grade is the most consistent prognostic indicator of disease-free interval, metastasis and survival time in dogs
Table 1: The Patnaik grading system for canine cutaneous mast-cell tumors (Patnaik, 1984)
Incidence of metastasis varies by studies, but in general, a 10 percent or less metastatic rate for grade I and II tumors
and a 50 percent or greater metastatic rate for grade III tumors is predicted. The tumor grade should be evaluated with other
clinical findings, such as the size and invasiveness of the tumor, the presence of local and distant metastasis, and tumor
Controversy exists among oncologists as to the definition of "clean" versus "close" margins for MCT. One report defines "clean"
margins as a 1-2 mm distance between tumor and normal tissue (Weisse, 2002) but other references are scarce. The author generally
defines "clean" margins for MCT as 10 mm or greater between tumor and normal tissue and "close" margins as 1-9 mm between
tumor and normal tissue. It is important to remember that tissues shrink and can become distorted in formalin, so it can be
difficult for the pathologist to give an accurate assessment of margins.
Post-operatively, further treatment options are based on biopsy results including evaluation of margins and tumor grade. For
low and intermediate grade MCT (Patnaik grades I and II), if the surgical margins are "dirty" (i.e., tumor cells extend to
margins of excision), then further treatment options include RT to the tumor bed or a second surgery if the site is amenable.
In addition, some investigators are trying to determine if chemotherapy administration in this setting of microscopic disease
will prevent local tumor regrowth. If surgical margins are "close" in this group of patients, then options include second
excision with scar revision, close monitoring of the site for recurrence or RT.