Surgical margin evaluation in veterinary medicine: an assessment of canine skin shrinkage and histopathologic reporting of canine mast cell tumors

When cutaneous mast cell tumors (MCT) are removed and evaluated for histopathology, the tissue undergoes many changes. Initially the mass is removed with planned surgical margins, which are a measure of grossly normal tissue around the tumor border. The cut margins are marked so that the pathologist can identify the margins in relation to the patient and the specimen is placed in formalin. Upon removal, however, the specimen undergoes noticeable shrinkage, which is further exacerbated by histologic processing. Upon being received by the laboratory, the gross appearance of the tissue has drastically changed from the time of surgery. This tissue is sectioned, stained and prepped for the pathologist to evaluate. Finally the pathologist prepares the histopathology report for the clinician. The histopathology reports, especially grade and margin reporting, are critically important for the clinician as they aid in the decision-making process regarding necessity and mode of adjunctive therapy recommended. However, the steps that occur from surgery until pathologic review can affect the pathologist’s ability to report accurate histologic margins if appropriate measures are not taken. Likewise, it is important for the surgeon to understand the tissue shrinkage that occurs if histologic and surgical margins are ever to be correlated. Therefore, the second project focused on evaluating MCT histopathology reports, which is the major communication tool between the pathologist and the surgeon. The specific objectives of the first project were to evaluate the effects of anatomic location, histologic processing, and sample size on shrinkage of surgically excised canine skin samples. The purpose of the second project was to describe and evaluate information presented within canine MCT histopathology reports specifically focusing on how information that is important for determining future treatment recommendations and patient prognosis is reported. In the first project, elliptical samples of the skin, underlying subcutaneous fat, and muscle fascia were collected from the head, hind limb, and lumbar region of 15 canine cadavers. Two samples (10 mm and 30 mm) were collected at each anatomic location of each cadaver (one from the left side and the other from the right side). Measurements of length, width, depth, and surface area were collected prior to excision (P1) and after fixation in neutral-buffered 10% formalin for 24 hours (P2). Length and width were also measured after histologic processing (P3). The results of the first project found that length and width decreased significantly at all anatomic locations and for both sample sizes at each processing stage. Hind limb samples had the greatest decrease in length, compared with results for samples obtained from other locations, across all processing stages for both sample sizes. The 30-mm samples had a greater percentage change in length and width between P1 and P2 than the 10-mm samples. Histologic processing (P2 to P3) had a greater effect on the percentage shrinkage of 10-mm samples. For all locations and both sample sizes, the percentage change between P1 and P3 ranged from 24.0% to 37.7% for length and 18.0% to 22.8% for width. Based on the results of this project we concluded that histologic processing, anatomic location, and sample size affected the degree of shrinkage of a canine skin sample from excision to histologic assessment. In the second project, MCT histopathology reports were collected from medical and surgical oncologists in 4 geographic regions of the United States: Midwest, Northeast, South and West. Up to 15 reports were obtained for cases presenting to these clinics between January 1st 2012 and May 31st, 2015. Inclusion criteria required that on the histopathology report the final diagnosis was MCT, a microscopic description was present, and the reports were from only cases where it was first attempt at surgical removal of the mass. Three hundred and sixty-eight reports were collected from 26 contributors. While the majority of the reports contained a clinical history (85.9%), information to evaluate for prognostic indicators was lacking. Both Patnaik (Patnaik, Ehler, & MacEwen, 1984) and Kiupel (Kiupel et al., 2011) grading systems were described in 76.5% of reports with a single system being used in 7.1% and 15.2% of reports respectively. Subcutaneous MCT were assigned a grading scheme in 67.2% of reports with 33.3% stating appropriate limitations. Surgical margins were reported in 92% of the reports with 77.2% describing both deep and lateral margins. Tissue composing the deep margin was only described in 10.9% of the reports. Based on this project, it was concluded that reporting of MCT has variability across pathologists with inconsistencies present in the information provided for clinical history, the reporting of margin evaluation and grading of subcutaneous MCT