Performance of Frozen Section Histopathology, Imprint Cytology and Fine‐Needle Aspirates for Detecting Canine Metastatic Mast Cell Tumour

Intra-operative staging of canine mast cell tumour (MCT) currently relies on routine cytology to determine nodal metastasis. While frozen section nodal histopathology is commonly used in humans, its applicability to veterinary settings is poorly characterised. The goal of this study was to determine the diagnostic performance of frozen section (FS) histopathology for diagnosing metastatic MCT, as compared to a formalin-fixed histopathologic gold standard. Performances of imprint cytology (IC) and fine needle aspirates (FNA) were also evaluated. Forty-one lymph nodes from 20 dogs with MCT were collected and stained with haematoxylin and eosin (HE) and Giemsa (formalin-fixed and frozen tissues), and Wright Giemsa and toluidine blue (IC and FNA). Nineteen out of 20 primary tumours were low grade. Frozen HE sections had poor agreement as compared to formalin-fixed HE histopathology (κ = 0.15); however, diagnostic performance increased to a good level of agreement when interpretation was combined with Giemsa (κ = 0.46). FNA and IC using Wright Giemsa had agreement comparable to combined frozen section histopathology (κ = 0.51 and 0.43, respectively). Combined frozen sections had a sensitivity of 65% and specificity of 93%, which was the same as FNA. Challenges encountered in morphologic interpretation of frozen sections included inadequate sectioning quality, architectural disruption, ruptured cells, and background metachromatic staining. These data provide support for FS histopathology as a feasible strategy for intra-operative detection of metastatic MCT, with diagnostic agreement similar to conventional cytology. Performance of FS histopathology is conditional upon a metachromatic stain evaluated in parallel with HE.