HISTOPATHOLOGICAL FEATURES OF CANINE MAMMARY GLAND LESIONS IN RELATION WITH SHEAR WAVE ELASTOGRAPHY FINDINGS.

Due to the heterogeneity of canine mammary gland lesions, the usefulness of ultrasound techniques, including Shear Wave Elastography (SWE), to discriminate between benign and malignant lesions is still unclear [1, 2]. In particular, SWE is the most advanced technique that allows imaging tissue elasticity enabling objective measurement of stiffness expressed in kPa or m/s, and it can differentiate benign and malignant human breast lesions with acceptable to excellent performance, reducing the frequency of unnecessary biopsies, particularly when coupled with B-mode ultrasonography [3]. The aim of this study was to evaluate possible correlations between several histopathological features of canine mammary gland nodular lesions such as percentages of fibrosis, non epithelial (myoepithelial/stromal) cells, chondroid and/or bone tissues, necrosis and intratubular secretion, and SWE values, as well as to investigate SWE diagnostic performance to distinguish benign vs malignant lesions, or its possible relationship with tumor histotype and/or grade. Mammary nodules were initially displayed in B-mode using a Logiq S8 (GE Healthcare, Milwaukee, WI, USA) and then examined with the Shear Wave Elastography tool, using a 9L-D multi-frequency linear probe (GE Healthcare) at a frequency of 8 MHz. Tissue stiffness was measured on a region of interest (ROI) manually defined by the same expert operator within the margin of the lesion and the value was recorded (ELStiff, kPa). Mammary nodules (n=27 from 16 dogs) were fixed in 10% neutral-buffered formalin and routinely processed for histology. Histological classification (hyperplasia n=3, benign tumors n=1, malignant tumors n=23) and grading (I n=15; II n=7; III n=1) were assessed according to Zappulli et al. (2019) and Penã et al. (2013), respectively [4, 5]. Fibrosis and non-epithelial cell components were evaluated through Masson Trichrome and Movat Pentachrome stainings, and immunohistochemistry for Vimentin, respectively, and quantified by means of specific tools of the ImageJ programme. The percentage of the other histopathological features was assessed by area measurement tools of Leica Application Suite X. Correlation coefficients were calculated through Pearson’s coefficient, whereas Mann-Whitney test was used to evaluate significant differences between groups. Statistical analysis revealed that % of fibrosis was the main feature showing a significant positive correlation with SWE values (Pearson's r=0.768; p<0.001) together with % of chondroid tissue (Pearson's r=0.772; p<0.039). A low but significant negative correlation was also present between % of fibrosis and % of Vimentin+ cells (Pearson's r=- 0.385; p<0.027). Although significant differences between benign vs malignant lesions, tumor histotypes or grades were non observed in our study, further investigations with a higher number of cases are still necessary to better evaluate the diagnostic performance of SWE for canine mammary gland tumors, and to establish optimal cutoff SWE values to differentiate benign vs malignant lesions.