Pathologic Characterization and Clinical Significance of Lymphovascular Invasion in Canine Thyroid Carcinomas

ABSTRACT The association between lymphovascular invasion (LVI) and outcomes in dogs with thyroid carcinoma is inadequately described, and a standardized definition of LVI is lacking. This study's objectives were to histopathologically and immunohistochemically characterize LVI and to evaluate associated clinical outcomes in dogs with thyroid carcinoma. Inclusion criteria were: dogs that underwent thyroidectomy for thyroid carcinoma with available histopathology samples, and a minimum of 1‐year postoperative follow‐up data or follow‐up to death. Thirty dogs with 35 distinct tumours, including 26 follicular and nine medullary, were evaluated. LVI was characterized as intracapsular endothelialized tumour emboli (without adhered fibrin) in 26 samples, intracapsular tumour emboli with adhered fibrin in seven samples, extracapsular tumour emboli without adhered fibrin in 10 samples, and extracapsular tumour emboli with endothelial disruptions and adhered fibrin in three samples. CD31 and Prox1 positive vessels contained endothelialized tumour thrombi in 17 and one sample, respectively. Extracapsular invasion with (non‐endothelialized) and without (endothelialized) associated fibrin were associated with a 10‐fold ( p = 0.001) and 3‐fold ( p = 0.013) increase in the likelihood of death, respectively. No significant difference in the likelihood of death was noted between cases of extracapsular invasion with or without fibrin ( p = 0.31). Intracapsular tumour thrombi with or without associated fibrin and non‐endothelialized tumour thrombi were not associated with outcomes in this cohort. These results support that in dogs with thyroid carcinoma, extracapsular LVI with or without associated fibrin thrombi, but not intracapsular LVI, is associated with worse outcomes. Further study is warranted on a larger population of dogs with standardized treatment and follow‐up.