Cytologic findings in mandibular and superficial cervical lymph nodes of dogs with thyroid carcinoma.

Objective: To describe cytologic findings from mandibular and superficial cervical lymph nodes in dogs with thyroid carcinoma and to determine prognostic factors associated with lymph node metastasis.

Animals: A total of 71 client-owned dogs with confirmed thyroid carcinoma that had cytologic results from at least 1 mandibular or superficial cervical lymph node between 2010 and 2020.

Procedure: Medical records from 2 referral veterinary hospitals were retrospectively reviewed. Cytology of lymph nodes was reviewed for presence of metastasis by diplomates of the American College of Veterinary Pathologists. Thyroid tumor diameter and volume, tumor fixation, bilateral location, vascular invasion, and stage were recorded to determine effects on nodal metastasis.

Results: A total of 154 lymph nodes (104 mandibular and 50 superficial cervical lymph nodes) from 71 dogs were cytologically evaluated, and 1/154 (0.6%) and 2/154 (1.3%) lymph nodes were noted to be definitively metastatic or probably metastatic, respectively. Given the infrequent rate of nodal metastasis (1.9% or less), statistical analysis of potential prognostic variables was not completed.

Conclusion and clinical relevance: Routine lymph node cytology of mandibular and superficial cervical lymph nodes appeared to be of low yield when assessing for metastasis of canine thyroid carcinomas. The medial retropharyngeal and deep cervical lymph nodes should continue to be evaluated as they appeared to have higher metastatic rates, based on historic reports. Additional studies are needed to determine prognostic factors associated with lymph node metastasis and effects on patient survival.