Prospective assessment of canine thyroid cancer-part I: nodal metastatic rate and impact of nodal immunohistochemistry in 70 dogs.

Objective: To determine the rate of nodal metastasis in dogs with thyroid cancer and evaluate whether immunohistochemistry (IHC) identifies additional metastases beyond evaluation with H&E.
Animals: 70 prospectively enrolled client-owned dogs with thyroid cancer managed with thyroidectomy.
Methods: Dogs underwent thyroidectomy with concurrent elective bilateral medial retropharyngeal (MRP) ± deep cervical lymphadenectomy. Thyroid tumors and associated lymph nodes were reviewed by a single board-certified pathologist. Immunohistochemistry was used for all primary tumors (thyroid transcription factor-1 and calcitonin) to support a diagnosis of follicular or medullary carcinoma. Lymph nodes without evidence of metastasis after H&E review were labeled with the antibody associated with the wider uptake in the primary tumor.
Results: 77 thyroid cancers were resected from the 70 dogs enrolled, including 61 (79.2%) follicular, 8 (10.7%) medullary, and 7 (9.3%) mixed follicular/medullary carcinomas, with 1 (1.3%) carcinosarcoma. Twelve dogs had evidence of nodal metastasis following H&E review. Occult micrometastasis was identified in 1 dog following nodal IHC, resulting in documented metastasis in 13 of 70 (18.6%) dogs. Metastasis was more common with medullary (5/8) and follicular/medullary carcinoma (3/7) than follicular carcinoma (5/61). All MRP metastases were ipsilateral (7/77 [9.1%]), without contralateral MRP metastases (0/62). Fourteen of 41 (34.1%) deep cervical lymph nodes were metastatic.
Clinical Relevance: Nodal metastasis was uncommon for follicular carcinoma but was seen in > 50% of dogs with thyroid cancer involving a medullary component. Routine nodal IHC appears to be low yield for thyroid carcinoma. Extirpation of ipsilateral MRP and identifiable deep cervical lymph nodes is recommended with thyroidectomy until detailed preoperative risk stratification becomes available.