34th Annual Scientific Meeting proceedings

Objectives:

Determine the rate of nodal metastasis in dogs with thyroid cancer and evaluate if immunohistochemistry (IHC) identifies additional metastases beyond hematoxylin and eosin (H&E) alone.

Methods:

70 dogs were enrolled and underwent thyroidectomy with elective bilateral medial retropharyngeal (MRP) +/- deep cervical lymphadenectomy. Thyroid tumors and associated lymph nodes were reviewed by a single pathologist. Immunohistochemistry (TTF-1 and calcitonin) was to facilitate diagnosis of all primary tumors. Lymph nodes without evidence of metastasis after H&E review were labelled with the antibody showing wider uptake in the primary tumor. 

Results:

Seventy-seven thyroid cancers were resected from 70 dogs, including 61 (79.2%) follicular, eight (10.7%) medullary, and seven (9.3%) mixed follicular/medullary carcinomas, with one (1.3%) carcinosarcoma. Twelve dogs showed evidence of nodal metastasis following H&E review. Occult micrometastasis was identified in one dog following nodal IHC, resulting in documented metastasis in 13/70 dogs (18.6%). Medullary carcinoma (5/8) and mixed follicular medullary carcinoma (3/7) were both associated with a significantly higher rate of nodal metastasis than follicular carcinoma (5/61; p = 0.001 and p = 0.03, respectively). All MRP metastases were ipsilateral (7/77, 9.1%), with no contralateral MRP metastases (0/62). 14/41 (34.1%) resected deep cervical lymph nodes were metastatic.

Conclusions:

Nodal metastasis was uncommon for follicular carcinoma but was seen in over 50% of dogs with thyroid cancer involving a medullary component. Deep cervical lymph nodes are an important site of metastasis. Ipsilateral MRP resection and extirpation of identifiable deep cervical lymph nodes are recommended concurrently with thyroidectomy until evidence-based, patient-specific guidelines can be developed.