# Unilateral Multifocal Follicular Thyroid Carcinoma with Vascular Invasion and Primary Hepatic Metastasis in a Dog: First Documented Case

## TL;DR

A 14-year-old dog had three thyroid tumors on one side and liver metastasis, marking the first documented case of this unusual cancer pattern in dogs.

## Contribution

First documented case of unilateral multifocal thyroid carcinoma with vascular invasion and primary hepatic metastasis in a dog.

## Key findings

- Three distinct thyroid tumors were found on the right thyroid lobe with vein invasion.
- Liver metastasis was confirmed without lung or lymph node involvement.
- Dog remained euthyroid and showed no recurrence after surgery.

## Abstract

Thyroid cancer in dogs usually appears as a single mass on one or both sides of the neck and can sometimes be found as an ectopic form at the base of tongue or heart. In this case, a 14-year-old female Jindo dog was found to have three separate tumors growing only on the right side of the thyroid gland. The tumors were carefully removed through surgery, along with a nearby neck vein that the tumor had invaded. Some nodules in the liver were also removed and were confirmed to be the first place where the cancer had spread, even though no metastasis was detected in the lungs or regional lymph nodes. The dog recovered well after surgery and did not show any changes in thyroid hormone levels. This is the first known report describing multiple thyroid cancers on only one side of the gland in a dog. The findings from this case may help veterinarians recognize unusual forms of thyroid tumor, choose the best surgical plan, and understand how these tumors can spread in unexpected ways.

A 14-year-old spayed female Jindo dog presented with a firm, non-painful right-sided cervical mass. Computed tomography identified three distinct, separate masses thought to be arising from the right thyroid lobe; the largest measured 66.6 mm × 42.0 mm × 37.6 mm with an estimated volume of 56 cm3 and showed invasion into the right internal jugular vein. Multiple hepatic nodules were detected without evidence of pulmonary metastasis and regional lymph node involvement. Right thyroidectomy with resection of the invaded vein and partial liver lobectomy were performed. The histologic results confirmed all three masses as follicular-compact thyroid carcinomas, and the hepatic lesion as metastatic thyroid carcinoma. The dog recovered uneventfully, remained euthyroid, and showed no local recurrence over a 5-month follow-up. In human medicine, multifocality is common in papillary thyroid carcinoma and is associated with a high rate of recurrence. This report documents the first canine case of multifocal thyroid carcinoma, featuring macroscopic vascular invasion and an uncommon metastatic pattern in which the liver was affected in the absence of detectable pulmonary lesion. The presence of multifocal disease within a single canine thyroid lobe necessitates comprehensive cross-sectional imaging, meticulous surgical planning with vascular considerations, and long-term monitoring to optimize the prognosis of this carcinoma.