Case report of a dog treated with radioactive iodine (I131) and stereotactic body radiation therapy (SBRT) for metastatic ectopic thyroid carcinoma

Thyroid carcinoma is a rare malignancy in middle-aged to older, medium to large breed dogs. Most are carcinomas or adenocarcinomas and are often detected incidentally via palpation or advanced imaging, such as computed tomography. Up to 48% of dogs present with metastatic disease, primarily in lymph nodes and lungs. Preferred treatment is thyroidectomy, but many dogs are poor candidates due to tumour immobility and metastasis. Stereotactic body radiation therapy and radioactive iodine (I131) are emerging treatments with promising outcomes in these cases. This report describes a dog with metastatic ectopic thyroid carcinoma treated with both SBRT and I131. This case describes a 10-year-old Jack Russell Terrier mix presenting with an ectopic thyroid carcinoma at the heart base and pericardial effusion. Initial treatment involved stereotactic body radiation therapy in three fractions totalling 21 Gy. Seven weeks later, with persistent tumour-related signs, a subcutaneous injection of 1600 MBq of I131 was administered. The dog clinically improved, while showing initial tumour reduction on cardiac ultrasound and thoracic radiographs. 37 months later, the dog presented again with recurrent pericardial effusion. Thoracic radiographs and ultrasound revealed metastatic nodules and increased size of the primary heart-based mass, indicating disease progression. A second dose of I131 (1600 MBq) was administered, but restrictive pericarditis and effusion necessitated a subtotal pericardectomy. Nine months after the second I131 treatment, the dog presented with similar signs and further disease progression evident on imaging including pleural effusion. Though treatment options, including a third I131 dose and a second SBRT course, were discussed, the owners chose to humanely euthanise the dog. This case highlights an unusually prolonged survival in a dog with metastatic ectopic thyroid carcinoma, suggesting the potential for extended disease control with multimodal external beam radiation and I131 therapies.