Introduction

Responses rates and duration of control of canine thyroid carcinomas treated with radioactive iodine (I131) have not previously been described. This study aimed to describe tumour response and progression-free interval (PFI) in dogs with thyroid carcinomas treated with I131. Secondary aims were to describe the overall survival time (OST) and identify prognostic factors.

Methods

A retrospective review of records from two institutions identified 66 dogs with thyroid carcinoma treated with I131 from January 2010—April 2020. Response was described using RECIST, or a subjective response assessment in cases without tumour measurements. Univariable and multivariable analysis was performed to identify prognostic factors.

Results

Forty-eight dogs (72.7%) were treatment-naïve and 18 dogs (27.3%) had prior therapy. RECIST responses were available for 33 dogs and subjective responses for 59 dogs. The overall response rates (ORR) were 35.3% (3 complete and, 9 partial responses) with RECIST, and 23.7% with subjective assessment (4 complete and 10 partial responses). Improvement in clinical signs was seen in 27 of 37 dogs (70.3%). Kaplan-Meier estimated overall median [95% confidence interval] PFI was 305 [249–578] (n=53), and OST was 616 [441–867] days (n=55). Dogs treated with prior therapy had a longer OST than treatment naïve dogs, and responders had a longer PFI and OST than non-responders.

Conclusion

Radioactive I131 can provide local tumour control in a proportion of dogs. Thyroidectomy prior to I131 may improve outcomes.

Funding Information

No funding was used for this project.