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Radioactive iodine ( 131 I) has previously been reported to prolong survival in dogs with thyroid carcinoma. This study aimed to describe tumour response and progressionfree interval (PFI) in dogs with thyroid carcinomas treated with 131 I. Secondary aims were to describe overall survival time (OST) and prognostic factors. A bi-institutional retrospective review of records identified 66 dogs with thyroid carcinoma treated with 131 I from January 2010 to April 2020. Response was described using RECIST or a subjective response assessment where specific tumour measurements were not available. Forty-eight dogs (72.7%) were treatment naïve and 18 dogs (27.3%) had received prior therapy at the time of 131 I treatment. Objective responses were available for 34 dogs and subjective responses for 58 dogs. The overall response rate was 35.3% (four complete and eight partial responses). Improvement of clinical signs was seen in 76.2% of dogs (32/42). Kaplan-Meier-estimated median PFI (95% confidence interval [CI]) was 301 (217-578) days and OST (95% CI) was 564 (421-865) days.Prior therapy was associated with a lower hazard for progression (hazard ratio[HR] 0.260 95% CI 0.123-0.548, p = .0004). Treatment of thyroid carcinoma using 131 I can effectively alleviate clinical signs and reduce disease burden in a proportion of dogs.

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