Prognostic factors and patterns of treatment failure in dogs with unresectable differentiated thyroid carcinomas treated with megavoltage irradiation.
OBJECTIVE
To determine quality and duration of progression-free survival (PFS) time in dogs with unresectable thyroid carcinomas treated with definitive megavoltage irradiation and analyze prognostic factors of PFS and patterns of failure (local recurrence vs metastasis).


METHODS
Prospective clinical trial.


METHODS
25 dogs with locally advanced thyroid carcinomas and no evidence of metastasis.


METHODS
Dogs were treated with 48 Gy during 4 weeks on an alternate-day schedule of 4 Gy/fraction.


RESULTS
Irradiation was safe and effective for treatment of large unresectable thyroid carcinomas. Progression-free survival rates were 80% at 1 year and 72% at 3 years. Time to maximum tumor size reduction ranged from 8 to 22 months. Factors affecting PFS were not found. Twenty-eight percent (7/25) of dogs developed metastasis. Dogs with bilateral tumors had 16 times the risk of developing metastases, compared with dogs with a single tumor. Dogs with no evidence of tumor progression had 15 times less risk of developing metastases. Radiation-induced hypothyroidism was suspected in 2 dogs 13 and 29 months after irradiation.


CONCLUSIONS
Irradiation is effective for local control of thyroid tumors, despite their slow regression rate. Results provided evidence that local tumor control affects metastatic outcome in dogs with thyroid carcinomas and is a strong basis for the development of new approaches that include irradiation in the management of dogs with advanced thyroid carcinomas. Improvements in local tumor control alone may be insufficient to improve survival times because of the high risk of metastatic spread before an initial diagnosis is made, which warrants initiation of early systemic treatment.
Théon
A P
AP
Department of Surgical and Radiological Sciences, School of Veterinary Medicine, University of California, Davis 95616-8745, USA.
Marks
S L
SL
Feldman
E S
ES
Griffey
S
S
D000818:Animals; D004283:Dog Diseases / Q000401:mortality / Q000532:radiotherapy*; D004285:Dogs; D005260:Female; D008297:Male; D011379:Prognosis; D011446:Prospective Studies; D011882:Radiotherapy, High-Energy / Q000662:veterinary*; D015996:Survival Rate; D013964:Thyroid Neoplasms / Q000401:mortality / Q000532:radiotherapy / Q000662:veterinary*; D017211:Treatment Failure