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
Design—Prospective clinical trial.
Animals—25 dogs with locally advanced thyroid carcinomas
and no evidence of metastasis.
Procedure—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 and Clinical Relevance—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. (J Am Vet Med Assoc 2000;216: