Page 831 - Small Animal Internal Medicine, 6th Edition

CHAPTER 48 Disorders of the Thyroid Gland 803

comfortable and have the potential for increased longevity RADIOACTIVE IODINE ( 131 I)
after treatment. In addition, local control of the tumor may Studies suggest that radioactive iodine therapy will prolong
VetBooks.ir halt or reduce metastatic dissemination, and the presence of survival time when used as sole therapy or in combination
with surgery for the treatment of thyroid tumors in dogs.
metastases may not ultimately affect outcome. Local control
of the thyroid carcinoma is of primary importance in manag-
months for dogs treated with radioiodine alone, 34 months
ing this disease. Worth et al. (2005) reported a median survival time of 30
when radioiodine was combined with surgery, and 3 months
SURGERY for dogs that did not receive treatment. Turrel et al. (2006)
Surgical excision of thyroid adenomas and small, well- reported median survival times of 839 days for dogs with
encapsulated, freely movable thyroid carcinomas is likely to local or regional tumors (i.e., stage II and III disease) and
be curative. Surgical removal of a fixed, locally invasive 366 days for dogs with metastasis. Tumor site (cervical versus
thyroid carcinoma, regardless of size, carries a guarded to ectopic), age, body weight, treatment protocol ( I alone or
131
poor prognosis for complete excision of the tumor. Radiation with surgery), and serum T 4 concentration were not signifi-
is the treatment of choice for these tumors. Chemotherapy cantly associated with survival time. Radioactive iodine
is indicated if distant metastases are identified, or in some therapy is useful for any thyroid tumor tissue that retains the
cases, postoperatively in dogs with vascular or lymphatic ability to trap iodine, including metastatic sites. A techne-
invasion, or “aggressive” histologic appearance. Surgical deb- tium scan can be used to determine whether the tumor can
131
ulking of fixed, locally invasive tumors may be indicated to trap iodine. Large doses of I (i.e., 30-100 mCi or more) are
relieve tumor-induced problems such as dysphagia or typically administered intravenously to treat canine thyroid
dyspnea and to allow more time for other therapies to work. carcinoma. Advantages of radioactive iodine treatment
Surgical cytoreduction may also be considered after radia- include the low risk of adverse effects and the potential to
tion or chemotherapy has caused large invasive tumors to treat both the primary tumor and metastatic lesions. One
decrease in size. Aggressive attempts at surgical removal, disadvantage is the need for prolonged isolation of the dog
especially of bilateral tumors, threaten the integrity of recur- after treatment. Potential adverse reactions include esopha-
rent laryngeal nerves, parathyroid glands, and normal gitis, tracheitis, and bone marrow suppression.
thyroid tissue. It is important to monitor serum calcium
concentrations before and for 2 to 3 weeks after surgery if CHEMOTHERAPY
there is any chance that the parathyroid glands have been Chemotherapy is indicated when total surgical removal or
excised or damaged. Vitamin D and calcium therapy should destruction with radiation therapy is not successful, when
be initiated if hypocalcemia develops postoperative (see distant metastatic lesions have been identified, and when the
Chapter 47). Serum T 4 and TSH concentrations should be size of the primary tumor is such that local invasion or
assessed 4 and 8 weeks after surgery and treatment initiated metastases are likely, even though it cannot be identified
if clinical signs and blood test results suggest hypothyroid- with diagnostic tests. A number of different chemotherapeu-
ism. (See Tobias and Johnston, 2018, for information on tic drugs have been used to treat thyroid tumors in dogs,
surgical techniques for the thyroparathyroid complex.) including doxorubicin, cisplatin, carboplatin, mitoxantrone,
toceranib phosphate, and chlorambucil. Doxorubicin and
EXTERNAL BEAM RADIATION THERAPY carboplatin are the two most commonly used to treat thyroid
Radiation therapy is the treatment of choice for locally carcinoma in dogs and may be administered on an alternat-
advanced thyroid carcinoma. External beam radiation can ing schedule at 3-week intervals for a combined total of 6
be used alone or in conjunction with surgery or chemo- doses. The response of canine thyroid tumors to doxorubicin
therapy. A slow regression rate of thyroid carcinoma has and carboplatin is variable and unpredictable. In most dogs
been noted after radiation therapy in dogs. In one study doxorubicin and carboplatin prevent further growth of the
involving 25 dogs with unresectable differentiated thyroid tumor and may cause the tumor to shrink, but total remis-
carcinoma and no evidence of metastasis, the time to attain sion is uncommon. One should always consult with an
maximum reduction in tumor size ranged from 8 to 22 oncologist whenever chemotherapy is being considered. (See
months after megavoltage irradiation (Theon et al., 2000). Chapters 76 and 77 for discussion of the use of these chemo-
Progression-free survival rates (defined as the time between therapeutic agents.)
completion of irradiation and detection of measurable local
tumor recurrence or death from causes unrelated to tumor ORAL ANTITHYROID DRUGS
progression) were 80% at 1 year and 72% at 3 years, with a Oral antithyroid drugs are used as palliative therapy to
mean progression-free survival time of 55 months in the 25 control the clinical signs of hyperthyroidism in dogs with
dogs. Acute adverse reactions to radiation include esopha- functional thyroid tumor; they are not used as primary treat-
geal, tracheal, or laryngeal mucositis causing dysphagia, ment because they are not cytotoxic. The therapeutic
cough, hoarseness, and transient neutropenia. These reac- approach is similar to that used in hyperthyroid cats (see p.
tions tend to be mild and self-limiting. Chronic radiation 794), beginning with 2.5 to 5 mg of methimazole adminis-
reactions include skin fibrosis, permanent alopecia, chronic tered twice a day, with subsequent increases in the dosage
tracheitis causing a dry cough, and hypothyroidism. and frequency of administration as needed to control clinical