Page 599 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 26 Tumors of the Endocrine System 577

as a consequence of RT for thyroid carcinoma, 232 therefore histologically in most thyrotoxic cats. 239,240,242 Both thyroid lobes
monitoring of the patient’s thyroid status after RT or surgery is are affected in 70% to 90% of cases, 240,242,243 although they may
recommended.
be asymmetrically enlarged at the time of diagnosis. Ectopic
VetBooks.ir of the facilities required for isolation after high-dose 131 I therapy. hyperplastic functional thyroid tissue also is found in a propor-
Radioactive iodine (
I) is not readily available for dogs because
131
Malignant carcinomas are the
242,244,245
tion of hyperthyroid cats.
Two reports of 131 I therapy for invasive thyroid carcinoma indi- least common cause of hyperthyroidism, occurring in only 1% to
cate relatively long STs. 233,234 One study reported 43 dogs treated 3% of thyrotoxic cats 239,240,242,245 ; however, one group recently
with 131 I, either as sole therapy or in combination with surgery. proposed that the prevalence of malignant disease increases with
The first 24 dogs were treated with a dose that was calculated by disease duration. 246 Feline thyroid carcinomas are more locally
means of scintigraphy, with a dose range of 555 to 1850 MBq; the invasive than their benign counterparts, and their metastatic rate
remaining 19 dogs were treated with a dose of 1600 MBq with- can be as high as 70%, with regional lymph nodes and lungs
out dose calculation or taking body weight into account. 233 The affected most commonly. 245,247,248 Unfortunately, no noninvasive
MSTs for 131 I used alone and in combination with surgery were tests are available that can reliably distinguish between benign and
30 months and 34 months, respectively, and no adverse events malignant thyroid disease in hyperthyroid cats, although clinicians
were noted. 233 A similar study reported 131 I therapy as primary may be more suspicious of thyroid carcinoma if thyroid palpation
therapy in 39 dogs with nonresectable thyroid carcinoma. 234 The or the appearance at surgery is atypical or if the patient does not
MSTs for dogs without metastatic disease and those with meta- respond as expected to medical therapy. 249 Nonfunctional thyroid
static disease were 839 and 366 days, respectively. The dose of 131 I carcinomas are uncommon. 249,250 Thyroid cysts also have been
was calculated using scintigraphy and body weight. The mean and reported in hyperthyroid and euthyroid cats in association with
median doses were 3.60 GBq and 3.70 GBq, respectively. Three both benign and malignant tumors. 251
dogs in this study developed fatal bone marrow suppression. 234 Hyperthyroidism was not recognized as a clinical disorder in
It is notable that the dose used in this study was more than twice cats until 1979, and evidence indicates that the prevalence has
that used in the previous study. The advantage of external beam increased since that time. 252 The prevalence has been reported to
RT is that the radiation dose can be evenly distributed through- be as high as 10% in older cats in the United States 253 and as high
out the tumor. The advantage of 131 I is that metastatic lesions as 12.3% in parts of Europe. 254 These high prevalence rates may
also can be treated with this therapy. RT should be considered in reflect a true increase in incidence over time, heightened aware-
cases of invasive thyroid carcinoma because prolonged STs may ness and testing by veterinarians, or both. If the incidence truly
be possible. has increased, environmental factors may have contributed. 239
The prognosis for dogs with unilateral mobile thyroid carci- Environmental factors (e.g., the availability of commercially pre-
noma is excellent with surgery alone, with a reported MST of 36 pared cat food) may also help explain why prevalence rates vary
months and a 70% 2-year survival rate. 214 Tumor diameter, tumor between geographic regions. 239 Several risk factors have been vari-
volume, and bilateral location have been associated with metastatic ably associated with hyperthyroidism, including the consumption
disease, and gross and histologic evidence of vascular invasion of commercially prepared canned cat food, the iodine content of
were associated with a reduced disease-free survival. 208 The role cat food, indoor residence, use of cat litter, exposure to bromi-
of chemotherapy is not well defined with this disease. In general, nated flame retardants, and use of flea-control products. 252,255–258
large tumors with evidence of vascular invasion or metastasis are None of these has been definitively shown to be a primary incit-
treated with systemic chemotherapy. Carboplatin most commonly ing cause, and the evidence for the role of some of these risk
is used. However, no definitive evidence exists that chemotherapy factors remains contradictory. The cause of feline hyperthyroid-
improves STs in dogs with thyroid carcinoma. One study showed ism may well be multifactorial; however, many authors consider
no benefit to the addition of chemotherapy for dogs that had been that the consumption of canned food likely plays a significant
treated surgically for thyroid carcinoma. 216 A recent study showed role. 239,254,259–263 The molecular events underlying the develop-
that dogs with thyroid carcinoma treated with isotretinoin 9-cis ment of feline hyperthyroidism have been investigated, 264–270 but
postoperatively had an increased survival rate compared to dogs currently no single unifying hypothesis explains the development
treated with surgery alone or surgery with adjuvant doxorubi- and increasing prevalence of this disease in cats.
cin. 235 The response to toceranib has been reported in dogs with The following discussion reviews the clinical features, diagno-
metastatic thyroid carcinoma in a phase I study of this therapy for sis, and treatment of feline hyperthyroidism, highlighting new
solid tumors in dogs. A clinical benefit was noted in 12 of 15 dogs information and including information pertaining to the malig-
with thyroid carcinoma, with four dogs having a partial response nant form of the disease. Many excellent and detailed reviews are
and eight dogs having stable disease. 219 This has not been reported available for the reader who wishes to read in more depth about
as first-line therapy but may be considered in cases of metastatic this common feline endocrinopathy. 239,240,253
disease. For dogs with functional thyroid tumors, if surgery, RT Hyperthyroidism is a disease of older cats with mean and
or 131 I therapy is not feasible, treatment with methimazole or an median ages in the range of 12 to 15 years. It is rarely diagnosed
iodine-restricted diet could be considered. 236–238 in cats younger than 8 years of age. 239 There is no sex predilection,
but several breeds of cat appear to be at decreased risk compared
Thyroid Gland Neoplasia in Cats to domestic short-hair and long-hair, nonpurebred cats appear to
be at increased risk. 259,271 The classic clinical signs of feline hyper-
Hyperthyroidism (thyrotoxicosis) is the most common endocrine thyroidism are well described and readily recognized by most cli-
disorder in cats. 239,240 The closest human counterpart to this dis- nicians. The “typical” hyperthyroid cat would be expected to be
ease is toxic nodular goiter. 241 Feline hyperthyroidism is most often losing weight despite polyphagia, hyperactive, and possibly have
caused by a primary thyroid abnormality that results in the pro- GI signs such as vomiting and/or diarrhea, in addition to polyuria
duction and secretion of excessive thyroxine (T ) and triiodothy- and polydipsia. Physical examination in affected cats may reveal a
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ronine (T ). Multinodular adenomatous hyperplasia is identified palpable thyroid nodule (goiter), tachycardia, cardiac arrhythmia,
3