Abstract

Surgery has a major role in the treatment of thyroid tumors in dogs. Whether surgery is curative depends on the histological tumor type and the disease stage. Radio-iodine treatment, external beam radiation, or chemotherapy are treatment options for cases with unresectable or metastasized disease.

Introduction

Clinically detectable thyroid tumors in dogs are most often large (> 3 cm) and malignant (> 85 % of cases). The mean age of dogs presented with a thyroid tumor is 9 years (range 5–15 years). There is no sex predilection. Boxers, Beagles and Golden Retrievers are reported to be overrepresented.^1-3

Canine thyroid tumors develop either from the follicular cells (follicular carcinomas) or from the parafollicular C cells (medullary thyroid carcinomas, less than 5 % of thyroid tumors in dogs). Epithelial (follicular) carcinomas commonly metastasize to the lungs and regional deep cervical lymph nodes and to many other organs.^2,3

A canine thyroid tumor which results in hyperthyroidism is found in only 10 % of cases. Most dogs with a thyroid tumor are euthyroid. Hypothyroidism associated with thyroid tumors in dogs was found to be caused by lymphocytic thyroiditis. Ectopic thyroid tumors may be found in the areas cranial to the larynx or in the cranial mediastinum.^2,4 The production of abnormal proteins from the C-cells of medullary thyroid carcinomas in humans may lead to profuse watery diarrhea in some cases. This effect has been described in canine patients as well.^2,5,6

Clinical Examination

Most patients present with a palpable cervical mass. The differential diagnosis includes abscesses, hematomas, lymphoma, lipomas or other tumors. Compression or invasion of surrounding structures (esophagus, trachea) by thyroid tumor growth will cause other clinical symptoms such as coughing, respiratory distress, dysphagia, vomiting/regurgitation, anorexia, and facial swelling. Tumor invasion of the recurrent laryngeal nerve may cause dysphonia and dyspnea.^3,7 Involvement of the sympathetic nerve may lead to Horner's syndrome.^3,8

Signs of invasive growth are diagnosed during physical examination by moving the solid tumor mass along the trachea during palpation. In cases with firmly fixed tumors surgical treatment may be considered contra-indicated.

The retropharyngeal areas are carefully palpated for the presence of regional metastases. Ectopic thyroid carcinomas may be present cranial to the larynx, or in the cranial mediastinum.^3,9 Ectopic mediastinal thyroid tumors may remain unnoticed with the exception of tumors that cause hyperthyroidism.

Diagnosis and Staging

The functional thyroid status can be established by measuring T₄ and TSH plasma levels.^3,9

Ultrasound of the neck with the animal in dorsal recumbency is helpful in the differentiation of thyroid tumors from abscesses, lymphadenopathies or mucoceles.^10,11 Signs of invasive growth of thyroid tumors into the trachea, esophagus or blood vessels are sometimes visible on ultrasound, but CT and MRI provide more detailed information in this respect.^12,13 For the detection of pulmonary metastases in dogs CT was found to be more sensitive than plain thoracic radiographs.¹⁴

Radionuclide scans are of use to determine the thyroid origin of a tumor. In cases where the primary thyroid tumor (and sometimes also distant metastatic thyroid tissue) is still able to concentrate iodine and therefore trap pertechnetate (^95mTcO₄^-) a 'hot scan' will visualize the tumor and its metastases. A 'cold scan' will be found in cases where tumor tissue has lost the ability to concentrate iodine: at best only the normal thyroid gland will be visible. In hypothyroid cases the thyroid scan shows no thyroid uptake at all.^9,15,16

Canine thyroid tumors are highly vascularized. Fine needle aspiration biopsies therefore contain excessive blood in most cases. The definitive diagnosis is provided by histology of the excised tumor.

Treatment Options

Malignant thyroid tumors with a tumor volume of > 100 cm³ are associated with metastatic disease in all cases.² However, clinical signs of distant metastasis may appear no sooner than after months or even years. Therefore, even in these cases, all freely-movable thyroid tumors are best surgically removed without delay to prevent local invasive growth.

Surgery is performed with the patient in dorsal recumbency with a small neck support and the front legs caudally retracted. The thyroid tumor is exposed via a ventral midline approach. The skin and subcutis are incised and the ventral musculature is separated in the midline. The tumor is carefully dissected from the surrounding structures. After the ligation of the caudal thyroid blood supply, tumor dissection is performed from caudal to cranial using bipolar electrosurgical equipment and scissors. The recurrent laryngeal nerve is protected. The carotid artery or jugular vein may be incorporated in the thyroid tumor mass. These vessels can be ligated and divided without further consequences for the patient. Finally, the cranial thyroid blood supply is ligated and divided to complete the tumor dissection. Special attention is paid to invasive tumor growth in the cranial thyroid vein and to possible regional metastases in the deep cervical lymph nodes in the retropharyngeal space. In cases of bilateral thyroid tumors an attempt is made to identify and spare one of the external parathyroid glands to prevent postoperative hypocalcemia. The surgical wound is sutured in layers with absorbable suture material.^17,18

In dogs with unresectable thyroid tumors external beam radiation therapy may be considered to provide long-term tumor control and a prolonged survival time. Side effects consist of lesions of the skin and oral mucosa.^19,20

The administration of radioiodine (¹³¹I) might be beneficial to extend survival times in cases with unresectable thyroid tumors that have iodine uptake.²¹ Regulatory requirements and prolonged hospitalization times limit the use of radionuclide treatment.

Doxorubicin and cisplatin have been used in chemotherapy protocols for metastasized canine thyroid tumors. However, large prospective studies that prove a prolonged survival time are lacking.^3,9,22

Prognosis

The long-term prognosis after surgical excision of a malignant thyroid tumor is guarded and depends not only on the histological malignancy grade. Undetected metastases might be present in many cases before the therapy is started and will be of major influence on the survival time. Surgery will prevent short-term complications from local invasive tumor growth and might be curative in cases with relatively small or well-encapsulated malignancies. The specific goals and limitations of surgery should be explained to the patient's owner.

References

1.  Brodey RS, Kelly DF. Thyroid neoplasms in the dog. A clinicopathologic study of 57 cases. Cancer 1968;22:406.

2.  Leav I, Schiller AL, Rijnberk A, et al. 1976. Adenomas and carcinomas of the canine and feline thyroid. Am J Path 83:61–122.

3.  Rijnberk A, Kooistra HS. Thyroids. In: Clinical Endocrinology of Dogs and Cats. 2nd ed. Hanover, Schluttersche, 2010:55–91.

4.  Carver JR, Kapatkin A, Patnaik AK. A comparison of medullary thyroid carcinoma and thyroid adenocarcinoma in dogs: a retrospective study of 38 cases. Vet Surg 1995;24:315–319.

5.  Kebebew E, Ituarte PHG, Siperstein AE, et al. Medullary thyroid carcinoma. Clinical characteristics, prognostic factors, and a comparison of staging systems. Cancer 2000;88:1139–1148.

6.  Leboulleux S, Baudin E, Travali J-P, et al. Medullary thyroid carcinoma. Clin Endocrinol 61:299–310.

7.  Harari J, Patterson JS, Rosenthal RC. 1986. Clinical and pathological features of thyroid tumors in 26 dogs. J Am Vet Med Assoc 2004;188:1160–1164.

8.  Melián C, Morales M, Espinosa de los Monteros A, et al. Horner's syndrome associated with a functional thyroid carcinoma in a dog. J Small Anim Pract 1996;37:591–593.

9.  Feldman EC, Nelson RW. Canine thyroid tumors and hyperthyroidism. In: Feldman EC, Nelson RW, eds. Canine and Feline Endocrinology and Reproduction. 3rd ed. St. Louis, Missouri, Saunders, 2004:219–249.

10. Wisner ER, Nyland TG. Ultrasonography of the thyroid and parathyroid glands. Vet Clin North Am Small Anim Pract 1998;28:973–991.

11. Wisner ER, Nyland TG, Mattoon JS. Ultrasonographic examination of cervical masses in the dog. Vet Radiol and Ultrasound 1994;35:310–318.

12. Wang J, Takashima S, Takayama F. Tracheal invasion by thyroid carcinoma; prediction using MR imaging. Am J Roentgenol 2001;177:929–936.

13. Takashima S, Takayama F, Wang J, et al. Using MR imaging to predict invasion of the recurrent laryngeal nerve by thyroid carcinoma. Am J Roentgenol 2003;180:837–842.

14. Nemanic S, London CA, Wisner ER. Comparison of thoracic radiographs and single breath-hold helical CT for detection of pulmonary nodules in dogs with metastatic neoplasia. J Vet Intern Med 2006;20:508–515.

15. Taeymans O, Peremans K, Saunders JH. Thyroid imaging in the dog: current status and future directions. J Vet Intern Med 2007;21:673–684.

16. Marks SL, Koblik PD, Hornof WJ, et al. 99mTc-Pertechnetate imaging of thyroid tumors in dogs: 29 cases (1980–1992). J Am Vet Med Assoc 1994;204:756–760.

17. Radlinsky MG. Thyroid surgery in dogs and cats. Vet Clin Small Anim 2007;37:789–798.

18. Peeters ME. Thyroidectomy. In: van Sluijs FJ, ed. Atlas of Small Animal Surgery. New York: Churchill Livingstone, 1992:20–22.

19. Théon AP, Marks SL, Feldman ES, et al. Prognostic factors and patterns of treatment failure in dogs with unresectable differentiated thyroid carcinomas treated with megavoltage irradiation. J Am Vet Med Assoc 2000;216:1775–1779.

20. Pack L, Roberts RE, Dawson SD, et al. Definitive radiation therapy for infiltrative thyroid carcinoma in dogs. Vet Radiol Ultrasound 2001;42:471–474.

21. Turrell JM, McEntee MC, Burke BP, et al. Sodium I131 treatment of dogs with nonresectable thyroid tumors: 39 cases (1990–2003). J Am Vet Med Assoc 2006;229:542–548.

22. Barber LG. Thyroid tumors in dogs and cats. Vet Clin Small Anim 2007;37:755–773.