Department of Clinical Sciences of Companion Animals, Faculty of Veterinary Medicine, Utrecht, The Netherlands
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.
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
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.
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
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.
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