Page 598 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

576 PART IV Specific Malignancies in the Small Animal Patient

of the neck is useful as a screening tool for a mass suspicious for common features noted at surgery. 220 The lymphatic drainage
thyroid carcinoma. However, ultrasound was not found to be as of the thyroid gland is to the cranial deep cervical lymph node
(cranial portion) and the caudal deep cervical lymph node (cau-
sensitive or specific as MRI or CT for determining the degree of
VetBooks.ir invasiveness and for confirming thyroid origin. 210 The decision to dal portion). 221 The lymph nodes are extremely small and are not
221
Because of this, elective
aspirate a mass in the region of the thyroid gland under ultrasound
present or identifiable in most dogs.
guidance is clinician dependent. Because a mass definitively aris- lymph node dissection is not routinely performed in dogs with
ing from the thyroid gland has a very high likelihood of being a thyroid carcinoma.
thyroid carcinoma and the mass ultimately will be removed and Surgical removal of a thyroid carcinoma includes a routine
submitted for histopathology, some clinicians do not perform ventral approach to the neck and trachea between the paired ster-
aspiration of the mass if imaging is to be performed. Ultrasound nohyoideus muscles. 220 The thyroid glands should be evaluated
is not as sensitive or specific as a CT scan in determining that a bilaterally. The thyroid mass is identified and removed by blunt
mass is originating from the thyroid gland, 210 so in the absence of dissection, taking care to preserve the recurrent laryngeal nerve
advanced imaging, cytology may be warranted. Fine-needle aspi- and with careful attention to hemostasis using hemoclips, electro-
ration should be done under ultrasound guidance to avoid hemor- cautery, or a vessel sealing device. In the author’s (SB) experience,
rhage; also, because this procedure is painful, the patient should gently retracting the thyroid mass ventrally and working outside
be sedated before thyroid mass aspiration. Thyroid carcinomas of the neck can help protect the critical structures that lie adjacent
are extremely vascular; therefore needle core biopsy and incisional to the thyroid gland. Potential complications of surgery include
biopsy should never be performed, because they carry a high risk hemorrhage and laryngeal paralysis arising from iatrogenic dam-
of severe hemorrhage in a noncompressible area. Aspiration or age to the recurrent laryngeal nerve. Postoperative care for these
biopsy also could lead to the formation of a large hematoma and patients should involve multimodal analgesia, intravenous fluids,
the spread of tumor cells within this area. and monitoring for dyspnea and hemorrhage. Most patients can
Thyroid carcinoma tends to metastasize to the draining lymph be discharged from the hospital within 24 hours.
nodes and lungs, but metastasis to abdominal organs also has been Bilateral thyroid carcinomas have been reported. Two recent
reported. 219 The local lymph nodes (mandibular) should be pal- reports of bilateral, noninvasive thyroid carcinoma treated with
pated and aspirated. Three-view thoracic radiographs also are rec- surgery alone demonstrated STs comparable to those for unilateral
ommended before therapy. Preoperatively a CT scan is very useful thyroid carcinoma. 222,223 One additional potential complication
for determining the origin and invasiveness of the tumor and for of total thyroidectomy is transient or permanent postoperative
staging the disease. CT has been shown to be more sensitive for hypocalcemia. 222 In one study of 15 dogs treated with total thy-
assessing the invasiveness of thyroid carcinomas than palpation roidectomy because of bilateral disease, parathyroid tissue was
or neck ultrasound. 210,211 Masses that have invaded dorsal to the preserved in two dogs, reimplanted in four dogs, and removed
trachea are difficult to assess ultrasonographically because of the completely in nine dogs. In that study postoperative hypocalce-
presence of air in the trachea. Furthermore, because of the high mia occurred in 11 dogs, with only seven dogs requiring long-
iodine content of precontrast and postcontrast thyroid tissue, CT term management of hypocalcemia. 222 Another study reported
has been shown to be useful in diagnosing nonthyroid cervical the intentional preservation of at least one external parathyroid
masses, such as carotid body tumors, because the normal thyroid gland in six cases of total thyroidectomy for thyroid carcinoma.
tissue is more easily identifiable using a contrast-enhanced CT The blood supply was preserved in five dogs, and the parathyroid
scan compared with ultrasound or MRI. 210 CT also is very useful gland was reimplanted in one dog. The MST was not reached,
for assessing the lungs for evidence of metastatic disease. and the mean ST was 920 days, with no evidence of recurrence. 223
Thyroid tumors are categorized as invasive or noninvasive. The Attempts should be made to preserve parathyroid tissue in cases of
invasiveness of a thyroid mass can be determined by a combina- total thyroidectomy in dogs.
tion of palpation characteristics and advanced imaging. In general, Ectopic thyroid carcinoma has been reported in dogs at the
a noninvasive mass is round to ovoid and mobile. Size alone is not base of tongue, involving the hyoid apparatus, cranial medias-
a predictor of whether a thyroid mass is resectable. On imaging, tinum, and the right heart base. 224–228 The treatment approach
evaluation of whether the thyroid mass remains encapsulated or to these tumors is similar to that for eutopic thyroid carcinomas,
invades adjacent structures is important for determining whether with long-term survival reported with surgery alone or with radio-
the mass can be removed. active iodine. 224
The thyroid gland is composed of two lobes that lie adjacent Invasive thyroid carcinomas generally are not amenable to sur-
to the left and right sides of the trachea from the fifth to eighth gery because of invasion of critical structures in the neck, such
rings. 220,221 The recurrent laryngeal nerves run dorsal to the thy- as the trachea, esophagus, carotid artery, and recurrent laryngeal
roid gland, and the carotid sheaths are lateral to the gland. Evalu- nerve. Treatment options for invasive thyroid carcinoma include
ation of laryngeal function before surgery is recommended as a external beam RT and radioactive iodine. External beam RT
baseline for potential damage to the recurrent laryngeal nerve, generally is more available at most facilities. Two reports exist on
as a result either of tumor invasion or of iatrogenic damage at the use of external beam RT to treat thyroid carcinoma in dogs.
the time of surgery. The external and internal parathyroid glands A hypofractionated protocol reported in 13 dogs resulted in an
are intimately associated with the thyroid gland 221 and often are MST of 96 weeks. 229 Interestingly, evidence of pulmonary metas-
obscured completely in cases of thyroid carcinoma. This is of little tasis at the time of diagnosis was not prognostic. 229 Definitive
consequence for unilateral tumors. The primary blood supply and fractionated RT was reported in eight dogs with invasive thyroid
drainage are the cranial thyroid artery and vein. The caudal thy- carcinoma, with a similar MST of 24.5 months. 230 A more recent
roid artery and vein also are significant, and the cranial and caudal study reported an MST of only 170 days for 20 dogs treated with
thyroid arteries anastomose across the surface of the gland. 221 This palliative radiation for advanced thyroid carcinoma. 231 In that
can be quite apparent in cases of thyroid carcinoma with signifi- study, achieving a partial or complete response was the only vari-
cant hypertrophy and neovascularization of the blood supply as able that was prognostic. 231 Hypothyroidism has been reported