Dr. Brett Darrow provides the surgery perspective on this challenging oncology case.
Brett Darrow, DVMIn addition to routine staging of dogs with thyroid carcinoma, preoperative considerations should include measurement of coagulation times to rule out a coagulopathy, as these tumors are highly vascular, and blood cross-matching to prepare for transfusion in the event of significant intraoperative hemorrhage.1
Thyroid carcinomas are highly vascular and may be locally invasive. The most important prognostic indicator for the success of surgical excision is the mobility of the tumor on palpation; a mobile tumor will be easily manipulated > 1 cm in all tissue planes.1,2 Immobility of the tumor is generally associated with infiltration into the surrounding tissues including musculature, trachea, esophagus, vascular structures and nerves. The anatomy of the area may be distorted as a result of the expansile nature of the tumor, making identification of normal structures difficult. The vascular supply to these tumors can be highly variable, although the carotid artery often supplies a large proportion of blood to the tumor. Additionally, the tumor capsule and associated neovascularization can be extremely fragile, resulting in hemorrhage with gentle manipulation. Hemostasis is best achieved with delicate use of bipolar electrocautery, stainless steel hemoclips or a bipolar vessel sealing device.1,2
General practitioners can confidently perform a thyroidectomy, but referral to a board-certified surgeon or radiation oncologist should be considered if the tumor is immobile or fixed (< 1 cm mobility in all planes), bilateral, large (> 7cm) or in an ectopic location.1,2 Surgery may not be a good treatment option for immobile and certain bilateral tumors, or if the esophagus or any other vascular, nervous, or muscular structure is involved.2
The surgical approach to the thyroid glands is relatively simple. A ventral mid-line incision is made from the caudal larynx to the manubrium of the sternum. Blunt and sharp dissection is performed to reach the paired sternohyoideus muscles lying on the ventral surface of the trachea. These muscles have a ventral midline separation, connected by a faint fascial plane, which must be separated. After separating these muscles, the thyroid tumor should be easily visible, although smaller tumors may require additional blunt dissection on the side of the tumor.
The contralateral thyroid gland should be assessed for normalcy.1 If it appears abnormal, a wedge biopsy should be performed. The area surrounding the tumor should be explored to identify critical structures and tumor blood supply. This may be difficult initially if the tumor is highly invasive. Wide surgical resection is not recommended as the tumor is usually contained with a capsule and is surrounded by peri-tracheal fascia.1 Additionally, a wide surgical excision may result in damage to the important local anatomy with unacceptable complications.
En bloc resection of the tumor is performed, which requires ligation of the cranial and caudal thyroid arteries, as well as any neovascular branches to the tumor. One entire thyroid, along with the associated internal and external parathyroid glands, may be removed without ensuing hypothyroidism or hypoparathyroidism. In a small number of dogs, the thyroid glands may be connected by a ventral isthmus of tissue.1 In this case, I prefer to perform a partial thyroidectomy with a 1-cm margin of normal thyroid tissue removed with the tumor. Additionally, the medial retropharyngeal lymph nodes lie just cranial to the thyroid glands and are a common site of metastasis. If they are enlarged or contrast-enhancing on preoperative imaging, removal is typically performed at the time of surgery.
Bilateral tumors may account for up to 30% to 60% of thyroid tumors.3 Recently, two publications have reported excellent clinical outcomes with bilateral thyroidectomy due to mobile tumors.4,5 Both studies described favorable survival times, no local recurrence and low metastatic rates despite the presence of numerous previously established negative prognostic factors including bilateral disease,6 gross evidence of tumor thrombi,7 and tumor volumes > 21 cm3.8 Both studies concluded that the parathyroid gland should be preserved for prevention of postoperative hypocalcemia via a modified extracapsular thyroidectomy when possible, although this was not critical to outcome. If the parathyroid vascular supply cannot be preserved, the parathyroid may be reimplanted into the sternohyoideus musculature (wholly intact or morsalized into 1 mm3 bits), where it can revascularize and function adequately.2,4,5 Postoperatively, few dogs required long-term calcium or calcitriol supplementation, but most dogs did require life-long levothyroxine supplementation.
Surgical removal of ectopic thyroid carcinomas depends on the location of the tumor and the surrounding anatomy. Recently, one study demonstrated that infiltration of the hyoid apparatus in the case of canine sublingual ectopic thyroid carcinoma could be treated successfully with partial hyoidectomy.9 No postoperative dysphagia or respiratory compromise was noted in these patients.
Complications associated with thyroidectomy for thyroid carcinoma occur when normal structures are not able to be preserved. The unilateral vagosympathetic trunk and recurrent laryngeal nerve can be sectioned with only a small risk of aspiration pneumonia; however, bilateral transection of both recurrent laryngeal nerves will result in laryngeal paralysis with a high likelihood of respiratory distress and aspiration pneumonia.1,2 Transection of both vagosympathetic trunks will likely result in severe cardiovascular and gastrointestinal alterations, resulting in poor quality of life and eventual death.10 Dogs (but not cats) can survive after ligation of both common carotid arteries11,12 or ligation of both external jugular veins.13 However, it is unknown whether simultaneous sacrifice of both common carotid arteries and both external jugular veins is tolerated. Other complications include hemorrhage (minor or life-threatening), infection, recurrence of neoplastic disease and, commonly, seroma formation.
Overall, surgery is the treatment of choice for freely moveable thyroid carcinomas; survival times have been reported to be approximately 1.5 to three years after surgical removal.14,15
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