Introduction

A 13 yr old male neutered Bengal cat was referred to BluePearl Veterinary Partners – Spring for evaluation of a ventral cervical swelling. The cat was initially presented to the referring veterinarian 4–5 days after the swelling was first noted. He was reported to be sleeping more than usual and would occasionally vomit, but he was otherwise apparently healthy. Approximately 12 mL of serosanguinous fluid was drained from the swelling through fine-needle aspiration. The mass was tentatively diagnosed as a seroma at that time. The swelling never fully resolved, and the patient was re-presented to the referring veterinarian ∼1 mo later for re-evaluation. The ventral cervical swelling had returned, and the patient also developed an edematous swelling associated with the left forelimb. There were also two large ulcerated areas now present on the lateral thorax. After performance of a serum biochemistry panel, which was entirely within normal limits, the patient was sedated to clip and clean the ulcerations, and the cervical swelling was drained for a second time. About 25 mL of serosanguineous fluid was collected and evaluated cytologically. There were red blood cells and occasional white blood cells, but there were no neoplastic cells seen on an in-house microscopic evaluation. The patient was given subcutaneous (SC) injections of cefovicin sodium^a (8 mg/kg) and dexamethasone (0.2 mg/kg) and instructed to start robenacoxib^b (0.8 mg/kg per os q 24 hr × 3 days) 2 days later. A thyroid cyst was suspected and referral for surgical removal was recommended.

Case Report

The patient was presented to BluePearl Veterinary Partners – Spring 6 weeks after the cervical swelling was initially noted. During this time, the only clinical signs reported from the owner were decreased activity and an increasing frequency of vomiting. His appetite had gone unchanged. On initial examination, a large, firm, SC, apparently right-sided mass was palpable in the ventral cervical area. The remainder of the physical examination was unremarkable, other than a few scabs over the left scapula. A complete oral examination was not performed as a result of the patient’s temperament. Complete blood count and serum biochemistry were performed, the results of which were all within the reference intervals. Differential diagnoses for the cervical swelling at this time included neoplasia, seroma, abscess, cyst, hematoma, salivary mucocele or other outflow obstruction, or granuloma.

The patient returned to the clinic the following day and was anesthetized for computed tomography of the cervical area and thoracic cavity. Under general anesthesia, a complete oral examination was performed and found to be unremarkable. Images with and without intravenous contrast (diatrizoate^c 2.2 mL/kg) were obtained and reviewed by a board-certified radiologist (Figure 1). Computed tomographic imaging revealed a large rounded-to-oval–shaped cystic structure in the right ventral aspect of the thorax in the region of the right thyroid gland. The mass measured ∼5.5 cm in length, 4.6 cm in width, and 4.4 cm in height and was causing significant leftward deviation of the trachea and hyoid apparatus. A small amount of gravity-dependent cellular debris was noted within the cystic structure. There were no abnormalities in the lung fields, including no evidence of nodular lung disease to suggest metastasis of any kind. The mass appeared well defined with clear margins on all sides, and surgical removal was recommended.

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Surgical removal of the mass was performed 2 days later. The patient was given maropitant citrate^d (1 mg/kg SC) and fentanyl^e (4 μg/kg intramuscularly) as premedications. Anesthesia was induced with fentanyl (4 μg/kg IV) and propofol^f (5mg/kg IV) and maintained with isoflurane^g in 100% oxygen. A standard ventral midline cervical approach was performed. The incision was continued on the right paramedian to midline between the sternocephalicus and the sternothyroideus muscles as a result of the deformation of the normal structures by the mass effect. These structures were retracted bilaterally with a Lonestar retractor, carefully avoiding damage to the carotid sheath. A large, ∼8 × 6 cm ovoid cystic structure was noted in the expected location of the right thyroid-parathyroid complex. It was well vascularized and deeply adhered to the trachea and caudoventral aspect of the larynx on that side. No leakage of fluid was noted from the mass itself during the procedure. The left thyroid complex was palpably normal at the time of surgery, but the surrounding tissues were not dissected to visualize directly. No palpable thyroid complex was noted on the right side after mass removal. The mass was carefully dissected free from its attachments with blunt dissection from a right-angle hemostat and curved mosquito hemostats, as well as bipolar electrocautery. Routine closure was performed. The fascia of the sternocephalicus and sternothyroideus muscles were closed with 2-0 polydioxanone^h in a simple continuous pattern and the subcutis was closed with 3-0 poliglecaprone 25ⁱ in a continuous intradermal pattern.

The patient recovered well from anesthesia. Initial postoperative care consisted of intravenous balanced replacement fluid^j (45 mL/kg/day), fentanyl (3 μg/kg/hr) intravenously for analgesia, and cold compress application to the incision site every 6 hr. Slight swelling associated with the incision was noted the morning following surgery, but the patient otherwise recovered well. The patient was discharged with instructions for strict activity restrictions and a soft e-collar to be worn until the incision was healed. Robenacoxib (0.8 mg/kg per os q 24 hr × 3 days) and buprenorphine^k (0.01 mg/kg transmucosally q 8–12 hr × 7 days as needed for pain) were prescribed to take at home.

The tissues excised were evaluated histologically and were consistent with thyroid carcinoma (Figure 2). Histologically, the mass was well circumscribed and partially surrounded by the thyroid capsule. The normal thyroid tissue was replaced by a highly cellular neoplasm of polygonal cells arranged in compact areas with a few cells forming follicles containing variable amounts of colloid. Cells had poorly distinct borders and small amounts of cytoplasm. Nuclei were oval to round, had finally stippled to homogenous chromatin, and had one-to-two small nucleoli. Invasion into the thyroid capsule with extension of cells into the surrounding connective tissue was occasionally seen. Mitotic figures averaged 5 per 10 high-power fields (40×). The mass also presented a central cystic cavity lined by cells similar to those described above. Angiolymphatic invasion by neoplastic cells was not detected histologically, and the margins were narrow at <1 mm.

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The cat in this report was referred to an oncologist for further treatment recommendations. Treatment with radioactive iodine (I131), which is often recommended for benign thyroid tumors was recommended. Other treatment options discussed at that time included injectable chemotherapy agents such as carboplatin or no further treatment with monitoring for regrowth of the tumor. No additional treatment was pursued by the owner. The patient was evaluated by his primary veterinarian 6 mo after surgery for reasons unrelated to the surgical procedure. At this visit, there was no evidence of regrowth of the cervical mass, and there were no obvious macroscopic signs of pulmonary metastasis noted on thoracic radiographs. The patient was reportedly clinically normal with no obvious signs of mass regrowth 1 yr post thyroidectomy; because of the patient’s age and otherwise apparently healthy status, the owners have elected to not perform any further diagnostics.

Discussion

Thyroid tumors are the most common endocrine tumor in canines, with incidence reports ranging from 1 to 4%; carcinomas represent 60–90% of these thyroid tumors.¹ Over a 10 yr period, 638 thyroid tumors were recorded from the information submitted to the Veterinary Medical Database that included data for 257,347 dogs.¹ Thyroid tumors accounted for 1.1% of neoplasms reported during this time period, with 90% of these thyroid tumors being classified as carcinoma or adenocarcinoma.¹ In dogs, there has been no reported sex predisposition, but by the time of diagnosis, most dogs are between the ages of 10 and 15 yr old.¹

A retrospective study performed at the German Zoological Gardens looked at spontaneous disease in 38 captive wild felids who had postmortem examinations performed between 2004 and 2013.¹ Kidney disease was the most frequently observed finding, seen in 87% of the wild felids, followed by various neoplasms in 50%. The most common group of organs affected by neoplasms in this study were endocrine organs (11/34, 32).² Thyroid tumors were the second most common tumor in this group of animals with 5 felids having adenomas and 1 having a carcinoma.² Additionally, a study looking only at neoplasms in felids was conducted at the Knoxville Zoological Gardens over a period of 24 yr (1979–2003).² In that study, endocrine tumors were the second most common tumor, accounting for 10 of the 40 tumors identified, with the thyroid being the endocrine organ most often involved (7/10). Only one thyroid carcinoma was diagnosed in this population of 26 wild felids.³ A similar study predating the Knoxville study was performed at the Philadelphia Zoological Gardens using data from 1901 to 1955.³ During this time, five thyroid carcinomas were identified in wild felids.⁴ Out of the three studies mentioned, the Philadelphia group had the most confirmed cases of thyroid carcinoma. The author of that study notes that this may be a result of lack of iodine supplementation in the wild felids’ diets as all cases occurred prior to 1950, when supplemental iodine was first introduced to feline diets.⁴ The median age of the felids with endocrine tumors at the German Zoological Gardens was 19 yr.² This number is slightly higher than previous averages of 14.5 and 14.9 yr of age for malignant and benign tumors, respectively, reported in the Philadelphia study and 15.8 yr of age in the Knoxville study.^3,4 These reported averages include cats who were identified with any form of neoplasia at the time of necropsy.^3,4

Thyroid carcinomas are rare in domestic cats and account for 1–3% of cats diagnosed with hyperthyroidism; these cats may exhibit clinical signs of hyperthyroidism such as polyphagia with weight loss and secondary hypertrophic cardiomyopathy.^2,4,5 One wild felid in the Knoxville collection had evidence of cardiac hypertrophy, which was speculated, but not proven, to be related to hyperthyroidism.³ The remaining wild felids in the three previously mentioned studies did not exhibit such clinical signs associated with thyrotoxicosis.^2–4 Although truly a domestic cat, the breed origin of the Bengal cat (domestic cat × Asian leopard cat [Prionailurus bengalensis]) should be considered when comparing these findings of wild felids and the traditional domestic cat. The Bengal cat described in this paper did not exhibit signs of weight loss or change in appetite. An echocardiogram was not performed to diagnose hypertrophic cardiomyopathy; however, no heart murmur was ever appreciated, suggesting that this tumor was less likely functional. Thyroid function tests were not performed on this patient, although other researchers have reported that thyroid tests, specifically on the cystic fluid did not correlate with that of serum T4 concentrations.⁶

The main clinical sign in the Bengal cat was occasional vomiting, which was likely a result of the mass effect of the large swelling. Miller et al. described similar findings in 40 cats who had thyroid cysts, with the number or severity of clinical signs correlating with the size of the cyst.⁷ To our knowledge, this is the largest study looking at cats with thyroid cysts, spanning over a time period of 11 yr with four institutions reporting data.⁷ This study included an exotic Savannah cat, but no Bengal cat was reportedly diagnosed with a thyroid cyst in this study population.⁷ The most common clinical signs described by Miller include dysphagia, regurgitation, cough, and laryngeal paralysis as a result of compression of surrounding structures by the large mass.⁷ Other findings, such as polyphagia, polyuria, and polydipsia, were attributed to hyperthyroidism, none of which the Bengal cat described in this paper had.⁷ The majority of palpable thyroid cysts in which histopathology has been performed have been reported histologically as adenomas; however, it is important to note that cystic changes can also be found with thyroid carcinomas.⁷ A cystic thyroglossal duct should also be considered for swellings in the ventral cervical area.⁷ In Miller’s study, thyroid cysts were most often associated with a history of hyperthyroidism, but the cysts were also found in cats with nonfunctional thyroid tumors with a lesser frequency.⁷ Thyroid cysts in this study were associated with both benign (4/40) and malignant (4/40) tumors.⁷ The percentage of functional and nonfunctional tumors was the same whether the tumor was benign or malignant, 75% and 25%, respectively.⁷ Because malignancy cannot be determined based solely on functionality, surgical removal is recommended; although rarely described in veterinary medicine, surgical removal is the treatment of choice in humans.⁷ Miller also mentions the possibility of treating thyroid cysts in cats with ethanol sclerotherapy based on the successful outcomes in cats with renal and hepatic cysts, but more research is needed before this should be considered.⁷

Distinguishing between an adenoma and a carcinoma with histopathology is important regarding treatment and prognosis.⁵ Nuclear scintigraphy is often used to diagnose hyperthyroidism in cats, but it cannot be used reliably to diagnose thyroid carcinoma. However, accumulation of pertechnetate or radioiodine within the mediastinum or pulmonary parenchyma is highly suggestive of metastasis.⁶ Ectopic thyroid and primary pulmonary tumors should be ruled out because these may also show increased uptake of radionuclide.⁶ There is little research available on thyroid carcinomas in cats; however, high malignancy is associated with thyroid carcinomas in dogs with 16–60% of dogs having regional lymph node or pulmonary metastasis at the time of diagnosis.¹ There are several treatment options proposed for dogs with thyroid carcinomas. Treatment choice is dependent on size and invasiveness of the tumor and if metastasis is present.⁸ Small tumors have been able to be surgically removed with reports of median tumor free time of 36 mo.⁹ However, for tumors that are nonresectable or incompletely excised, external beam radiation has been associated with median survival times of up to 2 yr.⁹ High-dose radioiodine combined with thyroidectomy is common therapy in humans with thyroid carcinoma.⁵ In humans, surgery followed by radioiodine therapy is often considered curative, with 60–95% of patients alive 10 yr after treatment.⁹ In a study including 39 dogs with nonresectable thyroid tumors treated with I131, median survival time for dogs with local tumors was longer than those with metastasis (839 and 366 days, respectively), but overall prolonged survival times were seen with I131 therapy.⁹ Radioiodine therapy acts on functional residual tumor as well as metastatic cells and can be used on any thyroid tumor that is able to accumulate organic iodine.^5,9 Higher doses are required for carcinomas compared with adenomas because malignant cells do not uptake or retain I131 as well as benign cells, and they are typically much larger in size.⁵ In a study by Hibbert et al., 6 out of 8 cats who received high-dose I131 therapy (a single dose of 1100 MBq of I131 administered SC) had a decrease in thyroid hormone levels and a median survival time of 814 days.⁵

Conclusion

Differentials for ventral cervical swelling in a cat should include cyst (thyroid, parathyroid, branchial, thyroglossal duct), adenoma (thyroid or parathyroid), carcinoma (thyroid or parathyroid), soft tissue sarcoma, abscess, and salivary mucocele.⁹ Until more research is made available in regards to feline thyroid carcinoma, veterinarians may look to the information available in canine literature to help guide their treatment plans, but no definitive statements regarding therapy and ultimate prognosis can be made. Appropriate diagnostics should be performed to determine the origin of the mass and the next steps in regard to treatment.