Abstract
A 5-year-old spayed female cat was evaluated for hyperthyroidism based on an elevated free thyroxine (T4) measurement and bilaterally enlarged symmetric subcutaneous masses in the area of the thyroid glands. Physical examination revealed bilateral subcutaneous masses on either side of the cervical trachea. Blood was obtained for serum biochemical profile and thyroid function analysis. Mild hyperalbuminemia, mild hypercalcemia, and mildly increased alanine aminotransferase activity were identified. Serum concentrations of total and free thyroxine were within the reference interval. Cytologic analysis of fine-needle aspirates from one of the masses was suspicious for neoplasia. Nuclear scintigraphy revealed no abnormalities. Surgically obtained excisional biopsies of both masses were submitted for histopathology and diagnosed as bilateral dermoid cysts. After excisional biopsy, the patient recovered without incident. The histopathologic diagnosis of completely excised bilateral dermoid cysts indicated that no further medical or surgical intervention was required. This is the first report of a cat presenting with bilateral dermoid cysts in the area of the thyroid glands. Histopathologic examination was necessary to make a definitive diagnosis. Practitioners should include cysts in their list of differential diagnoses for ventral neck masses in cats.
A 5-year old, 5.0 kg (11 lb) spayed female domestic shorthaired cat was referred to the Veterinary Teaching Hospital at the University of Georgia for evaluation and radioactive iodine treatment of hyperthyroidism. The referring veterinarian had diagnosed hyperthyroidism 8 weeks prior to presentation on the basis of an elevated serum free thyroxine by equilibrium dialysis (fT4) concentration (56 pmol/l; reference range, 10–50 pmol/l) and compatible clinical signs of polyuria and polydipsia and slight weight loss. The serum thyroxine (T4) concentration was 3.1 μg/dl (reference range 0.8–4.0 μg/dl).
The cat was bright, alert, and responsive on presentation. The heart rate, temperature, and respiratory rate were 160 beats/min, 101.1°F (38.4°C), and 32 breaths/min, respectively. The cat was in good body condition (body condition score 2.5/5) and two 0.5×1.0 cm subcutaneous masses were palpated on either side of the cervical trachea. There was no cervical lymphadenopathy. No other abnormalities were noted on physical examination. Differential diagnoses considered for the masses included thyroid adenomatous hyperplasia, cysts (ie, thyroid, parathyroid), neoplasia (thyroid or parathyroid adenoma/adenocarcinoma/cystadenoma, squamous cell carcinoma, lipoma, lymphoma), abscess, and salivary mucocele. Initial diagnostics included a complete blood count (CBC), serum biochemical profile, urinalysis, and serum T4 and fT4 measurements. Additionally, fine-needle aspiration with cytologic analysis of both masses was performed. The CBC revealed an elevated red blood cell count (12.59×106/μl; reference range, 5–10×106/μl), which was attributed to hemoconcentration secondary to dehydration. Serum biochemical abnormalities included hyperalbuminemia (4.4 g/dl; reference range, 3–4.3 g/dl), elevated alanine aminotransferase (ALT) activity (135 U/l; reference range, 31–104), and hypercalcemia (11.5 mg/dl; reference range, 9–10.8 mg/dl). The hyperalbuminemia and hypercalcemia were attributed to hemoconcentration, although hypercalcemia of malignancy was also considered. The mildly increased ALT activity was initially attributed to suspected hyperthyroidism. Urinalysis revealed trace proteinuria, an inactive sediment, and marked concentration (USPG>1.060) which was supportive of dehydration. Results of the serum T4 and fT4 were within the reference range at 2.6 μg/dl (reference range 0.8–4.0 μg/dl) and 13 pmol/l (reference range 10–50 pmol/l), respectively. An ionized calcium measurement, performed to more specifically evaluate the hypercalcemia, was within the reference range at 1.18 mmol/l (reference range 1.1–1.4 mmol/l), further supporting hemoconcentration rather than hypercalcemia of malignancy. Thoracic radiography was performed to evaluate for possible metastasis and findings revealed normal pulmonary vasculature and a normal cardiac silhouette with no evidence of metastasis. Cytologic analysis of modified-Wright-stained fine-needle aspirates from the left cervical mass revealed a sparsely cellular sample composed of a population of large cells that varied from round to polygonal to caudate with eosinophilic cytoplasm containing blue, and less frequently black or green, granules that ranged from small and discrete to large aggregates. There was moderate anisocytosis and anisokaryosis with frequent single nucleoli present. The cells present were suspected to be epithelial in origin, and the moderate cellular atypia was suggestive of a neoplastic population. Cytologic analysis of modified-Wright-stained fine-needle aspirates from the right cervical mass revealed abundant keratin and cellular debris that was compatible with aspiration of a keratin-producing cyst or tumor. As only a minimal amount of tissue was obtained via aspiration, a sample was not submitted for biochemical analysis. Because of the historically elevated fT4 and clinical signs compatible with feline hyperthyroidism, nuclear scintigraphy was recommended to evaluate for functional thyroid tissue and to provide for surgical planning prior to exploratory and excisional biopsies. Approximately 2.9 mCi of sodium pertechnetate Tc99m was administered intravenously and a gamma camera was used to obtain images with a low-energy collimator. The zygomatic salivary glands were more photoavid than the thyroid glands, and no ectopic thyroid tissue was identified. The examination was read as normal with a thyroid to salivary gland ratio of 0.5–0.75 on the left side and 0.48–0.6 on the right side. Taken together, these findings suggested that the cervical masses were not comprised hyperplastic or functional thyroid tissue. Surgery was performed following radioactive clearance. Two solitary masses (1×0.5 cm) were identified adjacent to, but independent of, both the thyroid and parathyroid glands which appeared grossly normal. The masses were superficial to the sternohyoideus muscles and were attached to the deep surface of the skin. Both masses were excised with blunt dissection ( Fig 1 ), and the attachment to the skin was ligated and excised with each mass. The cat recovered without incident. Histopathology revealed that the two masses were similar and consisted of a single large cyst composed of a wall of well differentiated stratified squamous epithelium encircling a large central cavity containing lamellar keratin and hair fragments. Small hair follicles and sebaceous glands, some of which clearly extended into the cyst cavity, surrounded the cyst ( Fig 2 ). The surrounding dermis contained loose collagen with focally dense mixed inflammatory infiltrates, comprised primarily of lymphocytes and macrophages with fewer mast cells and neutrophils, associated with infrequent keratin fragments.
Dissection of the right dermoid cyst.
Dermoid cyst, cat. The cyst wall contains a folliculosebaceous unit oriented perpendicular to the cyst cavity filled with lamellar keratin. Hematoxylin and eosin 40×.
Although no thyroid tissue was identified on the aspirates from the right cervical mass and cytologic findings from these aspirates were compatible with a keratin-producing cyst or tumor, the results of the cytologic analysis of aspirates from the left cervical mass were more confounding. Although the aspirates were sparsely cellular, the atypia detected within the epithelial cell population raised the concern for neoplasia. Excisional biopsy with histopathology was recommended and pursued, resulting in the final diagnosis of bilateral dermoid cysts. It is suspected that the atypical cells detected on cytology represented reactive epithelial cells with melanin and/or keratohyalin granules aspirated from the cells lining or surrounding the cyst.
Dermoid cysts (DC) are benign follicular structures, classified as congenital inclusion cysts, which are small solitary duplications of skin and, therefore, derived from mesodermal and ectodermal germ layers. 1–3 Histologically, DC are lined by stratified epithelium with surrounding mesodermal components, including sebaceous glands, hair follicles, smooth muscle, and/or fat tissue.3,4 The invagination of the dermoid cyst is completed by a surrounding layer of collagen which envelopes the cystic structure, and the cyst cavity is often filled with keratin debris and hair fragments and may also contain sebaceous secretions. 3 A dermoid cyst may resemble a follicular cyst or dilated pore of Winer clinically and must be differentiated by histopathologic exam. 5 Follicular cysts arise from the lining of the hair follicle and are further classified by the level of the follicle from which they arise. A key distinguishing feature of dermoid cysts is the presence of multiple hair follicles which are connected to the cyst wall and whose lumen opens into the cyst; these are notably absent in follicular cysts. 5
Dermoid cysts are rare in cats but this congenital anomaly has been reported in dogs, with the Kerry Blue terrier, Shih Tzu, Boxer, and Rhodesian Ridgeback breeds being over-represented.3,6,7 Dermoid cysts are suggested to be an inherited, simple recessive trait in Rhodesian Ridgeback dogs where they generally detected in younger animals and occur as solitary cystic structures on the dorsal midline and may extend into the spinal canal.3,7 Dermoid cysts are found most commonly on the shoulder and lateral neck in cats. 3 The origin of DC is controversial but they are thought to arise from an incomplete separation of the ectoderm from the neural tube during embryologic development.2,3,8 A classification scheme has been developed in human medicine which divides congenital dermoid cysts into four groups based on their location and origin of development. 9 Cysts belonging to group 1 arise in the periorbital region. Group 2 cysts develop over the dorsal surface of the nose. Group 3 cysts develop on the floor of the mouth, as in the region of first and second branchial arch fusion. Group 4 cysts are thought to develop during fusion of the thyroidal, suprasternal, and suboccipital regions. 9 The cat presented in the current case report would be best classified as group 4, which is characterized by cysts occurring in the dorsal or ventral neck region. Surgical excision is recommended for histopathologic diagnosis and to prevent secondary infection or complications associated with cyst enlargement.4,9
The initial finding of an elevated fT4 was most likely a result of daily fluctuation, non-thyroidal illness, or a false-positive result as both repeat measurements of T4 and fT4 and radionuclide imaging were within normal limits. 6,10–12 Non-thyroidal illness may cause an elevation in fT4 through displacement of thyroxine from serum binding protein.10,11 False-positive fT4 test results are reported to occur in approximately 5–15% of euthyroid cats. 11
For the cat in the current case report, the previous elevation of free thyroxine, the presence of subcutaneous masses in the ventral neck, and the cytologic evaluation suggestive of epithelial neoplasia provided some confusion which lead to a diagnostic workup for a thyroidal neoplasm. A literature search of dermoid cysts in cats revealed a paucity of cases, with no reported cases occurring in the vicinity of the thyroid gland.6,8 Despite the rarity of the condition, dermoid cysts should be included in the list of differential diagnoses for bilateral ventral cervical masses.