History

A 26-kg (57.2-lb) spayed female Labrador Retriever–crossbreed dog was presented for routine examination and vaccination. The dog's age was 7 years and 2 months. The owners reported that the dog had no history of major medical problems. However, the dog did have multiple subcutaneous masses that had been diagnosed as lipomas previously on the basis of cytologic examination of fine-needle aspirate specimens. The dog's vaccination status was current, and it was receiving a heartworm preventive. A flea and tick preventive was administered orally each month.

Clinical and Gross Findings

On physical examination, the dog was in good body condition and had an appropriate weight for its size. It was normothermic, had a heart rate of 96 beats/min and respiratory rate of 30 breaths/min, and was euhydrated. During the examination, a previously unidentified, 5-cm-diameter, subcutaneous mass was palpated on the right side of the dog's neck (Figure 1). The mass had been noticed by the owners approximately 1 month earlier, and it appeared to be getting larger. The mass was soft, fluctuant, and movable, similar to a lipoma in form and texture. During palpation of the mass, no signs of pain were elicited from the dog and no communication between the mass and underlying structures was felt. A fine-needle aspirate specimen of the mass was collected for examination.

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Photograph of a subcutaneous mass on the right side of the neck of an approximately 7-year-old Labrador Retriever–crossbreed dog. The dog had a history of multiple subcutaneous masses that had been diagnosed as lipomas previously on the basis of cytologic examination of fine-needle aspirate specimens.

Citation: Journal of the American Veterinary Medical Association 258, 2; 10.2460/javma.258.2.157

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Serum biochemical analyses, a CBC, urinalysis, and 3-view radiography of the dog's neck and thorax were performed. Serum biochemical analyses included assessment of concentrations of glucose, total protein, albumin, urea, creatinine, sodium, potassium, symmetric dimethylarginine, thyroxine, triiodothyronine, and free thyroxine and activities of alanine aminotransferase and alkaline phosphatase; all variables were within their respective reference interval. The CBC revealed a PCV of 50.1% (reference interval, 38.3% to 56.5%); WBCs and platelets were present in adequate numbers. The urinalysis revealed mild proteinuria (1+; 100 to 200 mg/dL) and a specific gravity of 1.037. Radiographically, mild changes consistent with aging or chronic airway disease were evident, and the cardiac silhouette was of normal size and shape. No evidence of metastatic disease was detected.

Cytologic and Histologic Findings

The cellular material in the fine-needle aspirate specimen was inconsistent with that of a typical lipoma. The specimen was submitted to a commercial laboratory for cytologic examination. Cytologic examination of the mass revealed indistinct aggregates of epithelioid cells with scattered individual cells. Cells each had a centrally placed, round nucleus with a finely stippled chromatin pattern. Cytoplasmic junctions within aggregates were poorly defined. Rarely, epithelial cells were arranged in acinar-like structures. Cells contained a moderate amount of basophilic cytoplasm with faint eosinophilic granulation. Anisocytosis and anisokaryosis appeared mild (Figure 2). Although it was difficult to definitively identify these cells, the suspicion was that the cells were of thyroid origin on the basis of the location of the mass and the morphology of the cells. Given the cytologic findings, a tentative diagnosis of epithelial neoplasia was made. The decision was made to progress with surgery to obtain a biopsy specimen for further histologic evaluation and, if possible, remove the mass in its entirety.

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Photomicrographs of a slide preparation of a fine-needle aspirate specimen obtained from the dog's subcutaneous neck mass. A—Notice the indistinct aggregates of epithelioid cells with scattered individual cells; cytoplasmic junctions within aggregates are poorly defined. Some epithelial cells are arranged in acinar-like structures. B—Cells each have a centrally placed, round nucleus with a finely stippled chromatin pattern and a moderate amount of basophilic cytoplasm with faint eosinophilic granulation. There is mild anisocytosis and anisokaryosis. Wright-Giemsa stain; bar in each panel = 10 µm.

Citation: Journal of the American Veterinary Medical Association 258, 2; 10.2460/javma.258.2.157

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An excision was made around the mass with 1-cm margins in all planes. During surgical exploration, the mass was noted to be ovoid in shape, measured 5.5 × 3 cm, had a smooth texture, and was heavily vascularized. In addition, the mass appeared well circumscribed, fluctuant, and freely movable. There was no obvious tissue connection or communication between the mass and underlying structures including the thyroid gland. No other tumors or nodules were identified near the mass or in association with the thyroid gland at the time of surgery.

Histologic analysis confirmed the mass to be a thyroid carcinoma and revealed proliferative epithelioid cells forming packets, cords, nests, and small follicular structures, all supported by a fine fibrovascular stroma. The cells were cuboidal to polygonal, each with eosinophilic cytoplasm and a round nucleus with > 1 small nucleolus. There was mild atypia with 11 mitotic figures observed in 10 hpf (100X). The mass was partially disrupted by zones of necrosis and hemorrhage. Small nests of tumor cells partially infiltrated the outer capsule but were not observed to extend into adjacent tissue. Several foci of suspected vascular invasion were present. A thin rim of compressed thyroid tissue was identified at the periphery of the mass. Small amounts of colloid production were noted. No parathyroid tissue was observed. There were narrow, tumor-free surgical margins, with the lesion extending to within < 1 mm from the borders (Figure 3).

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Photomicrographs of sections of the subcutaneous ectopic thyroid carcinoma. A—A thin rim of compressed residual thyroid tissue (star) is present at the periphery of the mass. H&E stain; bar = 700 µm. B—Many portions of the mass have a solid (compact) growth pattern. However, in some areas the cells form follicular structures containing eosinophilic secretory product consistent with colloid (arrows). H&E stain; bar = 200 µm.

Citation: Journal of the American Veterinary Medical Association 258, 2; 10.2460/javma.258.2.157

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Morphologic Diagnosis and Case Summary

Morphologic diagnosis and case summary: follicular thyroid carcinoma arising from subcutaneous ectopic thyroid tissue in the ventral aspect of the neck of a dog.

Comments

Thyroid tumors typically develop in older dogs and are considered relatively common, representing 1% to 3% of all neoplasms in dogs.^1,2 Although ectopic thyroid tissue is not uncommon in dogs, being identified in 50% of adult dogs during necropsy, tumors arising from ectopic thyroid tissue are rare.^2,3,4 Ectopic thyroid carcinomas in the cranial mediastinum, sublingual area and base of the tongue, and heart base and subcutaneously located in the ventral neck region of dogs have been reported.^{1,2,3,4,5,6,7} In dogs, most ectopic thyroid carcinomas develop in the heart base, cranial mediastinum, or both,^7,8 whereas subcutaneous ectopic thyroid tumors in the ventral neck region may be overtly visible or palpable. Ectopic thyroid tumors that are not visible, such as those located at the heart base, can be diagnosed with technetium Tc 99m pertechnetate scintigraphy.⁹

Similar to thyroid carcinomas in the thyroid gland, thyroid carcinomas in ectopic thyroid tissue usually do not secrete thyroxine; thus, affected dogs usually have serum thyroxine concentrations that are within the reference interval.² Thyroid carcinomas arising from ectopic thyroid tissue or from the thyroid gland are commonly malignant.^1,2,3 Ectopic thyroid tumors often do not metastasize; however, metastasis to the lungs has been reported.⁸ In dogs, clinical signs of thyroid carcinomas may include coughing, rapid breathing, dyspnea, dysphagia, change in bark tone or volume, and facial edema.^1,2,3 These signs are usually attributable to the mass effect of the tumor and may be a function of the tumor's location. Severe hemorrhage from a thyroid carcinoma that invaded major thoracic vasculature has been reported.¹⁰ A thyroid tumor in the neck region could also cause severe hemorrhage if the mass infiltrated the cranial vena cava. Dysphagia has been reported in ectopic thyroid carcinoma involving the base of the tongue.⁵ In the case described in the present report, the ectopic thyroid tumor was located near the vena cava but appeared encapsulated and noninvasive.

Ectopic thyroid carcinomas can be treated with radiation therapy, chemotherapy, or surgical excision, singly or in some combination. The dog of the present report was treated successfully with excisional surgery followed by systemic chemotherapy involving carboplatin at a conservative dosage of 250 mg/m², IV, once every 3 weeks for a total of 6 treatments.

The differential diagnoses for a freely movable, subcutaneous mass in the ventral neck region include lipoma, salivary mucocele, abscess or granuloma, thyroid tumor, and lymphoma. In the case described in the present report, the mass was initially suspected to be a lipoma because it was soft, fluctuant, freely movable and well circumscribed on palpation and because the dog had a history of lipoma development in that body region. However, findings of cytologic examination of a fine-needle aspirate specimen ruled out lipoma and supported a tentative diagnosis of epithelial neoplasia.

Cytologic findings can be used to diagnose a thyroid origin of tumors in < 50% of cases of thyroid carcinoma or ectopic thyroid carcinoma in dogs.^2,3 However, cytologic examination of fine-needle aspirate specimens is useful in that results may define the tumor as endocrine in origin, necessitating further diagnostic workup. Confirmatory diagnosis of ectopic thyroid carcinoma requires histologic evaluation of biopsy specimens.^2,3

Veterinary practitioners often diagnose lipomas on the basis of palpation alone. However, examination of a fine-needle aspirate specimen can help distinguish a lipoma from a more malignant subcutaneous tumor. Fine-needle aspiration is quick to perform, relatively noninvasive, and inexpensive. Fine-needle aspirate specimens of lipomas often appear as oily smears on a microscope slide and wash off with alcohol fixatives. Intact adipocytes on slides can sometimes be observed with the use of a commercially available triple stain. Although this method may not confirm that a subcutaneous tumor is a lipoma because adipocytes and fat cells may mask an underlying lesion, it can make the diagnosis of a lipoma more likely. If visible material remains on the test slide, it can be submitted to a commercial laboratory for histologic analysis. This may be especially important for freely movable masses in the ventral neck region because this area is a common site for ectopic thyroid carcinoma development. Overall, cytologic examination of fine-needle aspirate specimens is an important screening tool for suspected lipomas in clinical practice.