7-year-old spayed female Golden Retriever

Cervical radiographs – orthogonal projections provided. Findings: A poorly defined soft tissue opaque mass measuring ~ 3.1 x 4.7 cm is present in the right lateral cervical soft tissues ventral to the C3 and C4 vertebral bodies, mildly displacing the trachea ventrally at this location. The central portion of the mass on the right side has an amorphous region of mineral opacity. Focal ventral bulge in the soft tissues ventral to the hyoid apparatus, consistent with the previous surgical site. Primary differential is a neoplastic mass such as a thyroid carcinoma.

Cervical CT – pre-contrast (axial) and post-iodinated contrast images (axial, sagittal and dorsal) reformatted images. Volume data was acquired from a 16 slice MDCT and reformatted into 2 mm slices (even thicker slices for this format). Findings: A focal external bulge is present within the right ventrolateral subcutaneous tissues just caudal to the right mandible surrounding a distended right external jugular. This focal bulge has a density of ~ 25 HU and peripheral rim contrast enhancement, consistent with a pocket of dense fluid, that likely represents a seroma from the previous surgical site. A focal area of stippled mineral attenuation is identified in the cervical soft tissues at the plane of the C3 vertebra on the right side dorsolateral to the trachea, measuring ~ 2.2 cm in diameter. A few other focal punctate areas of mineral attenuation are present in the right ventrolateral soft tissues and vasculature of the neck adjacent to the trachea.

The external jugular veins are distended and displaced dorsolaterally from their expected anatomic locations. At the plane of C1-2, the right jugular vein contains a soft tissue attenuating mass within the lumen that extends cranially where the vein is severely distended ~ 2.6 cm. The majority of the vasculature surrounding the trachea contains soft tissue attenuating material. All of the tissues surrounding the trachea contrast enhance and soft tissue outside the vasculature cannot be differentiated from the dilated vessels filled with soft tissue. This area of soft tissue and vasculature contrast enhancement measures ~ 12.8 cm craniocaudally, 4.7 cm dorsoventrally, and 6.1 cm mediolaterally. Numerous tortuous vessels are identified throughout the cervical region and likely represent dilated collateral circulation. A portion of the mass abuts the cervical esophagus at the level of the C2 vertebra.

The medial retropharygneal and mandibular lymph nodes are normal in size and exhibit mild contrast rim enhancement. Normal thyroid glands are not identified, if they are present they do not contain enough iodine that would normally allow their identification.

Surgical pathology:

1. Thyroid carcinoma with osseous metaplasia.
2. Focal granulation tissue and hematoma (prior surgical site).
3. Invasive thyroid carcinoma into the cervical esophagus.
4. Chronic draining hemorrhage of the medial retropharyngeal and mandibular lymph nodes.

Comment: The primary neck mass is a thyroid carcinoma that has infiltrated the surrounding tissue, including the subepithelial tissue of the esophagus. There was also a chronic, healing focus consistent with the previous surgery site. Lymph nodes had evidence of draining hemorrhage, likely from the surgery site. Osseous metaplasia implies alternative differentiation of local cells into osteogenic cells as a response to some insult (eg traumatic, neoplastic, metabolic).

DISCUSSION

The incidence of thyroid carcinomas has been reported to be 1.1% in a recent study, and is the most common endocrine tumor in dogs. Carcinomas and adenocarcinomas represented 90% of the thyroid neoplasms. Older dogs (range of 5-18 years), golden retrievers, beagles and Siberian huskies are overrepresented. This patient was a 7 year old golden retriever and fits this population.

Thyroid carcinomas have been reported to invade into adjacent tissues such as the trachea, larynx, esophagus, cervical musculature and regional neurovascular structures. The tumor can invade into the cranial and caudal thyroid veins leading to tumor thrombi formation. Less than 25% of dogs have clinical or biochemical evidence of hyperthyroidism, like this dog with a mildly elevated T4. Metastasis is common, reported to be up to 33% with evidence at the time of diagnosis and 65-90% at necropsy. There was no evidence of pulmonary nodules on thoracic CT and no lymph node enlargement in this patient.

Lymph drains cranially from the thyroid glands, so the mandibular, parotid and medial retropharyngeal lymph nodes should be evaluated for evidence of metastatic disease. In this patient, there was no histopathological evidence of metastatic neoplasia in the lymph nodes, however the peripheral rim enhancement noted on CT may correlate with the histopathology of draining a site of hemorrhage (previous surgery or tumor site).

REFERENCES

1. Wucherer KL et al. 2010. Thyroid cancer in dogs: an update based on 638 cases (1995-2005). JAAHA; 46:249-254.
2. Liptak, JM. 2007. Canine Thyroid Carcinoma. Clinical techniques in small animal practice; 22(2): 75-81