CT of the canine thyroid for suspected carcinoma

Why a cervical mass deserves cross-sectional imaging

Not every ventral cervical mass is thyroid. In 23 dogs presented for suspected thyroid carcinoma (Taeymans et al., Veterinary Radiology & Ultrasound, 2013), 17 had thyroid carcinoma. The other six were four carotid body tumours, one paraoesophageal abscess and one undifferentiated squamous cell carcinoma.

Thyroid masses found on CT are usually malignant. In 4520 dogs scanned for any reason (Bertolini et al., Veterinary Radiology & Ultrasound, 2017):

• 96 had a thyroid mass (2.12%), and 79 of these (82.3%) were malignant
• 34 masses were found incidentally, and 24 of those (70.6%) were carcinomas

The authors advise assessing the neck in middle-aged and older dogs having body CT, and sampling incidental thyroid nodules.

Palpation versus CT for invasion

Whether a thyroid mass is mobile or fixed on palpation is often used to judge resectability. The evidence suggests palpation is not reliable on its own:

• In 19 dogs with a thyroid tumour and neck CT (Deitz et al., Journal of Small Animal Practice, 2014), palpable mobility had a sensitivity of 71% and a specificity of 0% for histological invasion. CT signs of invasion had a sensitivity of 70% and a specificity of 100%. All eight tumours with definite or possible invasion on CT were invasive carcinomas on histology.
• In the 23-dog comparison, palpation did not accurately separate locally invasive from encapsulated masses. MRI most often detected capsule disruption with invasion. For the diagnosis of thyroid carcinoma, CT had the highest specificity (100%) and MRI the highest sensitivity (93%). Ultrasound performed considerably less well. The authors recommend ultrasound as a screening tool and CT or MRI for preoperative diagnosis and staging.

Vascular invasion and resectability

Gross vascular invasion does not automatically rule out surgery. In 73 dogs with thyroid carcinoma and gross vascular invasion treated by thyroidectomy (Latifi et al., Veterinary and Comparative Oncology, 2021):

• 5 (6.8%) had intraoperative and 12 (16.4%) postoperative complications
• locoregional recurrence developed in 7 (9.6%) and distant metastasis in 9 (12.3%)
• median overall survival was 621 days, and the one-year disease-specific survival rate was 82.5%

For comparison, in 156 dogs that had unilateral thyroidectomy (Reagan et al., Journal of the American Veterinary Medical Association, 2019), complications occurred in 19.9%, with haemorrhage the most common intraoperative complication (7.7%). Perioperative mortality was 1.9%. Knowing in advance which vessels are involved is usually helpful for surgical planning.

Staging: lymph nodes and lungs

• Regional nodes. In a prospective study of 70 dogs with elective lymphadenectomy at thyroidectomy (Skinner et al., Journal of the American Veterinary Medical Association, 2024), 13 (18.6%) had nodal metastasis. It was more common with medullary (5 of 8) and mixed follicular/medullary carcinoma (3 of 7) than with follicular carcinoma (5 of 61). All medial retropharyngeal metastases were ipsilateral, and 14 of 41 deep cervical nodes were metastatic.
• Distant disease. In 58 dogs staged with CT of the neck and thorax (Giannasi et al., Veterinary Record, 2021), 31% were classed as WHO stage IV. WHO stage was not significantly associated with survival, while surgery was associated with longer survival. The authors note that CT staging found a higher distant metastatic rate than historical studies using other imaging.

Thyroid CT protocol at a glance

The widely used approach is one staging CT of the neck and thorax, as in the 58-dog staging study. The neck scan has to reach far enough to cover the whole range where these tumours occur, which in the 19-dog series ran from the level of the temporomandibular joint to C5. One session lets the report describe the primary tumour, both medial retropharyngeal and deep cervical node regions, and the lungs together.

What the radiologist looks at

In the 19-dog series (17 carcinomas, 2 adenomas), 12 masses were mineralised, 16 were heterogeneous and 16 were unilateral. The radiologist weighs these features:

Finding on CT	What the evidence shows
Attenuation	Carcinomas had lower attenuation than normal thyroid tissue.
Mineralisation, vascular invasion, tissue invasion	Seen only in malignant tumours in the 4520-dog series.
Intratumoural vessels	Significantly associated with malignancy.
Fluid and mineral	Across ultrasound, CT and MRI, carcinomas were large, heterogeneous and moderately to strongly vascularised, and most often contained mineralisation, fluid, or both.

The reader also looks for CT signs of invasion into surrounding tissue and vessels, checks the medial retropharyngeal and deep cervical nodes, and assesses the lungs for distant disease.

What your report will include

Every Veheri case is read by a radiologist or neurologist, and when you order you can choose Diplomate-only reporting or the full specialist panel. You can chat directly with the specialist assigned to your case, and a simplified pet owner report helps you explain the findings to the owner.

When you take the report forward:

• No invasion on CT does not exclude it. CT signs of invasion were specific but not very sensitive.
• Vascular invasion is not a dead end. Surgery remained an option in the 73-dog vascular invasion series, so a report of vessel involvement is a reason for surgical planning, not automatically for declining surgery.
• An incidental thyroid nodule deserves sampling, as the 4520-dog study authors advise.
• Histotype affects nodal risk. Medullary and mixed carcinomas metastasised to nodes more often, so the histology report and the CT node assessment are worth reading together.

What to include when you request a thyroid CT report

• signalment: age, breed and sex
• palpation findings: side, size and whether the mass feels mobile or fixed
• thyroid function results and any clinical signs of hyperthyroidism
• cytology or ultrasound findings, if available
• whether the mass was found incidentally
• the planned treatment, especially unilateral or bilateral thyroidectomy

How Veheri can help

Veheri provides veterinary teleradiology and reports neck and thoracic staging CT remotely for clinics. For staging more widely, see our practical guide to oncology imaging and thoracic CT for pulmonary metastases in dogs.