Thyroid neoplasia is relatively common, and masses can be quite large and aggressive by the time they are discovered. CT and MR imaging is often helpful in confirming the origin of the mass, and evaluating the extent of invasion into the surrounding tissues and vasculature. These findings can affect surgical approaches and options for treatment for the animal. The imaging features of thyroid neoplasia are described in this excerpt from Section 1.11—Thyroid and Parathyroid Disorders.
Feline functional thyroid nodular hyperplasia and adenoma
Functional benign adenomatous neoplasms and hyperplastic masses are common in the older cat. Because these are usually adequately characterized using other methods, descriptions of the CT and MR appearance of these lesions may have little clinical utility.
On both imaging modalities, thyroid glands are unilaterally or bilaterally enlarged and may include discrete mass lesions or diffuse lobar enlargement. Affected thyroid glands may have irregular margins and cystic components that appear hypoattenuating on CT images and hypointense and hyperintense on T1 and T2 MR images, respectively. The thyroid glands of affected cats are moderately to markedly contrast enhancing and may be nonuniform in appearance.
Thyroid adenocarcinoma
Canine thyroid carcinomas are most commonly unilateral, are usually poorly encapsulated, and aggressively invade adjacent tissues and vessels. A tentative diagnosis is often made before CT or MR imaging is performed, and these studies are most useful for confirming thyroid origin, determining operability, and for specific surgical planning.
On both CT and MR imaging, thyroid carcinomas are often large and may displace or invade adjacent cervical musculature, blood vessels, trachea, larynx, and esophagus. Some tumors appear to be well encapsulated, but others are unconstrained and highly invasive to adjacent tissues. Malignant tumors are typically highly vascular, and the parenchyma often appears heterogeneous and may have cystic and mineralized components. Regional lymph node metastasis is common.
On CT images, thyroid adenocarcinomas are generally isoattenuating to adjacent ventral cervical musculature on unenhanced images, with hypoattenuating and hyperattenuating regions within the parenchyma corresponding to cavitary lesions or mineralization, respectively, when present. Malignant neoplasms are markedly and nonuniformly contrast enhancing, and tumor thrombi may be evident in neoplasms with vascular invasion.
On MR images, tumors are generally T1 hyperintense on unenhanced images and of mixed hyperintensity on T2 images. Contrast enhancement on MR images parallels that seen on CT, with tumor parenchyma markedly and heterogeneously contrast enhancing. Ectopic thyroid tumors may occasionally be encountered in the ventral cervical region or cranial mediastinum. Ectopic thyroid tumors involving the hyoid apparatus are discussed in Chapter 1.10. CT and MR appearances of ectopic thyroid carcinomas are the same as described for in situ masses.
Other ventral cervical masses that can mimic the imaging appearance of thyroid carcinomas include carotid body tumors, hemangiosarcoma, undifferentiated carcinoma, granulomatous lymphadenitis, and paraesophageal abscess.
Homework
Review some of the imaging features and presentations of thyroid neoplasia, and discuss imaging approaches with your clinical team. The Atlas of Small Animal CT and MRI is available through Amazon and has over 700 figures showing case examples to further your learning. Check it out if this article was helpful!
See you next time,
Allison Zwingenberger
Co-Author, Atlas of Small Animal CT and MRI
P.S.
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