Thyroid Ultrasound > Top 10 Pathology > 4. Follicular Carcinoma

Follicular Neoplasms

Follicular adenomas are encapsulated true neoplasms of the thyroid gland and represent about 5% to 10% of all thyroid nodules. Differentiation of follicular adenoma from a follicular carcinoma is based on the presence of capsular or vascular invasion on histologic examination and thus cannot be made by sonography or by FNA cytology.

On sonography, follicular adenomas and follicular carcinomas are usually solitary encapsulated tumors, often with a well-defined peripheral hypoechoic halo representing the fibrous capsule. Two histologic patterns of growth of follicular carcinomas can be observed. The minimally invasive type is encapsulated, in contrast to the widely invasive type, which extends beyond the tumor capsule into blood vessels and adjacent parenchyma. Echogenicity is variable and follicular neoplasms can be echogenic, isoechoic, or hypoechoic. Echogenic adenomas are often smoothly marginated and ovoid in appearance, prompting use of the term ‘‘pseudotesticle’’ (see below image from Ultrasound Clinics Apr 2009) to describe their appearance. In contrast to papillary carcinomas, follicular carcinomas metastasize hematogenously to bone, lung, brain, and liver rather than by way of lymphatics.

Hurthle (oncocytic) cell tumors (see above image from Ultrasound Clinics Apr 2009) are a variant that is more aggressive. The benign vs malignant nature of these must be determined and if malignant, total thyroidectomy and I131 with thyroglobulin assays is needed.

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