Papillary Thyroid Carcinoma Arising Within a Follicular Adenoma: A Case Report, Ultrasound Features, and Considerations.

Papillary thyroid carcinoma (PTC) arising within a follicular adenoma is an exceptionally rare histopathological subtype that shows the nuclear features of PTC within a benign-appearing, circumscribed follicular adenoma. Although it is clinically significant because of its malignant nature, its ultrasonographic features have not been described previously. A 26-year-old man presented with a heterogeneous isoechoic solid nodule with a marked hypoechoic solid focus and a low echoic peripheral rim in the right thyroid gland on ultrasonography. On color Doppler images, markedly increased vascularity was seen in the hypoechoic solid component. An ultrasound-guided gun biopsy suggested a follicular neoplasm. In the evaluation of a hemithyroidectomy specimen, thyroid follicles and a thin fibrous capsule were seen, which favored a follicular adenoma with tumor cells in a focal area indicating papillary carcinoma. Here, we report a case of PTC arising within a follicular adenoma and discuss possible considerations in thyroid ultrasonographic examinations.