Ultrasound-guided fine-needle aspiration biopsy of thyroid neoplasms with lipomatous stroma: Report of two cases.

Thyroid tumors with abundant adipose tissue component are rare, reportedly accounting for 0.98-2.8% of all thyroid nodules, and include entities such as thyroid lipoadenoma and thyroid carcinoma with lipomatous stroma (TCLS). They may be encountered on fine-needle aspiration biopsy (FNAB), which is widely used in evaluation of thyroid nodules. However, due to their relative rarity, adipose elements rarely are recognized preoperatively in these tumors. Herein, we report two cases of thyroid tumors with abundant adipose tissue, along with cytologic, histologic, and ultrasonographic features. Although an intermixture of adipose tissue and thyroid follicular cells is the key cytologic feature of thyroid tumors with adipose stroma, other cytologic findings, such as abundant fat droplets or isolated fragments of adipose tissue, also should raise the possibility of a fat-containing tumor, particularly when a biopsy is performed by a cytopathologist under ultrasonographic guidance and adequate radiologic-pathologic correlation. Cytopathologists should be aware that overlooking lesional adipose tissue within a thyroid neoplasm might give the false impression of a non-diagnostic or sparsely cellular FNAB specimen.