Cytological features of clear cell thyroid tumors, including a papillary thyroid carcinoma with prominent hobnail features.

Primary thyroid tumors with extensive clear cell changes are extremely rare. More than 10,000 ultrasound-guided thyroid fine needle aspirations examined over a period of 17 years by the first author, only one of the 530 (<0.2%) papillary thyroid carcinomas (PTCs) and three of the 42 (7.1%) follicular thyroid carcinomas (FTCs) had extensive cytoplasmic clearing. Thyroidectomies were performed for these cases at four different hospitals in New York City. Final pathology was available for review in each of the four cases. Histology showed a 1.5-cm PTC with prominent hobnail features and clear cell change in a 31-year-old woman, a 4.3-cm FTC without angioinvasion in a 31-year-old woman, a 4.5-cm angioinvasive FTC in a 45-year-old man, and a 2-cm FTC with extensive angioinvasion in a 41-year-old woman with McCune-Albright syndrome (previously published). On ultrasound, the FTCs were solid circumscribed nodules and the PTC had an irregular margin. In these cases, the etiology for the cytoplasmic clearing included accumulation of glycogen in the PTC, accumulation of vesicles in two of the FTCs, and accumulation of lipid droplets in a FTC with extensive angioinvasion. Review of the cytologic literature showed 17 cases of follicular-derived thyroid tumors with extensive clear cell change. To the best of our knowledge, this is the first cytologic report of PTC with hobnail features and extensive clear cell change.