Cytomorphologic and molecular features of hobnail variant of papillary thyroid carcinoma: case series and literature review.

Recent reports indicate that hobnail papillary thyroid carcinoma (HPTC) is a rare, but very aggressive variant of papillary thyroid carcinoma. The authors describe the cytological features of five HPTC on fine-needle aspiration biopsies (FNAB). Moreover, their immunophenotype and the presence of B-RAF mutation by pyrosequencing were investigated. The patients' (three females and two males) age ranged from 27 to 86 (mean 65) years. Tumor size ranged from 2 to 9 cm (mean 4.2 cm). FNAB were highly cellular with a bloody background and scant colloid. The cells were arranged in papillary-like clusters or in micropapillary groups. The cell population consisted of medium-sized cells with "tear-drop" cytoplasm, apically placed nuclei that produced a surface bulge leading to a hobnail appearance. At higher magnification, nuclei showed variable degrees of atypia, occasional pink intranuclear pseudoinclusions, and grooves. Nuclear stratification and atypical mitotic figures were usually present. Immunocytochemistry revealed positive staining for thyroglobulin, thyroid transcriptor factor-1, Hector Battifora Mesothelial Antigen-1, partial loss of E-cadherin expression, and nuclear expression of p53 protein. B-RAF mutation was present in three out of five cytological cases. Immunohistochemical and molecular results were confirmed on histological sections. Recognizing the unique cytological features of HPTC should help to avoid misdiagnosis of this rare variant.