Metastatic hobnail variant of papillary thyroid carcinoma: A diagnostic challenge in cell block preparation.

Hobnail variant of papillary thyroid carcinoma (HV-PTC) is an unusual entity recently included in WHO classification of endocrine tumors (2017) and proposed as an aggressive variant of PTC. Compared to patients of classical counterparts, HV-PTC frequently has extrathyroidal extension, exhibits nodal or distant metastasis, and responds poorly to radioiodine treatment, leading to increased mortality. We hereby describe the cytohistological and immunohistochemical features of a metastatic HV-PTC in 55-year-old male, previously diagnosed as poorly differentiated papillary thyroid carcinoma in thyroidectomy specimen. Five years after total thyroidectomy with radical neck dissection the patient presented with gross pleural effusion showing multiple lung parenchymal and pleural based lesions with complete collapse of lung on computed tomography scan. The conventional cytology of pleural fluid showed dyscohesive cells arranged in micropapillary form gave the suggestion of metastatic papillary carcinoma. But the cell block preparation highlighted >30% hobnail cells arranged in micropapillary pattern showing increased atypical mitosis and occasional pseudoinclusions. Supplemented with immunohistochemistry (CK19, TTF-1, and p53), final diagnosis HV-PTC was made.