Occult carcinoma confirmed to be a diffuse sclerosing variant of papillary thyroid carcinoma with unusual immunohistochemical features: A pitfall of clinicopathological diagnosis.

Immunocytochemistry in a 78-year-old man diagnosed as having systemic metastatic cancer of unknown primary origin revealed atypical cells positive for napsin A and TTF-1, suggesting adenocarcinoma of the lung. However, there was no evidence of a primary lesion in the lung on positron emission tomography/computed tomography or at autopsy. Meanwhile, both the left and right thyroid lobes were firm and grayish white with marked fibrosis. Histology identified a diffuse sclerosing variant of papillary thyroid carcinoma that was positive for TTF-1 and napsin A but negative for PAX8. This disease entity is often misdiagnosed clinically as chronic thyroiditis. This is the first report of napsin A-positive and PAX8-negative thyroid carcinoma and highlights the pitfalls of clinicopathological diagnosis.