A potential diagnostic pitfall for hobnail variant of papillary thyroid carcinoma

Aims Hobnail variant of papillary thyroid carcinoma (PTC) is an aggressive PTC subtype characterised by a hobnail cytomorphology. However, some classic PTC have a ‘hobnail‐like’ cytomorphology associated with thick, hyalinised, variably oedematous fibrovascular cores that appears to be a form of ischaemic/degenerative atypia. Methods and results We studied three cohorts to compare the histopathological characteristics and clinical outcome of ‘hobnail‐like’ classic PTC and true hobnail variant of PTC: cohort 1, PTC consecutively resected between 2016 and 2017 (to assess frequency of ‘hobnail‐like’ cytomorphology); cohort 2, 20 ‘hobnail‐like’ classic PTC resected between 2005 and 2007 (to assess clinical outcome); and cohort 3, seven true hobnail variant of PTC. A ‘hobnail‐like’ cytomorphology was identified in 16% of consecutively resected PTC. Compared with true hobnail variant, ‘hobnail‐like’ classic PTC occurred in younger patients (mean age 40 years versus 68 years, P