Laryngotracheal presentation of anaplastic thyroid carcinoma with squamous differentiation: seven cases demonstrating an under-recognized diagnostic pitfall.

To describe a series of anaplastic thyroid carcinomas that mimicked primary head and neck squamous cell carcinoma (HNSCC) by virtue of both morphology and clinical presentation. Seven cases were identified in a 15-year period where a biopsy of an airway lesion that appeared to be squamous cell carcinoma was, in fact, anaplastic thyroid carcinoma. The tumours had squamous and/or spindle cell morphology, with only the squamous component being apparent in the airway biopsy. Some tumours arose within metaplastic (n = 3) or atypical (n = 3) epithelium, supporting the diagnosis of a primary mucosal tumour. Positive PAX8 (n = 5) and TTF-1 (n = 4) staining was identified. An endotracheal presentation of anaplastic thyroid carcinoma with squamous morphology may be misdiagnosed as a primary head and neck squamous cell carcinoma. PAX8 and TTF-1 expression are helpful in making the distinction, but the problem lies in suspecting a thyroid carcinoma in what appears to be a straightforward diagnosis of HNSCC.