Follicular squamous cell carcinoma is an under-recognised common skin tumour

Current thinking assumes most cutaneous squamous cell carcinoma arise from the surface epidermis. Uncommon pilar and tricholemmal carcinomas are well recognised and recently authors have drawn attention to uncommon squamous cell carcinoma with predominant follicular infundibular differentiation. In contrast we propose that follicular (infundibular-tricholemmal) squamous cell carcinoma is exceedingly common and can be defined as follows: cytologically malignant tumour, abrupt connections to the epidermis (at follicular infundibula), infundibular and/or tricholemmal differentiation and lacking co-existent Bowen's disease or distinctive clinical-pathological features of keratoacanthoma. Recognition that many cutaneous squamous cell carcinoma are of follicular origin has major implications for differential diagnosis, staging, prognosis, management and future research.