How do we recognize a difficult squamous cell carcinoma? A retrospective analysis of clinically and dermoscopically misdiagnosed tumours

Background In people with lighter skin phototypes, squamous cell carcinoma (SCC) is typically nonpigmented and must be distinguished from other nonpigmented tumours. Although the dermoscopic features of SCC are well-known, some SCCs are challenging to recognize even with dermoscopy. Objectives To investigate the clinical and dermoscopic features responsible for an inaccurate clinical diagnosis of invasive SCC and potential clues that could help in the recognition of this tumour. Methods We retrospectively screened our institutional database for clinically misdiagnosed SCCs over a 10-year time period (2013-2023). We then presented 10 expert dermoscopists with a series of clinical and dermoscopic images of misdiagnosed invasive SCCs and sought their opinion. Results In total, 73 SCCs from 73 patients (55 men and 18 women) aged 37-97 years (mean 78.8) were included. Most tumours were located on the cheek (21%), followed by the forehead (16%), nose (12%) and scalp (12%). Thirty-seven SCCs were misdiagnosed as basal cell carcinoma, 15 as actinic keratosis, 10 as irritated seborrhoeic keratosis, 7 as Bowen disease, 2 as viral warts and 2 as cutaneous horn. White scales and keratin were voted by the experts as the main features (n = 34/73) that might have helped in the accurate clinical diagnosis of the included SCCs. Conclusions The dermoscopic characteristics of invasive SCC might overlap with other types of tumours. In challenging tumours, the presence of white scales and keratin might guide the accurate recognition of invasive SCC.