Abstract

Cutaneous squamous cell carcinoma in situ (SCCIS) is a common malignant neoplasm, typically presenting in older patients as a slow-growing, well-demarcated plaque on sun exposed skin. At least 4 histologic patterns of SCCIS have been described, including full-thickness (FT), hypertrophic actinic keratosis (HAK)-type, acantholytic, and Bowenoid-type (BT). While estimated that 3-5% of all SCCIS lesions will progress to an invasive squamous cell carcinoma (iSCC), the risk of progression by subtype has not been assessed. The primary aim of our study is to quantitatively evaluate the relationship between each histologic pattern of SCCIs and iSCC. We queried our pathology information management system with search terms “invasive AND squamous AND in situ,” from 2020-2022. Only cutaneous SCC was included. Anogential sites and keratoacanthoma-type SCCs were excluded. Glass slides were then independently reviewed by three pathologists who categorized the predominant histologic pattern of SCCIS in each case as either FT, HAK-type, acantholytic, or BT. iSCCs were classified as low-grade (LG = well-differentiated) or high-grade (HG = moderately to poorly differentiated). 62 cases from 60 patients met the inclusion criteria. The percentages of iSCCs by pattern of SCCis were as follows: 43.5% FT, 46.8% HAK-type, 8.1% acantholytic, and 1.6% BT. Correlation of iSCC grade with FT and HAK-types was assessed by Chi-square with Fisher’s exact correction. There were insufficient acatholytic and Bowenoid type of SCCis to analyze. Five LG and 22 HG iSCCs occurred in association with FT SCCis compared with 19 LG and 12 HG in association with HAK-type SCCis (p < 0.01). Our data suggests that all histologic patterns of SCCIS can give rise to iSCC with a higher risk of high grade invasive squamous cell carcinoma associated with the full-thickness type of SCCis than the HAK-type SCCis. More data is being acquired to establish the relative risk of each SCCIs subtype for invasion.

Financial Disclosure:
No current or relevant financial relationships exist.