P151 Tumour thickness and depth of invasion in cutaneous squamous cell carcinoma

Abstract Cutaneous squamous cell carcinoma (cSCC) is the second most common invasive human cancer. Total deaths attributable to cSCCs are now similar to those for melanoma. Current staging poorly predicts which tumours will become locally advanced or metastatic. The Royal College of Pathologists reporting dataset for cSCC includes both tumour thickness (TT) and depth of invasion (DoI). TT and DOI are frequently used interchangeably; however, they are distinctly different pathological parameters. TT is measured from the surface of the invasive squamous cell carcinoma for an exophytic tumour, and from the ulcer base for an ulcerated tumour to the deepest point of invasion. While four methods of measuring DoI are commonly used, the American Joint Committee on Cancer (AJCC) 8th edition recommends measuring from the granular layer of adjacent, normal-looking skin to the deepest tumour cell. Both parameters have been shown to correlate with disease-specific mortality. Since June 2022 we have reported both TT and DoI for cSCCs. All cSCCs are reviewed at the multidisciplinary team, with outcomes recorded. We conducted a retrospective database review to investigate the relationship between DOI and TT at our centre. In total, 627 cSCCs were included, with AJCC stages pT1 (68.1%), pT2 (16.3%) and pT3 (15.6). The values for the TT, DoI and difference are shown in the Table. Using the Brigham and Women’s Hospital (BWH) tumour staging for cSCC, 63.5% were stage T1, 25.4% T2a, 9.7% T2b and 1.4% T3. The TT, DoI and difference are shown in the Table. This study highlights the variance between DOI and TT and the utility of considering both when staging cSCC.TableTumour thickness (TT), depth of invasion (DoI) and difference by tumour classificationStageTT (mm), mean (range)DoI (mm), mean (range)Difference (mm), mean (range)AJCC pT13.6 (0.6–35)2.3 (2–6)1.29 (−1.1 to 32) pT26.7 (1.7–25)3.2 (0.5–6)3.48 (0–20) pT311.8 (2.5–61)8.2 (0.1–40)3.7 (−0.2 to 57.4)BWH T13.6 (1–28)2.4 (0.6–10)1.2 (−1 to 32) T2a7.8 (1.7–53)4.2 (0.6–10)3.7 (0.1–18) T2b10 (1.8–61)7.1 (1.8–40)3.1 (0–57.4) T3 9.1 (4–17)7.8 (4–14)1.3 (−0.2 to 4)AJCC, American Joint Committee on Cancer; BWH, Brigham and Women’s Hospital.