Supplementary Material for: Single versus Multiple Level Sectioning for the Subtyping of Basal-Cell Carcinoma: A Retrospective Study

Background: The histological subtype of basal-cell carcinoma (BCC) is often based on a punch biopsy; only a small part is evaluated, possibly leading to misclassification. Consensus on the optimal approach to process punch biopsies is lacking, though accurate subtyping is important for appropriate treatment. Objective: The aim is to investigate whether evaluating 4 levels of a punch biopsy instead of 1 or 2 levels leads to more accurate subtyping of BCC. Methods: In a retrospective study we evaluated 87 punch biopsies of histologically confirmed BCCs. The primary outcome was the proportion of “more aggressive” BCCs (nonsuperficial vs. superficial, infiltrative vs. nodular subtype) that was missed by evaluation on 1 or 2 levels, using 4-level diagnosis as reference standard. Results: Eighty-five cases were available for analysis. Subtyping based on 1 level resulted in discrepancies with 4-level diagnosis in 16.5% of all cases. Underdiagnosis occurred in 14 of 58 nonsuperficial BCCs (24.1%, 95% CI: 13.9–37.2). Seven of 38 nodular BCCs (18.4%, 95% CI: 7.74–34.3) were diagnosed as superficial in 1 level, and 7 of 20 infiltrative BCCs (35%, 95% CI: 15.4–59.2) were diagnosed as superficial (n = 2) or nodular (n = 5) in 1 level. Conclusion: In order to maximize correct subtyping and plan appropriate treatment, we advise to evaluate at least 2, but preferably more, levels of a punch biopsy to determine the BCC subtype.