Mohs Margins in Stage-1-Positive Basal Cell Carcinoma Are Often Histologically Shallow Irrespective of Biopsy Subtype: A Retrospective Analysis Using Histologic Depth as a Surrogate for In Vivo Imaging Detectability.

BACKGROUND: Presurgical biopsy subtype guides basal cell carcinoma (BCC) risk stratification, but how well it predicts histopathology at the Mohs margin is unclear. Noninvasive imaging with reflectance confocal microscopy (RCM), line-field confocal optical coherence tomography (LC-OCT), and optical coherence tomography (OCT) may detect margin-localized tumor within imaging depth. OBJECTIVE: The aim of this study was to quantify low-risk-only histopathology at stage-1 positive Mohs margins by biopsy subtype and estimate histology-based amenability to RCM, LC-OCT, and OCT. METHODS: This retrospective analysis included 108 primary BCCs treated with Mohs micrographic surgery from January 2021 through November 2022. Cases with first-stage margin positivity and retrievable Mohs slides were reviewed. Potential imaging amenability was estimated as a histologic surrogate, using the depth from the granular layer to the tumor top with modality-specific thresholds of RCM ≤ 150 μm, LC-OCT ≤ 300 μm, and OCT ≤ 720 μm. RESULTS: Overall, 63.9% of cases showed low-risk-only histopathology at the stage-1 positive margin, whereas only 33.3% were classified as low-risk-only on biopsy. Histology-based imaging amenability was 55.6% for RCM, 65.7% for LC-OCT, and 76.9% for OCT. Among high-risk biopsy cases, estimated amenability remained substantial: 51.4%, 63.9%, and 75%, respectively. CONCLUSION: Biopsy subtype underpredicted the low-risk-only Mohs margin phenotype. Using histologic depth as a surrogate, many first-stage margin breaches were shallow enough to fall within modality-specific depth thresholds, including in high-risk biopsy tumors. As no in vivo imaging was performed, these findings estimate potential detectability and support feasibility and trial design rather than demonstrating imaging performance.