Facing the Block and False Positives in Mohs Surgery: A Retrospective Study of 2,198 Cases

BACKGROUND: "Facing the block" is defined as trimming tissue on a microtome from peripheral or deep surgical margins before sectioning. Facing includes trimming performed intentionally but unnecessarily, accidentally, or out of necessity because of one's choice of embedding and freezing method. OBJECTIVE: To assess whether facing the block is associated with a greater number of stages to clear tumor and, by inference, a significant false positive rate. MATERIALS AND METHODS: A retrospective analysis was performed comparing the average number of stages per Mohs case at two surgical facilities with the same physician over a 1-year period. Site A histotechnologists intentionally face the block, whereas site B histotechnologists do not. Tissue thickness lost during trimming at each site was recorded for 4 weeks. RESULTS: Comparing Sites A and B, stages per case were 1.92 and 1.53, respectively (p < .01) and trimming depths before the first section were 325 and 187 μm (p < .01). CONCLUSION: Facing the block is associated with 0.39 more stages per case and, by inference, a false-positive rate of approximately 39%. Mohs surgeons should evaluate the methods of tissue preparation that their histotechnologists use. Facing the block should not be performed.