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BACKGROUND Management of basal cell carcinoma (BCC) varies by histopathologic subtype; however, biopsies may inadequately characterize them as nonaggressive, risking potential suboptimal treatment. OBJECTIVE To characterize the rate of undetected aggressive BCC subtypes by size, location, and histopathology type. MATERIALS AND METHODS Retrospective cohort study of 928 BCCs treated with Mohs Micrographic Surgery (MMS) at a tertiary academic institution from 2015 to 2017, comparing patient and tumor characteristics and histopathologic subtype on biopsy versus Mohs. RESULTS Among the 825 BCCs with known subtypes on biopsy, 68% (561/825) were classified as nonaggressive, 28% (159/561) of which were subsequently found to have aggressive subtypes on MMS. Aggressive features were more often underrepresented in biopsy samples taken from Area H compared with Area M/L (odd ratio [OR] 2.65, 95% confidence interval [CI] 1.73-4.08, p , .001) or those with nodular subtypes (OR 2.19, CI 1.08-4.45, p 5 .03). Of concern, these unsuspected aggressive BCCs required more Mohs stages for clearance (mean 2.37, SD 0.72, p , .001) compared with BCCs that remained nonaggressive on both biopsy and Mohs (mean 1.50, SD 0.75). CONCLUSION Given the high percentage of BCCs with unsuspected aggressive subtypes, higher clinical suspicion for undiagnosed high-risk BCCs should be given to nodular BCCs and to BCCs on Area H.

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