Background Squamous cell carcinoma (SCC) in situ (SCCIS) is often treated without any pathologic confirmation of tumor clearance. It is unclear how often an invasive SCC is harbored within a lesion in which the initial biopsy demonstrated SCCIS because of inadequate sampling. This study examines the final histologic diagnosis of cases in which the initial biopsies were diagnosed as SCCIS and evaluates factors that may correlate with a histologic upstaging of the diagnosis. Methods We prospectively recruited 29 consecutive patients with biopsy‐proven SCCIS sent for Mohs micrographic surgery (MMS). Each tumor underwent MMS, and the central blocks of the Mohs debulking specimens were horizontally sectioned at 30‐μm intervals until exhausted. A fellowship‐trained Mohs surgeon and a board‐certified dermatopathologist processed and examined these sections to determine the final histologic diagnosis of the tumor. Results Of the 29 subjects with biopsy‐proven SCCIS, nine were found to harbor invasive SCC on final histology. Of the remaining lesions, seven had residual SCCIS, whereas the rest exhibited only actinic keratoses or scars. Approximately 31% of lesions showed evidence of invasive SCC. Correlating the clinical characteristics of the lesions with their corresponding final histologic diagnoses, the lesions harboring invasive SCC were more likely to demonstrate clinical signs of residual tumor (scales and papular changes) and be larger than 1.4 cm in diameter. Limitations Our experience at a single institution in the northeastern United States may not be reflective of a wider population. There is also a possible referral bias, because only lesions with high clinical suspicion for invasive SCC were referred for MMS. Conclusion Although biopsy‐proven SCCIS is often treated with modalities that are best suited for superficial disease and do not involve a final pathologic confirmation of clearance (e.g., cryotherapy, electrodesiccation and curettage), this study demonstrated that up to 31% of biopsy‐proven SCCIS lesions may harbor invasive SCC. Clinical signs of residual tumor and a diameter larger than 1.4 cm are statistically significant predictors of underlying invasive SCC. These data suggest that treatment modalities that include histologic control of tumor removal should also be strongly considered for the treatment of select biopsy‐proven SCCIS meeting the above criteria.

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