Anecdotal experience and data from multiple retrospective studies have suggested that a significant percentage of nonmelanoma skin cancers (NMSCs) display an aggressive histologic subtype that is not diagnosed on initial biopsy.

To prospectively determine the proportion of NMSCs upgraded at the time of Mohs micrographic surgery (MMS) and examine the surgical parameters of upgraded lesions.

In this prospective, cross-sectional study, all patients undergoing MMS for NMSC at our institution over the course of 1 year were screened for inclusion. Frozen sections were reviewed independently by 2 fellowship-trained Mohs surgeons.

In total, 265 of 2578 (10.3%) tumors displayed a more aggressive skin cancer histologic subtype on frozen-section analysis at the time of surgery than at the initial biopsy. Upgraded tumors required significantly more stages to reach tumor clearance, had a larger postoperative defect size, and more often required complicated repairs than nonupgraded tumors.

Single center study, limited time period, and cross-sectional design.

A significant portion of MMS cases were upgraded at the time of surgery to a more aggressive subtype than that seen at the initial biopsy. Upgraded cases were larger and more surgically challenging than nonupgraded ones. This finding has important implications for primary dermatologists' referral practices and Mohs appropriate use criteria guidelines.