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Unusual Calcification Mimicking Periosteal Tumor Invasion in a Squamous Cell Carcinoma Treated With Mohs Micrographic Surgery.

Mohsmicrographic surgery (MMS) has become the treatment of choice for high-risk nonmelanoma skin cancers (NMSCs) because of its decreased recurrence rates and minimization of deformity. Step-wise, intraoperative histologic evaluation gives performing surgeons both complete microscopic control of surgical margins and confidence in normal tissue preservation. Although markedly successful at clearing tumors, MMS relies on the accurate microscopic interpretation of histology on frozen sections. Most problems seem to occur in 1 of 3 areas: (1) specimen orientation when excising or marking; (2) poor staining; and (3) microscopic interpretation of processing artifacts and cancer mimickers on frozen section. The lattermost problem-area provides a source of diagnostic error, and greater exploration of potential frozen section confounders is needed. The presence of processing artifacts on a frozen section can present diagnostic challenges and Mohs surgeons need to be aware of these mimickers when performing MMS. Herein, we present a rare case of focal dystrophic periosteal calcification mimicking periosteal tumor invasion. We discuss the role of dystrophic calcification in the context of cutaneous tumors and the importance of permanent sections in helping make the diagnosis. A 93-year-old Caucasian woman was referred for MMS for treatment of a moderately differentiated squamous cell carcinoma located on her right superior lateral forehead (Figure 1A). Clinically, the lesion was a 3.03 3.0-cm exophytic, nonmobile tumor with central crust. She had no other notable lesions on the head and neck and she had no lymphadenopathy. Clinically visible gross tumor was removed requiring 3 stages with the resultant 4.73 4.2cm defect down to calvarium (Figure 1B). Frozen sections of the first 2 stages revealed moderately differentiated squamous cell carcinoma (Figure 2). Note the linear growth pattern of deeply invasive tumor cells on higher power (Figure 2). After the first 2 layers, the peripheral margins were clear, but the deep margin remained positive for tumor. The third, and deepest galeal layer was adherent to the periosteum and intraoperativemicroscopic examination of the excised tissue revealed hematoxylin-rich cellularappearing structures concerning for malignant cells (Figure 3). Intraoperative tele-consultation with a dermatopathologist was concerning for residual tumor. This suggested possible tumor extension into the periosteal layer of the frontal bone, and the patient was therefore referred to Otolaryngology for definitive treatment with removal of the outer table. To confirm the diagnosis, permanent formalin-fixed nonmarginal Mohs sections were reviewed by the dermatopathologist. These revealed that the deep, hematoxylinrich material represented focal dystrophic calcification (Figure 4A), supported by a positive Von Kossa stain (Figure 4B). AE1/AE3 and P63 immunostains on permanent