PMC9910816_01
SCC right (R) forearm. A 2.6 x 2.3 cm scaly nodule on the right dorsal forearm. A shave biopsy revealed invasive well-differentiated SCC.. SCC: Squamous cell carcinoma
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FIG1
The physical exam was significant for a 2.6 x 2.3 cm scaly nodule on the right dorsal forearm (Figure 1).
Mohs Stage I frozen sections. Sections were positive for full-thickness epidermal keratinocyte atypia with large hyperchromatic and pleomorphic nuclei. Dermal cystic structures are observed. Top left and right: Hematoxylin and eosin (H&E) 4x; Bottom left and right: H&E 10x.
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FIG2
I was positive for full-thickness epidermal keratinocyte atypia with large hyperchromatic and pleomorphic nuclei. In addition, dermal keratin-filled cystic structures lined by stratified squamous epithelium with a preserved granular layer were observed in the margins, histologically emulating EICs versus milia (Figure 2). 
Mohs Stage II frozen sections. Persistence of dermal cystic structures without keratinocyte atypia. Left: H&E 4x; Right: H&E 10x.
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FIG3
In stage II, no keratinocyte atypia was noted, but the benign-appearing dermal cystic structures in the margins persisted (Figure 3).
Mohs Stage III frozen sections. Renewed areas of keratinocyte atypia as well as finger-like epidermal projections resembling pseudoepitheliomatous hyperplasia. H&E 10x.
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FIG4
Interestingly, stage III showed renewed areas of keratinocyte atypia projecting from the undersurface of the epidermis and extending into the dermis, suspicious for SCC, as well as finger-like epidermal projections resembling pseudoepitheliomatous hyperplasia (Figure 4).
Mohs Stage IV frozen sections. Similar findings to Stage III with recurrence of a cystic structure. Top left: cystic structure, H&E 10x; Top right and bottom left: Finger-like pseudoepitheliomatous hyperplasia projections, H&E 10x; Bottom right: Renewed areas of keratinocyte atypia suspicious for squamous cell carcinoma (SCC), H&E 10x.
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FIG5