Supplemental Data 2

Proliferating KA shows an endophytic, multilobulated squamous tumor with central prominent, keratin-filled cavity or cavities (1). The cells show low nuclear to cytoplasm ratio containing eosinophlic glassy cytoplasm (2). The deep edge of tumor is often pushing rather than infiltrative (3). Regressing KA may display remnants of involuted squamous epithelium with thinned epidermis and loss of the glassy eosinophilic cells seen in a proliferating KA (4). Increased basket-weave flaky keratin may fill remnant cystic spaces (5). There may also be scarring, and lymphocytic inflammation and aggregates (6).
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