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Keratoacanthoma is a benign neoplasm composed of keratinizing squamous cells, which frequently grows rapidly and resolves spontaneously if untreated. Secondary ossification arises in association with several cutaneous tumors or inflammatory conditions; however, the presence of ossification in keratoacanthoma is uncommon. We herein report the first case of keratoacanthoma with secondary ossification.

A 47-year-old man visited our hospital, complaining of a nodule on the cheek that had been present for six months (Fig. 1a). No history of trauma or infection was referred. A physical examination showed a firm, dome-shaped nodule, 1 cm in diameter, with a horn-filled crater in its center. Histopathological examination revealed an irregular, epidermal hyperproliferation with hyperkeratosis extending into the mid-dermis, with a central, keratin-filled crater (Fig. 1b). The epidermis extended like a buttress over the bilateral sides of the crater. An inflammatory infiltrate was also seen in the dermis. A lamellar bony structure was found in the surrounding stroma (Fig. 1c).

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Figure 1.  (a) Clinical appearance of the nodule on the cheek. (b) Histological view showing hyperkeratosis and epidermal hyperproliferation with central crater. Bony structure is located (arrow) (hematoxylin–eosin, original magnification ×12.5). (c) A fragment of well-developed lamellar bone is present close to the nodule of keratoacanthoma (hematoxylin–eosin, original magnification ×200)

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So far, secondary ossification is reported in association with various benign as well as malignant tumors, such as melanocytic nevi (nevus of Nanta), blue nevi, organoid nevi, acne, cutaneous mixed tumors, epidermal and dermoid cysts, trichoepithelioma, pilomatricoma, lipoma, dermatofibroma, pyogenic granuloma, basal cell carcinoma, squamous cell carcinoma, and melanoma.1–3 To the best of our knowledge, this is the first case of keratoacanthoma with secondary ossification.

Osteoblasts play a central role in the bone formation. Osteoblasts secrete several inducing factors such as bone morphogenetic protein (BMP)-2, BMP-4, β-catenin, osteopontin, osteonectin, and osteocalcin,4 which exert effect on the precursor cells derived from mesenchymal cells. BMPs are members of transforming growth factor-β (TGF-β) and transform the fibroblasts, primitive mesenchymal cells, or progenitor cells in the bone marrow into osteoblasts.5 In the nevus of Nanta, which shows secondary ossification associated with dermal nevus, TGF-β and connective tissue growth factor (CTGF) expression is detected on the nevus cells, while intradermal nevus without ossification was not stained. CTGF is regulated by TGF-β in the fibroblasts and plays an important role in anchoring the independent growth of fibroblasts that is induced by TGF-β.6 CTGF plays a role in the proliferation and differentiation during ossification.7 They suggest that TGF-β and CTGF may stimulate primitive mesenchymal cells or displaced embryonic cells in the stroma to differentiate into osteocytes.

The majority of the lesions exhibiting secondary ossification occur on the face, as was found in our case. Stromal cells residual in the face or facial fibroblasts may have higher potentiality for osteocytes, but further studies will be necessary to clarify the pathogenesis of secondary ossification.

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