Secondary Ossification Associated with Seborrheic Keratosis Adjacent to Basal Cell Carcinoma.

Sir,

An 80-year-old female was referred to our hospital, complaining of a facial tumor which had existed for last 8 years. She denied either prior local trauma or infection at the site. Physical examination showed a relatively well-circumscribed erythematous plaque with infiltration on the left cheek, and in addition, a brownish keratotic slightly-elevated nodule was located adjacently [Figure 1]. A biopsy specimen showed well-circumscribed basaloid tumor nests in the upper to mid-dermis. The tumor cells were uniform in size with oval nuclei and scant cytoplasm with peripheral palisading, and the tumor nests were surrounded by myxomatous stroma [Figure 2a], which was positively stained by alcian blue, suggestive of basal cell carcinoma. A total resection including the adjacent keratotic tumor was performed. Histological features of the keratotic nodule showed reticular epidermal proliferation containing pseudocysts, extending to the upper dermis [Figure 2b]. Of note, a lamellar bony structure of homogeneous materials containing osteoblastic cells was found in the surrounding stroma [Figure 2c and d].

Figure 1

Clinical appearance of the nodule on the cheek presenting with an erythematous plaque (arrowhead) and keratotic nodule adjacently (*).

Figure 2

(a) A biopsy revealed basal cell carcinoma nests in the dermis. (H and E, x40) (b) Reticular tumor nests connecting to the epidermis. (H and E, x40) (c) A fragment of well-developed bony structure is present close to the keratotic horn. (H and E, x40) (d) Higher magnification showing homogeneous materials-containing osteoblastic cells. (H and E, x200)

Secondary ossification is sometimes observed in several skin tumors and inflammatory skin disorders.[1] Among skin tumors, basal cell carcinoma is the most common.[2] The present case developed basal cell carcinoma and seborrheic keratosis in a close location, however, secondary ossification (osteoma) was observed just beneath the seborrheic keratosis, but not associated with basal cell carcinoma. Benign skin tumors showing secondary ossification are most common in melanocytic nevus and pilomatricoma, whereas other benign tumors are rare, i.e., apocrine hidrocystoma, lipofibroma, neurofibroma, pyogenic granuloma, and keratoacanthoma.[13] To the best of our knowledge, only one case of seborrheic keratosis with secondary ossification was reported;[1] however, details are unknown because it was reported in a large review of a single institute.

The mechanism of secondary ossification is still unknown. Osteoblast plays a central role in the bone formation. Osteoblast secrets several inducing factors such as bone morphogenetic protein (BMP)-2, BMP-4, β-catenin, osteopontin, osteonectin and osteocalcin,[4] which exert an effect on the precursor cells derived from mesenchymal cells. BMPs are members of transforming growth factor (TGF)-β, and transform the fibroblasts, primitive mesenchymal cells, or progenitor cells in the bone marrow into osteoblasts.[5] BMPs interact with the extracellular matrix, which may contribute to the transformation of primitive mesenchymal cells into osteoblasts forming bony tissues. In addition, previous studies suggested that TGF-β and connective TGF may stimulate primitive mesenchymal cells or displaced embryonic cells in the stroma to differentiate into osteocytes. Our results of the immunohistological study showed that BMP-4 was detected in the sebaceous glands (data not shown). Sebaceous glands are abundant in the face, which may explain why the majority of skin tumors with secondary ossification occur on the face, but the reason why the frequency of secondary ossification is low still remains unknown.

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Conflicts of interest

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