A Case of Desmoplastic Trichoepithelioma with Ossification

Abstract

Sir, Desmoplastic trichoepithelioma was first described as a distinct clinicopathological entity in 1976 by Brownstein and Shapiro.[1] The clinical features of desmoplastic trichoepithelioma are solitary, hard, and annular lesions on the face of a woman.[2] Histologically, desmoplastic trichoepithelioma has three characteristic findings: Narrow strands of basaloid cells, desmoplastic stroma, and keratinous cysts,[3] and is associated with ossification in approximately 6% of cases.[2] We report a case of desmoplastic trichoepithelioma with ossification. A 30-year-old Finnish man presented to our hospital with an asymptomatic 7 × 5 mm, skin-colored, and slightly-depressed plaque on his left cheek [Figure 1]. He had been aware of the plaque for 2 years, and the lesion had recently increased in size. Figure 1 Clinical features of the patient. A slightly-depressed plaque on his cheek Histopathological examination of the lesion revealed small nests of basaloid cells with focal connections to the overlying epidermis [Figure 2a], keratinous cysts and desmoplastic stroma [Figure 2b]. The basaloid cells were arranged in narrow strands, with one to three rows of cells [Figure 2c]. The stroma consisted of collagen and there were no clefts between the nest of tumor cells and the stroma. Palisading was not observed. At the base of the lesion, a circular bone formation and a foreign-body giant-cell granuloma were present [Figure 2d]. Based on these histological findings, the diagnosis of desmoplastic trichoepithelioma with ossification was made. One month after the skin biopsy, total resection was performed. Histopathological findings of the total resection specimen revealed the same findings as the biopsy. Figure 2 Histopathological features. (a) Small strands of basaloid cells and a small bone formation were present (H and E, original magnification ×40). (b) Keratinous cyst and desmoplastic stroma (original magnification ×100). (c) The basaloid cells were arranged in narrow strands, with one to three cell thicknesses (original magnification ×100). (d) Adjacent to the bone, a foreign-body giant-cell granuloma was present (original magnification ×100) Desmoplastic trichoepithelioma is rare, benign adnexal tumor that usually presents as an asymptomatic, firm, annular plaque, which can closely mimic morpheaform basal cell carcinoma.[4] Brownstein and Shapiro found only three cases of desmoplastic trichoepithelioma with ossification among their series of 50 cases of desmoplastic trichoepithelioma.[2] The differential diagnosis between desmoplastic trichoepithelioma and morpheaform basal cell carcinoma is important, yet difficult. Histopathologically, desmoplastic trichoepithelioma contain horn cysts, commonly exhibit epidermal hyperplasia, foreign-body keratin granulomas and calcification, all of which are unusual in morpheaform basal cell carcinoma.[2] In contrast, nodular masses of tumor cells with peripheral palisading and abundant mitotic figures, frequently present focally in morpheaform basal cell carcinoma, are absent in desmoplastic trichoepithelioma.[2] The term osteoma cutis denotes the development of focal ossification in the dermis and the subcutaneous tissue. Ossification can be either primary, arising de novo in healthy skin, or secondary, developing in association with pre-existing neoplastic or inflammatory skin lesions. Secondary osteoma cutis is relatively frequent and is responsible for about 85% of bone found in the skin.[5] Pilomatricomas most commonly undergo ossification. Osteoma cutis frequently occurs in acne scars and is typically found in young women with scarring acne.[6] The pathophysiologic mechanisms of ossification are unclear. The findings of heterotopic bone formation in acne scars[7] and folliculitis[8] suggest that inflammation of the pilosebaceous unit may play a pivotal role. Some authors believe that inflammation of degenerating keratin pearls results in calcification and subsequent ossification.[9] Other authors have reported that bone-forming cells in secondary ossification may arise from osteogenic stromal elements.[10] It has been reported that in osteoma cutis, fibroblasts may have the ability to differentiate into osteoblastic cells that have properties similar to those of osteoblasts, such as high alkaline phosphatase activity and high expression of osteonectin.[11] Several bone-forming growth-regulating factors that may also participate in secondary ossification have been identified. In the present case, adjacent to the bone, a keratinous cyst, which is one of the characteristic histopathological findings of desmoplastic trichoepithelioma, and a foreign-body giant-cell granuloma were present. Thus, our findings support the hypothesis that inflammation of degenerating keratin pearls results in calcification and subsequent ossification.[9]