Cutaneous chondroma with overlying pigmentation clinically mimicking dermatofibroma
Authors: Y. Shinojima, H. Hara, H. Shimojima, T. Terui
Journal: British Journal of Dermatology (2005)

Conflicts of interest: none declared. Sir, Cutaneous chondromas are comparatively rare benign tumours that occur in the extraocceous tissues. We report an unusual case of cutaneous chondroma with overlying pigmentation clinically mimicking dermatofibroma, diagnosed with the aid of immunohistochemical staining, and discuss the histogenesis. An 18‐year‐old man presented with a 3‐month history of a lesion on his left thigh. The lesion was asymptomatic and had progressively enlarged. There was no prior history of trauma to this area. On examination, a firm, nontender, 1·0 × 1·0 cm mass could be palpated on the medial aspect of the left thigh, near to the knee joint (Fig. 1a). The lesion was well circumscribed and freely movable around the tissues. The skin overlying the mass had a brownish colour. The tumour was clinically diagnosed as dermatofibroma and was excised under local anaesthesia. Its cut surface was yellowish‐white in colour and showed a nodular configuration (Fig. 1b). Microscopically, the tumour was located in the deep dermis through the subcutaneous tissue, and was surrounded by a fibrous capsule (Fig. 2a). It was chiefly composed of chondrocyte‐ or chondroblast‐like cells, a homogeneous basophilic extracellular matrix and fine collagen fibres. The cells contained oval hyperchromatic nuclei, but not atypical mitotic figures (Fig. 2b). The epidermis was moderately acanthotic, and the basal layer was hyperpigmented, resembling that of dermatofibroma. To investigate the constituent structural components in detail, we performed immunohistochemical staining for chondroitin 4‐sulphate proteoglycans and chondroitin 6‐sulphate proteoglycans using antiproteoglycan ΔDi‐4S mouse monoclonal antibodies (2‐B‐6) and antiproteoglycan ΔDi‐6S mouse monoclonal antibodies (3‐B‐3) (Seikagaku‐kogyo, Tokyo, Japan), respectively, and for collagen type I and collagen type II using antihuman collagen type I goat polyclonal antibodies (SouthernBiotech, Birmingham, AL, U.S.A.) and antihuman collagen type II rabbit polyclonal antibodies (Novocastra Laboratories, Newcastle upon Tyne, U.K.), respectively. The chondroitin 6‐sulphate proteoglycans and collagen type I were intensively localized in the extracellular matrix and around the tumour cells (Fig. 2c,d), whereas the chondroitin 4‐sulphate proteoglycans and collagen type II were hardly detected in and around the tumour. Based on these pathological findings, we diagnosed this tumour as cutaneous chondroma, which is mainly composed of fibrocartilages, that is to say fibrochondroma. Furthermore, to investigate the mechanism of pigmentation, we performed several examinations. Fontana–Masson staining revealed increased melanin granules in the basal layer of the epidermis (Fig. 2e). Immunohistochemical analysis using polyclonal antibody against stem cell factor (SCF) (Nichirei, Tokyo, Japan) revealed no marked immunostaining in the tumour, unlike dermatofibroma, and toluidine blue stain showed that a significant increase in the mast cells induced by SCF was not seen in the dermis.