Solitary nodule on the forearm

Dermatofibroma (DF), also known as benign fibrous histiocytoma, is a soft-tissue tumor usually occurring in middle-aged adults and showing a slight female predominance. The typical dermatofibroma generally occurs as a single or multiple skin-colored to reddishbrown or dark-brown firm nodule located most commonly on the lower legs [1]. While its diagnosis poses little trouble in the presence of classical clinicopathological features, a dermatofibroma may show a wide variety of clinicopathological variants and, therefore, its diagnosis may become difficult. The lipidized dermatofibroma represents a rare variant of dermatofibroma. Herein, we report a typical case of this type of tumor.


INTRODUCTION
Dermatofibroma (DF), also known as benign fibrous histiocytoma, is a soft-tissue tumor usually occurring in middle-aged adults and showing a slight female predominance. The typical dermatofibroma generally occurs as a single or multiple skin-colored to reddishbrown or dark-brown firm nodule located most commonly on the lower legs [1]. While its diagnosis poses little trouble in the presence of classical clinicopathological features, a dermatofibroma may show a wide variety of clinicopathological variants and, therefore, its diagnosis may become difficult. The lipidized dermatofibroma represents a rare variant of dermatofibroma. Herein, we report a typical case of this type of tumor.

CASE REPORT
A 58-year-old male presented himself with a single, slowly progressive, asymptomatic lesion on the right forearm persistent for the last three years. Past medical and family history was unremarkable. A clinical examination revealed a well-circumscribed erythematous nodule 15 mm in diameter (Fig. 1). The lesion, which was tender and firm on palpation, was excised under local anesthesia. Histological evaluation revealed mild epidermal acanthosis and basal pigmentation. The dermis showed a cellular proliferation of foam cells and histiocytes surrounded by hyalinized collagen bundles. Touton giant cells were also observed in some parts of the tumor (Fig. 2). These histopathologic findings led to the diagnosis of lipidized fibrous histiocytoma-one of the rare subtypes of DF.

DISCUSSION
Dermatofibroma (DF) is one of the most common types of cutaneous soft-tissue lesions [1]. The typical dermatofibroma generally occurs as a single or multiple firm reddish-brown nodule. It usually occurs on the lower extremities of young to middle-aged females [1]. Apart from its classical features, different variants have been described. Lipidized fibrous histiocytoma is a rare variant of dermatofibroma that represents approximately 2.1% of all dermatofibromas [2]. Lipidized DF was described by Iwata as ankle-type fibrous histiocytoma (dermatofibroma) due to its characteristic location on the ankles [3]. Subsequent evaluation by Wagamon demonstrated no significant difference in location between lipidized and nonlipidized dermatofibromas. Compared to ordinary dermatofibromas, patients with lipidized dermatofibromas tend to be older, most commonly in the fifth and sixth decades of life, and predominantly male [3]. Clinically, it manifests itself as a solitary exophytic yellow nodule usually larger than the common variant [1,3,4]. Apparently, it is not associated with hyperlipidemia [4]. It is frequently confused with other nodular tumors such as dermatofibrosarcoma protuberans, epidermoid carcinoma, sarcoma, and cutaneous leiomyoma [5].
Lipidized fibrous histiocytoma is defined by histological features. Its lesions are histologically characterized by the accumulation of numerous foamy cells dissected by distinctive hyalinized wiry collagen fibers. Foamy cells can be round, polygonal, or stellate in shape. These characteristic features in combination with the typical features of common dermatofibromas fulfill the criteria for the diagnosis of a lipidized dermatofibroma [3,4]. Touton giant cells are also frequently present [4]. The differential diagnosis includes eruptive xanthoma, granular cell tumors, tuberous xanthoma, and xanthogranuloma, but the presence of unique features such as distinctive stromal hyalinization are clues to the correct diagnosis of lipidized dermatofibroma [3].

CONCLUSION
Lipidized dermatofibroma is a rare histological variant of dermatofibroma that should be distinguished from other cutaneous foamy histiocytic lesions, which may impact patient management.

Consent
The examination of the patient was conducted according to the Declaration of Helsinki principles.