When an old pigmented lesion becomes serious: the problematic diagnosis of low-grade skin malignancies – Bednar tumor with fibrosarcomatous transformation

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Abstract Bednar tumor is a rare cutaneous neoplasm, regarded as a variant of dermatofibrosarcoma protuberans with melanotic colonization and it usually affects young and middle-aged adults or children. This lesion is considered a low-grade malignant tumor, which can be associated with multiple local recurrences after surgical excision. Although a rare phenomenon, these lesions may undergo fibrosarcomatous transformation, which implies a poorer prognosis of the disease, as the tumor has a more locally aggressive behavior and patients might also develop distant metastases. We present the case of a 53-year-old female patient, with no significant medical history, who presented with a subcutaneous nodule on her upper back, for which a wide surgical excision was performed. The gross examination of the specimen showed a solitary protuberant grey-white nodule with a bluish shade and flecked with pigment. The microscopic examination revealed a malignant proliferation with a predominantly fascicular growth pattern, composed of spindle cells with highly pleomorphic nuclei and high mitotic rate, as well as the presence of dendritic cells with abundant melanin. Upon immunohistochemical analysis, the proliferation showed negative staining for CD 34 and AE 1/3, whereas the scattered dendritic cells stained positive with S100 protein. Ki 67 was positive in 15% of the tumor cells and the absence of p53 expression was noted. Thus, the diagnosis of Bednar tumor with fibrosarcomatous transformation was established. The aim of this paper was to gain further knowledge about the histopathological and immunohistochemical features, as well as about the treatment of Bednar tumor, especially considering its rarity.