Cutaneous leiomyosarcoma originating in a symplastic pilar leiomyoma: a rare occurrence and potential diagnostic pitfall

A case of cutaneous leiomyosarcoma originating in a symplastic pilar leiomyoma is described. The patient was a 60-year-old man who presented with a painless nodule on his chest. Thorough clinical examination did not reveal any evidence of tumor elsewhere. A punch biopsy was performed which showed a benign spindle cell neoplasm with focal cellular pleomorphism that had smooth muscle differentiation and was clearly originating from the arrector pili muscle, consistent with a cutaneous symplastic pilar leiomyoma. Immunohistochemical studies were performed and the tumor cells were strongly positive for smooth muscle actin (SMA) and desmin. Given the clinical information of a mass on the chest, clinical evaluation was recommended and a re-excision was performed. Histologically, the re-excision showed two distinct populations of cells. The upper portion of the tumor mirrored the initial biopsy; however, the base of the tumor showed hypercellular areas composed of spindle cells with marked pleomorphism and increased number of mitoses. The diagnosis of a cutaneous leiomyosarcoma originating in a symplastic pilar leiomyoma was given. This is the first report of this association of such occurrence reported in the literature.