Answer: Leiomyosarcoma

Histology: The lesion consists of well circumscribed proliferation of malignant spindle cells involving the dermis and the subcutaneous tissue. The epidermis is unremarkable. The cells are arranged in long intersecting fascicles with marked pleomorphism, high mitotic activity and necrosis.

Discussion: The differential diagnosis includes sarcomatoid carcinoma (squamous cell), spindle cell melanoma, and sarcoma. The absence of an epidermal lesional component and the negative immunostaining for keratins and S100 in the tumor cells makes the first two diagnoses less likely. The tumor cells are positive for actin and desmin. In this case the diagnosis of leiomyosarcoma is favored. Primary leiomyosarcoma occurring in the skin dermis is usually small in size (<2.0 cm) with rare areas of necrosis and a good prognosis if completely excised. It is a rare lesion and has to be differentiated from leiomyosarcoma arising in the subcutis (usually from vessel walls) which is a more aggressive tumor and behaves more like soft tissue leiomyosarcoma and from metastases of leiomyosarcoma to skin from other sites (mainly when multiple skin lesions are present).