Leiomyosarcoma With Dedifferentiation Mimicking a Benign Neoplasm.

Superficial leiomyosarcoma is a rare soft tissue sarcoma that typically presents as a firm, painful nodule in adults older than 50 years. We report a unique case of a 35-year-old woman with a slow-growing, mobile, subcutaneous nodule on the upper arm, initially presumed to be a benign lesion, most likely an angiolipoma. After excisional biopsy, histopathologic evaluation revealed an intermediate-grade leiomyosarcoma with focal dedifferentiation, demonstrating an abrupt transition from well-differentiated leiomyosarcoma to a high-grade, immunophenotypically undifferentiated sarcoma. Immunohistochemical staining showed loss of smooth muscle actin and desmin in the central dedifferentiated region, with retention of these stains at the peripheral lower-grade components. Wide local excision achieved clear margins, and imaging revealed no metastasis. Only 2 prior cases of cutaneous dedifferentiated leiomyosarcoma have been reported, both in the head and neck and with a more classic clinical presentation of a firm and immobile nodule. Our case is unusual in its location and deceptively benign clinical presentation. Given its aggressive potential and high reported rates of metastasis and mortality, prompt recognition and excisional biopsy are essential for diagnosis. This case highlights the importance of maintaining clinical suspicion for malignancy in persistent subcutaneous nodules and adds to the limited literature on dedifferentiated cutaneous leiomyosarcoma.