Extraskeletal Ewing Sarcoma Mimicking Benign Soft Tissue Tumors: A Rare Case.

Extraskeletal Ewing sarcoma (EES) is a rare subtype within the Ewing sarcoma family of tumors (ESFT), typically presenting as a large soft tissue mass. Imaging features of EES are generally non-specific and vary depending on the tumor's location and the tissues involved. Magnetic resonance imaging (MRI) plays a crucial role in the initial evaluation, local staging, and ongoing management. A definitive diagnosis requires core needle biopsy, immunochemistry, and histopathological confirmation. Treatment typically involves systemic chemotherapy combined with surgical excision when feasible. Due to the tumor's tendency for large size and local invasion, a multidisciplinary approach is essential for optimal management. A 23-year-old female patient with no significant medical and surgical history presented with a complaint of a slowly enlarging mass over the lateral right thigh that has been growing for an estimated duration of three years, according to the patient. The mass measured approximately 2 × 2 to 3 × 4 cm and was firm, round, non-tender, and associated with occasional episodes of bloody discharge. Ultrasound (US) imaging showed a hypoechoic, oval, subcutaneous lesion measuring 1 × 1.4 cm with regular margins. Based on clinical and imaging findings, differential diagnoses included hemangioma, angiolipoma, leiomyoma, neurolipoma, and dermatofibroma. The patient underwent surgical excision of the lesion through an elliptical incision. Gross examination revealed a 6 × 4 cm skin ellipse with an underlying 7 × 4.5 × 2.5 cm mass, appearing whitish and lobulated, with a focal ulceration. Histopathological analysis confirmed the diagnosis of EES, grade 3, with a mitotic rate of 25/10 HPF. Immunohistochemistry demonstrated strong diffuse CD99 positivity along with positivity for vimentin, BCL2, Cyclin D1, and C-Kit, with a high Ki-67 proliferation index (85%-90%). No lymphovascular invasion, nodal involvement, or distant metastasis was detected, and staging was pT1. Following multidisciplinary discussion, the patient was initiated on adjuvant chemotherapy using the CAV-IE (cyclophosphamide, doxorubicin, vincristine alternating with ifosfamide and etoposide), with a plan to complete 12 cycles, while radiotherapy was not indicated. She commenced her treatment with Cycle 1 of the IE component, accompanied by supportive care including intravenous fluids, ondansetron, omeprazole, and dexamethasone. As of the latest update, the patient remains admitted and is currently on Day 4 of her first IE cycle. She developed dysuria during hospitalization, and the treatment plan was adjusted to include an increased dose of mesna to enhance uroprotection. This case highlights the importance of considering EES in the differential diagnosis of persistent soft tissue masses, even in slow-growing lesions. It underscores the pivotal role of immunohistochemistry and multidisciplinary management in achieving a definitive diagnosis and favorable outcomes.