Subcutaneous low-grade fibromyxoid sarcoma of the thigh in a pediatric patient: a case report

Introduction and importance: Low-grade fibromyxoid sarcoma (LGFMS) is a rare soft tissue sarcoma that is exceptionally uncommon in children and often mistaken for a benign lesion owing to its bland histology and indolent course. We report a pediatric case of subcutaneous LGFMS of the thigh, radiologically misdiagnosed as a hemangioma, highlighting this diagnostic pitfall. Case presentation: An 11-year-old girl presented with a 6-month history of a painless, enlarging right thigh mass. Ultrasonography and magnetic resonance imaging showed a well-circumscribed, 25 × 17 × 15 mm subcutaneous lesion adjacent to the vastus medialis, favoring a hemangioma. Given its small size and well-defined margins, marginal excision was performed without prior biopsy. Histopathology revealed a fibromyxoid neoplasm with bland spindle cells and a low Ki-67 index (1%); immunohistochemistry was positive for MUC4 and Bcl-2 and negative for CD34, CD99, S100, desmin, and Smooth Muscle Actin(SMA), confirming LGFMS. After staging excluded metastasis, wide re-excision, including adjacent muscle and fascia, was performed; no adjuvant therapy was given. Clinical discussion: LGFMS is diagnostically challenging because its bland histology and indolent presentation frequently mimic benign lesions. MUC4 immunohistochemistry, supplemented by FUS-CREB3L2 fusion testing when needed, is essential for diagnosis. Because initial excision is often inadequate when it is mistaken for a benign process, prompt wide re-excision is recommended once it is diagnosed. Despite its low-grade appearance, LGFMS carries a risk of late recurrence and metastasis, warranting long-term surveillance. Conclusion: LGFMS should be considered in the differential diagnosis of subcutaneous thigh masses in children, even when imaging favors a benign lesion. Histopathological confirmation, adequate margins, and prolonged follow-up are essential for optimal outcomes.