Purpose or Case Report: Malignant ectomesenchymoma (MEM) is a rare, rapidly growing soft tissue tumor composed of mesenchymal and neuroectodermal elements. It usually develops in the pelvis, retroperitoneum, or genitourinary tract of children. According to WHO, MEM is classified among skeletal muscle tumors containing embryonal rhabdomyosarcoma mixed with neuroblastic components, typically positive for desmin, myogenin, and synaptophysin. The tumor most often occurs during the first years of life and is characterized by aggressive behavior. Fewer than one hundred cases have been described worldwide.
Methods & Materials: A 19-month-old boy presented with a firm, painless nodule on the right forearm. The skin above the lesion was unchanged. Ultrasound revealed a well-defined, heterogeneous intramuscular mass (50 × 20 × 27 mm) with irregular vascular flow on Doppler imaging, suggesting malignancy. MRI showed a hyperintense lesion on T1-weighted images with hemorrhagic areas and mild surrounding edema. Biopsy confirmed malignant ectomesenchymoma composed of rhabdomyoblastic and ganglioneuroblastic elements. Immunohistochemistry was positive for desmin, myogenin, MyoD1, synaptophysin, and NSE, with a high proliferation index (Ki-67 = 70%). Due to inoperability, chemotherapy according to the CWS-VAIA III non-RMS-like protocol was initiated. The initial stage was HRG, IRS III, N1.
Results: MEM is an extremely rare tumor showing dual mesenchymal and neuroectodermal differentiation. Although typically located in the pelvis or retroperitoneum, it may also appear in the extremities. Imaging may reveal irregular vascularization and necrotic areas, but final diagnosis relies on histopathology and immunostaining. Treatment includes surgery, chemotherapy, and sometimes radiotherapy, although the latter is avoided in small children. Complete resection followed by adjuvant chemotherapy offers the best outcomes, whereas disseminated disease requires systemic treatment. The rarity of MEM limits the establishment of uniform protocols and complicates prognosis.
Conclusions: diagnostic and therapeutic challenges. Imaging provides essential information, but biopsy remains crucial for confirmation. Early recognition and a multidisciplinary approach are key to improving outcomes in affected pediatric patients.