Evaluation of an intramuscular nodular lesion in a pediatric patient: Diagnostic challenges and imaging findings

Malignant ectomesenchymoma (MEM) is an uncommon and rapidly progressing malignant tumor affecting the nervous system or soft tissues, composed of both mesenchymal and neuroectodermal components. It is mostly found in the pelvic region, retroperitoneal area, and genitourinary system of pediatric patients. A 19-month-old child was hospitalized for an ultrasound and lab tests due to a painless nodular mass on the right forearm. The ultrasound showed a well-defined intramuscular lesion measuring 57 × 20 × 27 mm with a chaotic blood flow pattern, raising suspicion of a neoplastic condition. A biopsy confirmed ectomesenchymoma. The patient was scheduled for chemotherapy based on the CWS — VAIA III protocol and categorized as high-risk group (HRG); and staged as Group III according to the Intergroup Rhabdomyosarcoma Study classification (IRS III), with involvement of regional lymph nodes (N1). Diagnosis of MEM is challenging due to its rarity and variability, leading to prolonged treatment timelines. In the case analyzed, ultrasound showed malignant features, such as a subfascial position and a lesion over 5 cm, with irregular vascular patterns suggesting necrosis. Surgical intervention is the primary treatment, while chemotherapy and radiotherapy are generally avoided in children due to neurodevelopmental risks. Understanding MEM tumorigenesis may help identify better systemic treatments in the future.