Metastatic Retroperitoneal Rhabdomyosarcoma in a Child: A Case Report Highlighting the Role of Imaging in Diagnosis, Staging, and Follow-Up.

Retroperitoneal masses in children encompass a heterogeneous group of benign and malignant tumors with a largely nonspecific clinical presentation. Rhabdomyosarcoma (RMS) is a soft tissue sarcoma that uncommonly arises primarily in the retroperitoneum, a site associated with a comparatively unfavorable prognosis related to delayed diagnosis, large tumor volume at presentation, and frequent invasion of adjacent vascular and digestive structures. We report the case of a five-year-eight-month-old boy who presented with a one-month history of progressive abdominal distension and left lower limb edema secondary to iliac vein compression. Cross-sectional imaging revealed a large heterogeneous retroperitoneal mass with pulmonary and osseous metastases at diagnosis. The initial percutaneous biopsy was morphologically consistent with embryonal RMS, while a repeat biopsy was non-representative. The patient received induction chemotherapy with a 96.2% estimated volumetric response, followed by conservative local surgical management. Approximately one year after diagnosis, he experienced severe clinical deterioration and subsequently died. This case illustrates the diagnostic challenges of retroperitoneal RMS in children, the pitfalls of percutaneous biopsy in large heterogeneous tumors, and the central role of imaging at every stage of diagnosis, treatment response assessment, and follow-up, despite the unfavorable prognosis of this rare tumor site.