Massive Retroperitoneal Cystic Lymphangioma (18.5 L) Mimicking an Ovarian Tumor in an Adolescent: A Case Report Highlighting the "Crowding Effect" and Diagnostic Pitfalls

Introduction : Retroperitoneal cystic lymphangioma (CL) is a rare benign malformation, predominantly pediatric, and exceptionally uncommon in adolescents. Due to the expansive nature of the retroperitoneal space, these tumors can reach massive dimensions. The resulting "mass effect" frequently mimics gynecological malignancies, creating a significant diagnostic pitfall. Case Presentation : We report a rare case of a 16-year-old female presenting with progressive abdominal distension resembling a full-term pregnancy. Preoperative CT and MRI revealed a giant cystic mass occupying the entire abdominopelvic cavity. Radiologically, the lesion was indistinguishable from a giant ovarian cystadenoma. However, a critical discordance was noted: despite the massive tumor burden, serum tumor markers (CA-125, HE4, AFP) were strictly within normal limits. During surgical exploration, the mass was identified as originating from the retroperitoneum, strictly separating it from the adnexa. Approximately 18,500 mL of fluid was drained. The cyst wall encased the left renal hilum and was densely adherent to the renal artery, leading to an intraoperative vascular injury. A multidisciplinary approach facilitated vascular repair and complete cyst excision while successfully preserving the left kidney. Pathological examination confirmed a cystic lymphangioma. Conclusion : This case highlights the diagnostic challenge of massive abdominal cysts in adolescent females. The discordance between imaging features suggestive of an ovarian origin and negative serum tumor markers should serve as a "red flag," prompting consideration of retroperitoneal etiologies. Furthermore, this report demonstrates that even when the tumor involves complex hilar vasculature, organ-sparing resection is achievable through meticulous surgical technique and multidisciplinary collaboration.