Castleman disease mimicking accessory spleen on imaging: A case report.

Castleman disease (CD) is a nonclonal lymphoproliferative disorder that causes non-neoplastic lymph node enlargement. With an incidence of approximately 21-25 cases per million, CD presents variably, often mimicking both benign and malignant conditions across various body regions. Clinically, it ranges from asymptomatic lymph node enlargement in Unicentric Castleman's Disease (UCD) to aggressive, multicentric presentations affecting multiple organs. Accurate diagnosis relies on surgical pathology due to the disease's diverse clinical and imaging manifestations. We report a rare case of UCD in a 19-year-old male who presented with mild, nonspecific left upper quadrant pain. Initial examinations, including ultrasonography, computed tomography, and magnetic resonance imaging, showed a hypervascular retroperitoneal mass that was initially suspected to be an accessory spleen or pancreatic tail neuroendocrine tumor. Surgical resection and histopathological analysis established the diagnosis of hyaline-vascular type UCD. This case highlights the diagnostic challenges of UCD, particularly when presented in uncommon locations like the retroperitoneal peripancreatic region. Imaging often fails to conclusively differentiate CD from other vascular lesions, necessitating a histopathological evaluation. Prior case studies have also reported similar diagnostic challenges and the efficacy of surgical resection for treating UCD. This case report adds to the existing literature by outlining the diagnostic procedure and challenges associated with retroperitoneal UCD. This highlights the need for increased awareness, advanced imaging techniques, and histopathological confirmation to achieve accurate diagnosis and effective treatment. A multidisciplinary approach is critical in managing such complex cases, ultimately leading to favorable patient outcomes.