Case Report: Pelvic splenosis confused with malignancy-a reminder for differential diagnosis.

Background: Splenosis is a rare, benign disorder that arises after splenic trauma or splenectomy. Because its imaging features are nonspecific and often mimic those of malignant tumors, it is frequently misdiagnosed as malignant. The condition often presents without specific symptoms or elevated tumor markers, further complicating the diagnostic process. Case presentation: A 47-year-old woman with a prior history of splenectomy 20 years earlier after trauma presented with a progressively enlarging pelvic mass. Serial imaging modalities, including magnetic resonance imaging, contrast-enhanced ultrasound, and computed tomography, indicated a potential gastrointestinal malignancy, although all tumor markers remained within normal limits. Image-guided percutaneous biopsy was considered but deemed unsuitable due to the complex anatomical location of the dominant mass in the rectouterine pouch, where a safe needle trajectory avoiding bowel and major vessels could not be guaranteed. Exploratory surgery was performed, and intraoperative frozen-section analysis of an approximately 5-cm purple-brown nodule confirmed the diagnosis of splenosis. Histopathological examination with immunohistochemical staining (CD8+, CD68+, CD117-, DOG1-) confirmed the diagnosis of splenosis and excluded malignancies such as gastrointestinal stromal tumor (GIST) and carcinoma. Smaller implants located in the omentum and mesentery were preserved. The patient's postoperative course was uneventful, and she expressed both relief and satisfaction with the benign diagnosis as well as the avoidance of unnecessary extensive resection. Conclusion: This case highlights the necessity of considering splenosis in the differential diagnosis of pelvic masses, especially in patients with a history of splenic trauma or splenectomy. A comprehensive evaluation that incorporates clinical history, advanced imaging modalities-including magnetic resonance imaging and ^99mTc-labeled heat-damaged red blood cell scintigraphy-as well as multidisciplinary consultation, can enhance diagnostic accuracy and help prevent overtreatment. Current evidence indicates that asymptomatic splenosis does not necessitate surgical intervention, and precise preoperative recognition is crucial for optimizing patient management.