Uncovering the True Diagnosis: A Case Report of Multilocular Peritoneal Inclusion Cyst.

Peritoneal inclusion cysts (PICs) are rare, benign cystic tumors predominantly found in premenopausal females. They often present diagnostic challenges due to their asymptomatic nature and non-specific imaging features, requiring biopsy and immunohistochemistry for a definitive diagnosis. This report presents a case of a large, multilocular PIC initially misdiagnosed as a lymphangioma. A 29-year-old asymptomatic nulliparous female presented for follow-up after an abdominal ultrasound, conducted during a complicated cystitis workup, incidentally revealed a notable fluid collection adjacent to the spleen. Computed tomography of the abdomen and pelvis showed a complex cystic lesion in the left upper quadrant, measuring 7.6 x 11.7 cm. The finding was interpreted as a lymphangioma. Over six months, the lesion doubled in size, prompting further evaluation and doxycycline sclerotherapy. Despite eight months of treatment, the patient began to experience persistent symptoms of abdominal pressure, sharp groin pain, and decreased appetite. Magnetic resonance imaging revealed that the lesion had grown to 7.0 x 17.1 x 34.6 cm. An incisional biopsy confirmed PICs through pathology and immunohistochemistry. Genetic testing for hereditary cancer was negative. The patient underwent extensive surgical resection involving multiple organs, ultimately achieving no gross residual disease. This case underscores the diagnostic challenges posed by PICs and the necessity of biopsy for accurate diagnosis, differentiating them from lymphangiomas. It emphasizes the importance of a multidisciplinary approach and individualized treatment plans when managing PICs. Continued research and long-term follow-up are essential for refining treatment strategies for this rare condition.