Primary retroperitoneal hydatid cyst masquerading as malignancy.

Hydatid cysts, usually found in liver and lungs, are rare to be primarily located in the retroperitoneal region. It may mimic malignant mass and differentiation may be difficult. Surgical exploration in undiagnosed cases may lead to adverse complications. High index of suspicion is crucial, especially in patients from endemic region. Diagnosis can be aided by imaging and serologic testing for Echinococcus IgG. We present a 61-year-old Tibetan female visiting to North America and found to have a complex cystic and solid capsulated right retroperitoneal mass. Echinococcus IgG western blot assay was positive, and the patient was diagnosed to have primary retroperitoneal hydatid cyst.