RESUMEN
INTRODUCTION: The retroperitoneal Müllerian cysts are extremely rare lesions seen mostly in female patients but their occurrence in males is quite possible. Their exact etiology stills unknown, its histological and immunohistochemical characteristics may serve as an evidence that reflects its origin. CASE PRESENTATION: We present a case of a resected retroperitoneal Müllerian cyst that reoccurred severely after 3 months of the first laparotomy in a male patient with a significant history of testicular teratocarcinoma. Due to the cyst's localization and its proximity to vital retroperitoneal structures, total surgical resection couldn't be made and the recurrence was minimized by post-operative oral chemotherapy. DISCUSSION: The retroperitoneal Müllerian cyst is a benign cystadenoma that could be confused with other retroperitoneal lesions, which makes the pathological examination with the immunohistochemical study of the cyst's wall essential to make the diagnosis. The immunopositivity to Cytokeratin 7 and the immunonegativity to Cytokeratin 20 is a key feature that confirm the diagnosis whenever the retroperitoneal Müllerian cyst is suspected. Due to its high vascularity, we highly recommend the administration of chemotherapy which targets the proliferative cyst's cells. CONCLUSION: The RMCs are rare benign lesions that tend to reoccur if total surgical resection isn't made, when the total resection couldn't be achieved, the-unspecific anti-mitotic drugs may help in minimizing the recurrence and improve the life quality of the patient.