A 33-year-old woman was referred to our institution due to left lumbar pain. Past medical history included a cholecystectomy and an appendectomy. The patient had taken oral contraceptives for menstrual irregularities. No previous history of trauma or pancreatitis was reported. The physical examination was normal and the routine laboratory tests were unremarkable.
CT scan was performed and showed a well-defined hypoattenuating cystic mass on the left retroperitoneum (Figures 1 and 4), measuring 93 x 58 x 31 mm (longitudinal x anteroposterior x transversal diameters). There were no calcifications, and non-significant contrast-enhancement was observed.
The lesion embedded and displaced anteriorly the left renal artery and vein without invasion (Figure 2). It also caused compression of the renal pelvis with mild dilatation of the calyces, inducing a junctional syndrome (Figure 3).
The patient underwent a partial resection and histological analysis revealed a Müllerian cyst. Toral resection was not possible because the cyst embedded the renal vessels, but dilatation of the renal pelvis was reverted.
Müllerian cyst of the retroperitoneum is a rare disease that is thought to arise from urogenital cysts. They occur generally in women from 19 to 47 years of age . The incidence in men is extremely rare . The size ranges from 8 to 25 cm in diameter . Urogenital cysts arise from vestiges of the embryonic urogenital tissue and can be categorized based on their embryonic lines into pronephric, mesonephric, metanephric, and müllerian types .
The retroperitoneal tissue may contain an aberrant Müllerian duct remnant, which might grow in the response to abnormal hormonal stimuli. Certain hormonal estrogenic stimulation for menstrual irregularities has been associated with the development of Müllerian cysts . Müllerian cysts are usually discovered in obese women with menstrual irregularities . The patients could be asymptomatic or presenting abdominal pressure, nausea, and vomiting . On CT, the Müllerian cyst appears as a well-circumscribed unilocular or multilocular cyst with fluid attenuation. After the administration of intravenous contrast, enhancement was not valuable. These findings are not specific enough to allow the differentiation of Müllerian cysts from other retroperitoneal cystic masses, such as cystic mesothelioma or lymphangioma [1,3].
It is a benign disease that can be treated with complete resection . Surgical excision is necessary to establish the diagnosis and avoid complications [1,2].
Aspiration of the contents may reveal a cytologic diagnosis but presents a high rate of recurrence. Partial resection increases the risk of local recurrence once remnants of the cyst are left in the retroperitoneum .
In a woman with menstrual irregularities and with a cystic retroperitoneal mass, Müllerian cyst should be considered in the differential diagnosis.
 Yang D, Jung D, Kim H, et al (2004) Retroperitoneal Cystic Masses: CT, Clinical, and Pathologic Findings and Literature Review. RadioGraphics 24(5):1353-1365. PMID: 15371613
 Naem A, Dlewati A, Alhimyar M, et al (2019) A rare presentation and recurrence of a retroperitoneal Müllerian cyst in a male patient: A case report. International Journal of Surgery Case Reports 65:301–304. PMID: 31760217
 Parmentier E, Valk J, Willemsen P, Mattelaer C (2021) A large retroperitoneal Mullerian cyst: case report and review of the literature. Acta Chir Belg 121(4):278-285. PMID: 31795845
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