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Although much less common, primary retroperitoneal processes must be considered along with lesions of appendiceal or ovarian origin when considering the differential diagnosis of cystic masses in the right lower quadrant. Cystic masses of the retroperitoneum are uncommon clinical entities. The differential diagnosis is broad and includes lesions of a possible neoplastic origin, such as cystic lymphangioma, cystic teratoma or dermoid tumor, mucinous cystadenoma or cystadenocarcinoma, endometriosis, cysts of mesonephric duct remnants (such as Müllerian cysts), bronchogenic cysts, and epidermoid cysts. Other nonneoplastic cystic lesions, such as pancreatic pseudocysts, urinoma, hematoma, or infectious cysts (such as hydatid cysts), may also be found in the retroperitoneum. Computed tomography along with clinical features may help distinguish between different types of lesions.¹

In this case, pathologic analysis revealed a mass measuring 11 × 7 × 2.5 cm (Figure 2A). There was a smooth pink-tan lining and the cyst was filled with serous fluid. The wall thickness ranged from 0.1 to 0.2 cm. There were no gross excrescences. Microscopic examination revealed a predominantly mesothelial-lined cyst. There was also a component with a ciliated cuboidal to columnar lining. Of note, there was a clear transition between the mesothelial lining of the cyst and the columnar component. Multiple ciliated columnar cells were observed (Figure 2B).

Benign peritoneal cystic mesothelioma, also referred to as multilocular benign peritoneal inclusion cyst, is 1 type of cystic retroperitoneal lesion that is characterized by thin-walled cysts lined by flat or flattened cuboidal epithelium.^2- 3The etiology is unclear but has been hypothesized to be either neoplastic or reactive in nature.⁴Surgical resection has been advocated.⁴

A Müllerian cyst is another rare type of cystic lesion that may occur in the retroperitoneum. These thin-walled cysts are lined by cuboidal to columnar epithelium with cilia.⁵A number of hypotheses have been suggested as possible origins for these cysts. These have included the idea that the lesions are derived from remnants of the Müllerian duct or from the developing Müllerian duct itself. Alternatively, they may be derived from remnants of the urogenital ridge, ectopic ovarian tissue, or metaplasia of cysts derived from coelomic epithelium.^5- 6Surgical excision is also recommended for these lesions.⁶

In the case described here, the cyst displayed a predominantly flat or mesothelial-type lining, similar to that observed in benign peritoneal cystic mesothelioma. However, there was a distinct component with ciliated columnar epithelium, similar to what has been described in Müllerian cysts. The clear transition between the two suggests that a metaplastic process may have occurred in this case, with the Müllerian component arising from mesothelial epithelium.

In summary, although they are rare, retroperitoneal lesions must be considered in the differential diagnosis of cystic lesions of the right lower quadrant. Computed tomography may be helpful in differentiating some of these lesions, but diagnosis can be challenging. Laparoscopy, as in this case, can prove useful for both diagnostic and therapeutic purposes.

Correspondence:Giselle G. Hamad, MD, Department of Surgery, University of Pittsburgh, Magee Womens Hospital of UPMC, 3380 Boulevard of the Allies, Ste 390, Pittsburgh, PA 15213 (hamadg@upmc.edu).

Accepted for Publication:October 21, 2006.

Author Contributions:Study concept and design: Kaczorowski and Hamad. Acquisition of data: Kaczorowski and Hamad. Analysis and interpretation of data: Kaczorowski and Hamad. Drafting of the manuscript: Kaczorowski and Hamad. Critical revision of the manuscript for important intellectual content: Kaczorowski and Hamad. Administrative, technical, and material support: Hamad. Study supervision: Hamad.

Financial Disclosure:Dr Hamad has been a consultant for Cardinal Health and a speaker for US Surgical and sanofi-aventis.