Answer: Mucinous cystadenoma of the retroperitoneum

Histology: The specimen consists of a cystic structure lined by mucinous epithelium on one surface and mesothelium on the opposing surface. The mucinous epithelium is bland with no micropapillary formation and no dysplasia. No mitotic figures are apparent. No ovarian type stroma is evident. No smooth muscle is present in the cyst wall. Per the surgical report, the lesion is not attached to the ovary.

Discussion: Primary retroperitoneal mucinous tumors are rare lesions that primarily occur in adult women. They have no physical connection to or involvement of the ovary, distinguishing them from large cystic ovarian mucinous tumors. Similar to the mucinous tumors of the ovary, the mucinous tumors of the retroperitoneum are subdivided into: mucinous cystadenoma, mucinous borderline tumor or tumor with low malignant potential, and mucinous carcinoma. Complete surgical excision of the cystadenoma is curative.

The actual origin of primary retroperitoneal mucinous tumors is unclear, but there are several hypothesis. The most popular hypothesis is that invagination of the peritoneal mesothelium undergoes mucinous metaplasia with cyst formation. This hypothesis might be supported in the present case by the finding of focal mesothelial invaginations within the cyst wall of this case. Other hypothesis include that the tumors arise from ectopic ovarian tissue, however there is no ovarian-type stroma present; that the tumor arises from a teratoma (e.g., pre-sacral), however no other non-mucinous components are present; and finally that the tumor arises from an intestinal duplication cyst, however there is no smooth muscle layer in the cyst wall.

Reference(s):
– Hart WR. Mucinous tumors of the ovary: a review. Int J Gynecol Pathol. 2005 Jan;24(1):4-25.
– Roma AA, Malpica A. Primary retroperitoneal mucinous tumors: a clinicopathologic study of 18 cases. Am J Surg Pathol. 2009 Apr;33(4):526-33.