Nodular smooth muscle metaplasia in multiple peritoneal endometriosis

We report here an unusual presentation of peritoneal endometriosis with smooth muscle metaplasia as multiple protruding masses on the lateral pelvic wall. Smooth muscle metaplasia is a common finding in rectovaginal endometriosis, whereas in peritoneal endometriosis, smooth muscle metaplasia is uncommon and its nodular presentation on the pelvic wall is even rarer. To the best of our knowledge, this is the first case of nodular smooth muscle metaplasia occurring in peritoneal endometriosis. As observed in this case, when performing laparoscopic surgery in order to excise malignant tumors of intra-abdominal or pelvic organs, it can be difficult for surgeons to distinguish the metastatic tumors from benign nodular pelvic wall lesions, including endometriosis, based on the gross findings only. Therefore, an intraoperative frozen section biopsy of the pelvic wall nodules should be performed to evaluate the peritoneal involvement by malignant tumors. Moreover, this report implies that peritoneal endometriosis, as well as rectovaginal endometriosis, can clinically present as nodular lesions if obvious smooth muscle metaplasia is present. The pathological investigation of smooth muscle cells in peritoneal lesions can contribute not only to the precise diagnosis but also to the structure and function of smooth muscle cells and related cells involved in the histogenesis of peritoneal endometriosis.