Case Report: Uterine mesothelial cyst: a report of three cases and literature review.

The patient is a 42-year-old woman admitted for evaluation of a pelvic mass discovered on routine examination 1 month prior. A pelvic ultrasound at that time revealed a 75 mm*60 mm anechoic cystic mass anterior and to the right of the uterus ( Figure 1A ), though she reported no abdominal pain or distension. The patient had kidney stones for over 10 years. Gynecological examination indicated the uterus was normal in size, and a smooth, mobile, non-tender mass approximately 6 cm in diameter with well-defined borders was palpated in the right adnexa; the left adnexa was unremarkable. Tumor markers, blood routine, liver and kidney function were normal. Laparoscopic exploration demonstrated an abnormal uterine contour and identified a 7.0 cm diameter cyst on the right fundus with an intact, smooth capsule and no adhesions. The cyst was excised in its entirety, revealing a smooth inner wall and pale yellow clear fluid without papillary nodules. The final pathological diagnosis was a uterine mesothelial cyst, with focal areas of active epithelial cell proliferation ( Figure 1B The capsule is covered by a single layer of flat squamous epithelial cells, and the stroma is composed of fibrous connective tissue, accompanied by a small number of chronic inflammatory cells). Postoperative recovery was good and there was no recurrence. (A) Ultrasonography image (UT: Uterus; BL: Bladder; CY: Cyst) and (B) postoperative pathological result of the first patient. A 41-year-old female patient presented with a 4-month history of intermittent, dull lower abdominal pain. The pain began without an identifiable cause and was not associated with fever, nausea, vomiting, diarrhea, constipation, or urinary symptoms. Pelvic ultrasonography revealed a 6 mm endometrial thickness and an anechoic area on the posterior uterine wall, measuring approximately 48*32*45 mm, which raised suspicion of a uterine cyst or liquefied malignant tumor due to its poor transparency and peripheral blood flow signals ( Figure 2A ). The patient had a prior history of one cesarean section. Gynecological examination indicated a normally sized, non-tender uterus, slight thickening of the right adnexa without tenderness, and no palpable masses or tenderness in the left adnexal region. Tumor markers, blood routine, liver and kidney function were normal. Laparoscopic exploration demonstrated an abnormal uterine contour and a smooth, approximately 5 cm diameter cystic mass on the right posterior uterine wall that was not adherent to surrounding tissues; the bilateral fallopian tubes and ovaries appeared normal. The cyst was completely excised and was found to have a smooth wall containing pale yellow, clear fluid. Postoperative pathological examination diagnosed a uterine mesothelial cyst ( Figure 2B , The capsule is covered by a single layer of columnar epithelium, accompanied by papillary folds, and the stroma is a fibrous vascular tissue). Postoperative recovery was good and there was no recurrence. (A) Ultrasonography image and (B) postoperative pathological result of the second patient. A 56-year-old female patient was admitted following the incidental discovery of a pelvic mass during a routine medical examination 3 months prior. No intervention was undertaken at that time. Subsequent pelvic ultrasonography revealed a perimenopausal uterus and a multilocular cystic mass on its left side, the nature of which was indeterminate and considered a possible tubal hydrocele ( Figure 3A ). The patient had a prior history of uterine fibroid removal with unilateral salpingectomy. Gynecological examination indicated slight uterine enlargement and thickened left adnexa without tenderness, while the right adnexa were unremarkable. Tumor markers,blood routine, liver and kidney function were normal. Laparoscopic exploration demonstrated a uterus equivalent in size to a 2-month gestation, featuring a 2-cm diameter, smooth-surfaced fibroid on the anterior wall and a 6-cm multilocular cyst on the posterior wall; the left fallopian tube was absent. The posterior wall cyst and the anterior wall fibroid were completely excised. Postoperative pathology confirmed a periuterine serous cyst and a uterine smooth muscle tumor with hyaline degeneration ( Figure 3B , The capsule is covered by transitional epithelium, and the stroma consists of smooth muscle and fibrous vascular tissue, with a small amount of chronic inflammatory cells infiltration). Postoperative recovery was good and there was no recurrence. (A) Ultrasonography image and (B) postoperative pathological result of the third patient.

Mesothelial cysts constitute a form of cystic mesothelioma. They are considered benign, encapsulated lesions rather than malignant tumors. Typically small and solitary, these cysts are also referred to as isolated mesothelial cysts. In women, uterine mesothelial cysts are relatively uncommon, present with atypical clinical features, and are frequently missed or misdiagnosed. The precise incidence remains unclear, and no unified diagnostic or therapeutic standard exists. This report describes three cases of intrauterine cysts managed via laparoscopic exploration, all with favorable postoperative outcomes.

Mesothelial cysts can occur on any abdominal membrane surface, including the round ligament, adnexa, mesentery, and peritoneum ( 1 ). As a form of cystic mesothelioma, these lesions are benign colloid cysts rather than malignant tumors. Typically solitary and small, they usually measure less than 5 cm and are also described as isolated mesothelial cysts. The cysts are often unilocular, though they may contain two or three cystic cavities. Their walls are very thin and lined by well-differentiated flat mesothelial cells without significant mesothelial hyperplasia ( 2 ). This condition is relatively uncommon in women. We identified nine cases of uterine mesothelial cysts after Pubmed search and China National Knowledge Infrastructure (CNKI) published nearly 20 years ( Tables 1 , 2 ). Its exact incidence remains unclear, and no unified standard exists for diagnosis and treatment. Case reports. Case series of uterine mesothelial cysts in the literature The pathogenesis of uterine mesothelial cysts remains unclear. A developmental origin is generally suspected, though some researchers attribute the condition to chronic peritoneal inflammation ( 3 ). Established risk factors include pelvic endometriosis, a history of pelvic inflammatory disease, and prior pelvic surgery ( 4 ). Among the three cases mentioned in the text, two of them had a history of pelvic surgery, which is consistent with the high-risk factor of a previous pelvic surgery. One case had a history of kidney stones for over 10 years, which might be related to long-term abdominal membrane irritation. Mesothelial cysts lack typical clinical manifestations and are usually diagnosed following the detection of an abdominal mass or discovered incidentally during other abdominal surgeries. Only a minority of patients present with symptoms such as menorrhagia. Both imaging and laboratory findings are non-specific, complicating preoperative diagnosis and frequently leading to misdiagnoses such as cystic degeneration of uterine fibroids, ovarian cysts, inflammatory encapsulated effusions, or pelvic cystic lymphangioma. Cystic degeneration of uterine fibroids typically appears as patchy anechoic areas with thick, irregular walls. Uterine cystic lymphangiomas are often located within the myometrium and may be associated with irregular vaginal bleeding. Endometriosis usually manifests as multiple small anechoic areas containing fine internal echoes, with the surrounding myometrium exhibiting enhanced and heterogeneous echogenicity, frequently accompanied by dysmenorrhea ( 5 ). Currently, definitive diagnosis relies on postoperative pathological examination. Microscopically, the cysts are typically lined by a single layer of cuboidal epithelium without hyperplasia, exhibiting a moderate nuclear-to-cytoplasmic ratio, an absence of mitotic figures, and normal cellular polarity. Immunohistochemical markers can aid in distinguishing mesothelial cysts. Calcium-binding proteins demonstrate good specificity and sensitivity for mesothelial cells, while other useful markers include h-caldesmon, cytokeratin 5/6, WT-1, HBME, and mesothelin ( 6 , 7 ). No single marker, however, provides 100% sensitivity and specificity for mesothelial cysts ( 8 ). Mesothelial cysts are extremely rare in clinical entities, and no clear treatment guidelines currently exist. Given their benign nature, some authors advocate for conservative management with close follow-up when the diagnosis is certain. In practice, however, preoperative differentiation from ovarian tumors, uterine fibroids, and other cystic conditions is often difficult due to a lack of specific laboratory or imaging findings. Consequently, surgical excision remains the primary treatment. Unlike multicystic mesothelioma, which has a high recurrence rate (27–75%) and a potential for malignant transformation, benign mesothelial cysts seldom recur and show no malignant tendency ( 9 ). All three cases of uterine mesothelial cyst diagnosed at our institution had a favorable prognosis following surgical resection, with no recurrence observed.

In conclusion, endometrial seromeningitic cysts are rare and should be included in the differential diagnosis of pelvic lesions. Enhancing the understanding of this rare disease is conducive to achieving individualized management.