Benign hemorrhagic mesenteric cyst mimicking peritoneal dissemination in early-stage ovarian clear cell carcinoma: a case report.

A 58-year-old postmenopausal woman presented with a 10-day history of progressive lower abdominal pain and marked abdominal distension. She had no significant personal medical history or family history of gynecological malignancy. Physical examination revealed a massive, fixed abdominopelvic mass with ill-defined margins, extending superiorly to the xiphoid process (Fig.  1 A). Following successful surgical resection, the abdominal distension markedly resolved, and the abdominal contour returned to normal (Fig.  1 B). Fig. 1 Clinical appearance before and after surgical resection. ( A ) Preoperative marked abdominal distension caused by a large abdominopelvic tumor. ( B ) Marked improvement in abdominal contour after surgical resection Clinical appearance before and after surgical resection. ( A ) Preoperative marked abdominal distension caused by a large abdominopelvic tumor. ( B ) Marked improvement in abdominal contour after surgical resection Laboratory investigations revealed elevated serum CA125, HE4, and CA19-9 levels, with values of 196.99 IU/mL (normal range: <35 IU/mL), 121.45 pmol/L, and 182.11 U/mL, respectively. Other tumor markers were not markedly elevated, including CEA at 1.92 ng/mL, AFP at 2.99 ng/mL, and CA15-3 at 28.90 U/mL. Contrast-enhanced computed tomography (CT) of the abdomen and pelvis demonstrated multiple large multilocular cystic-solid abdominopelvic masses, the largest measuring approximately 225 × 160 × 310 mm (Fig.  2 ). Patchy solid densities were also observed within the omentum and mesentery, raising concern for peritoneal dissemination and possible gastrointestinal involvement. Given the elevated CA19-9 level and suspicious mesenteric and omental findings, gastrointestinal endoscopy was performed to exclude a gastrointestinal primary tumor or intraluminal involvement. Gastroscopy showed chronic gastritis, and colonoscopy revealed multiple colonic and rectal polyps, with no evidence of gastrointestinal malignancy. Chest CT showed emphysematous changes in both lungs, without definite evidence of thoracic metastasis. Taken together, these findings led to a preoperative clinical suspicion of advanced ovarian malignancy with extraovarian involvement. Fig. 2 Preoperative contrast-enhanced computed tomography findings. ( A ) Axial contrast-enhanced CT image showing a large multilocular cystic-solid abdominopelvic mass with internal septations. The white arrows indicate septated cystic components, and the blue arrow indicates a solid component within the lesion. ( B ) Coronal CT image demonstrating extensive occupation of the abdominopelvic cavity by the mass. ( C ) Sagittal CT image showing marked cranio-caudal extension of the lesion Preoperative contrast-enhanced computed tomography findings. ( A ) Axial contrast-enhanced CT image showing a large multilocular cystic-solid abdominopelvic mass with internal septations. The white arrows indicate septated cystic components, and the blue arrow indicates a solid component within the lesion. ( B ) Coronal CT image demonstrating extensive occupation of the abdominopelvic cavity by the mass. ( C ) Sagittal CT image showing marked cranio-caudal extension of the lesion The patient underwent exploratory laparotomy with intraoperative frozen section assessment. According to the intraoperative findings, comprehensive surgical staging was completed during the same procedure. No staged or interval procedure was performed. Intraoperatively, a massive right ovarian tumor measuring approximately 30 cm in maximum diameter was identified. The tumor was densely adherent to the omentum and adjacent bowel loops. After careful adhesiolysis, the ovarian tumor was excised intact. Systematic exploration of the abdominal cavity revealed a separate, firm, well-circumscribed, mobile, exophytic mesenteric lesion measuring approximately 20 × 15 × 5 cm (Fig.  3 ). Incision of the lesion yielded thick, dark-brown hemorrhagic fluid (Fig.  4 C). Fig. 3 Intraoperative and gross findings of the mesenteric lesion. ( A ) Intraoperative view showing a separate, firm, mobile, exophytic mass in the small bowel mesentery, without gross continuity with the ovarian tumor. ( B ) Gross appearance of the resected mesenteric lesion, measuring approximately 20 × 15 × 5 cm Intraoperative and gross findings of the mesenteric lesion. ( A ) Intraoperative view showing a separate, firm, mobile, exophytic mass in the small bowel mesentery, without gross continuity with the ovarian tumor. ( B ) Gross appearance of the resected mesenteric lesion, measuring approximately 20 × 15 × 5 cm The hemorrhagic appearance of the mesenteric lesion complicated intraoperative assessment. Given the association between OCCC and endometriosis, extraovarian OCCC implants may occasionally present with hemorrhagic or endometriosis-like features; therefore, the gross appearance of this mesenteric lesion could mimic a metastatic implant and was difficult to distinguish macroscopically from malignant dissemination. The lesion was closely related to the mesentery and adjacent bowel loops; however, intraoperative assessment showed no direct bowel wall invasion, and the mesenteric blood supply to the adjacent bowel could be preserved. Frozen section analysis of the primary ovarian tumor indicated an epithelial malignancy. Frozen section analysis of the mesenteric lesion showed a benign fibrous cyst wall without evidence of malignancy. Based on these intraoperative and frozen section findings, the surgical team proceeded with comprehensive staging surgery rather than cytoreductive surgery for presumed advanced disease. The procedure included total abdominal hysterectomy, bilateral salpingo-oophorectomy, pelvic and para-aortic lymphadenectomy, omentectomy, appendectomy, and complete excision of the mesenteric lesion. The surgery was completed with no macroscopic residual disease. Gross examination of the surgical specimen confirmed a multilocular cystic-solid ovarian tumor containing hemorrhagic material (Fig.  4  A–B). Histopathological examination confirmed ovarian clear cell carcinoma. Microscopically, the tumor showed papillary and tubulocystic architectural patterns. The tumor cells were polygonal to hobnail-shaped, with abundant clear to eosinophilic cytoplasm and enlarged atypical nuclei (Fig.  5 A). Nuclear atypia was evident, and mitotic activity was present. Fig. 4 Gross appearance of the resected specimens. ( A ) External view of the resected ovarian tumor showing a large cystic-solid mass. ( B ) Cut surface of the ovarian tumor demonstrating a multilocular cystic-solid appearance with hemorrhagic contents and papillary projections. ( C ) Cut surface of the resected mesenteric lesion showing a dense fibrous cyst wall and thick, dark-brown hemorrhagic contents Gross appearance of the resected specimens. ( A ) External view of the resected ovarian tumor showing a large cystic-solid mass. ( B ) Cut surface of the ovarian tumor demonstrating a multilocular cystic-solid appearance with hemorrhagic contents and papillary projections. ( C ) Cut surface of the resected mesenteric lesion showing a dense fibrous cyst wall and thick, dark-brown hemorrhagic contents Immunohistochemical staining showed focal positivity for Napsin A and positivity for PAX8 and P16, supporting the diagnosis of OCCC (Fig.  5 B–C). The tumor cells were negative for ER, PR, PTEN, SALL-4, and Calretinin, with partial positivity for P53 and Vimentin. The Ki-67 proliferation index was approximately 20%. Fig. 5 Histopathological and immunohistochemical findings of the primary ovarian tumor and mesenteric lesion. ( A ) Hematoxylin and eosin (H&E) staining of the primary ovarian tumor showing ovarian clear cell carcinoma with papillary and tubulocystic architectural patterns. The tumor cells are polygonal to hobnail-shaped, with abundant clear to eosinophilic cytoplasm and enlarged atypical nuclei (×200). ( B ) Immunohistochemical staining showing focal cytoplasmic positivity for Napsin A in the tumor cells (×200). ( C ) Immunohistochemical staining showing nuclear positivity for PAX8 in the tumor cells (×200). ( D ) H&E staining of the mesenteric lesion showing a fibrous cyst wall without epithelial lining, cytological atypia, or malignant cells, consistent with a benign hemorrhagic cyst (×200) Histopathological and immunohistochemical findings of the primary ovarian tumor and mesenteric lesion. ( A ) Hematoxylin and eosin (H&E) staining of the primary ovarian tumor showing ovarian clear cell carcinoma with papillary and tubulocystic architectural patterns. The tumor cells are polygonal to hobnail-shaped, with abundant clear to eosinophilic cytoplasm and enlarged atypical nuclei (×200). ( B ) Immunohistochemical staining showing focal cytoplasmic positivity for Napsin A in the tumor cells (×200). ( C ) Immunohistochemical staining showing nuclear positivity for PAX8 in the tumor cells (×200). ( D ) H&E staining of the mesenteric lesion showing a fibrous cyst wall without epithelial lining, cytological atypia, or malignant cells, consistent with a benign hemorrhagic cyst (×200) The resected mesenteric lesion was diagnosed as a benign hemorrhagic mesenteric cyst. Histological examination showed a dense fibrous cyst wall without epithelial lining, cellular atypia, or malignant cells. No ovarian carcinoma component was identified within the mesenteric lesion. Peritoneal washing cytology revealed a few cells with nuclear atypia; however, no definitive malignant cells were identified, and the finding did not provide sufficient evidence of malignant peritoneal dissemination. The postoperative course was uneventful. Based on the final surgical and histopathological findings, the tumor was classified as FIGO stage IA ovarian clear cell carcinoma (T1aN0M0). The patient subsequently received adjuvant chemotherapy with paclitaxel 240 mg plus carboplatin AUC 5 every 21 days. At the most recent clinical evaluation, 12 months after surgery, she remained free of clinical and radiological evidence of recurrence and continued regular oncologic surveillance.

Ovarian clear cell carcinoma (OCCC) is a distinct subtype of epithelial ovarian cancer characterized by specific clinicopathological features, including an association with endometriosis and relatively low sensitivity to platinum-based chemotherapy [ 1 – 3 ]. Accurate FIGO staging is essential because it guides surgical management, prognostic assessment, and the need for adjuvant therapy [ 4 ]. Preoperative evaluation of suspected OCCC commonly includes serum tumor markers and cross-sectional imaging, such as computed tomography (CT) or magnetic resonance imaging (MRI). However, these modalities are not specific for malignancy or peritoneal dissemination. Benign extraovarian lesions, including inflammatory masses, hemorrhagic lesions, and cystic mesenteric lesions, may mimic metastatic disease on imaging, potentially leading to clinical overstaging and unnecessary surgical morbidity [ 5 ]. Herein, we report a case of early-stage OCCC in which preoperative imaging raised concern for peritoneal dissemination because of mesenteric abnormalities. During exploration, the hemorrhagic appearance of the mesenteric lesion further confounded the diagnosis, as it closely resembled an OCCC metastatic implant with endometriosis-like hemorrhagic features, making macroscopic distinction from malignant dissemination difficult. This case highlights a diagnostic pitfall and underscores the importance of integrating intraoperative findings with histopathological confirmation to ensure accurate staging and appropriate clinical management.

This case illustrates a diagnostic and staging pitfall in which a benign hemorrhagic mesenteric cyst mimicked peritoneal dissemination in early-stage OCCC. When imaging and tumor markers suggest extraovarian involvement, benign hemorrhagic or endometriosis-like lesions should also be considered in the differential diagnosis. Careful integration of radiologic findings, intraoperative assessment, frozen section analysis, and final histopathological confirmation is essential to avoid overstaging and guide appropriate management.

Ovarian clear cell carcinoma (OCCC) is a distinct histological subtype of epithelial ovarian cancer characterized by specific clinical and biological features [ 6 ]. Although early-stage OCCC is generally associated with a more favorable prognosis than advanced-stage disease, accurate staging remains essential because it directly influences surgical planning, prognostic assessment, and the need for adjuvant therapy [ 4 , 7 , 8 ]. In the present case, contrast-enhanced CT demonstrated mesenteric and omental abnormalities, which, together with elevated serum CA125, HE4, and CA19-9 levels, raised concern for advanced-stage ovarian malignancy with extraovarian involvement. However, final histopathological evaluation confirmed FIGO stage IA OCCC, and the mesenteric lesion was diagnosed as a benign hemorrhagic cyst. This discordance between the preoperative radiologic impression and final pathological staging highlights the difficulty of distinguishing true metastatic spread from benign mimickers in selected clinical scenarios. OCCC may exhibit heterogeneous patterns of spread, ranging from disease confined to the ovary to transcoelomic dissemination in more advanced cases [ 9 ]. In addition, OCCC is closely associated with endometriosis, and hemorrhagic or endometriosis-like lesions may be encountered in this disease context [ 10 , 11 ]. These biological and clinicopathological features can complicate the interpretation of extraovarian hemorrhagic lesions during preoperative imaging and intraoperative exploration. In the present case, the mesenteric lesion contained thick, dark-brown hemorrhagic fluid, which increased the concern that it could represent an extraovarian OCCC implant with endometriosis-like hemorrhagic features rather than a benign process. This intraoperative ambiguity emphasized the need for histopathological confirmation. The diagnostic challenge in this case extended beyond the general distinction between benign and malignant intra-abdominal lesions. Benign hemorrhagic mesenteric lesions may present as complex cystic masses with internal hemorrhage, and their gross appearance may overlap with that of hemorrhagic extraovarian implants in the context of OCCC [ 5 , 12 , 13 ]. In the present case, the convergence of a mesenteric location, old hemorrhagic content, and an underlying OCCC background created a misleading clinical scenario. As summarized in Table 1 [ 5 , 8 , 12 – 15 ], a broad differential diagnosis should be considered when evaluating mesenteric abnormalities in patients with suspected OCCC. Table 1 Differential diagnosis of mesenteric lesions mimicking peritoneal dissemination in ovarian clear cell carcinoma (OCCC) Diagnosis Typical imaging features Intraoperative findings Key distinguishing features OCCC peritoneal metastasis Irregular peritoneal nodules, ascites, peritoneal thickening, or complex cystic lesions Diffuse peritoneal seeding or discrete hemorrhagic implants Histopathology showing malignant clear or hobnail cells; often associated with other signs of carcinomatosis Hemorrhagic mesenteric cyst Complex cystic lesion with mixed attenuation or internal hemorrhagic density Smooth-walled cystic lesion containing dark-brown hemorrhagic fluid Fibrous cyst wall without epithelial lining, cellular atypia, or malignant cells Endometriosis-related lesion Cystic or nodular lesion with variable density due to hemorrhage “Chocolate-like” fluid, fibrotic adhesions, or endometriotic foci Endometrial glands and stroma on histopathology Inflammatory pseudotumor Solid or heterogeneous mass with variable enhancement Firm lesion with adhesions or inflammatory changes Prominent inflammatory cell infiltration without malignant epithelial components Gastrointestinal stromal tumor (GIST) Well-defined enhancing mass adjacent to bowel loops Exophytic mass arising from or closely related to the bowel wall Gastrointestinal origin; immunohistochemical positivity for CD117 or DOG1 Mesenteric hematoma Hyperdense or heterogeneous lesion on CT, depending on bleeding stage Organized clot-like lesion or hemorrhagic mass History of trauma, anticoagulation, coagulopathy, or vascular injury; absence of tumor cells Differential diagnosis of mesenteric lesions mimicking peritoneal dissemination in ovarian clear cell carcinoma (OCCC) Intraoperative frozen section analysis can provide important real-time information during surgery for ovarian tumors. Previous studies have reported that the overall diagnostic accuracy of intraoperative frozen section analysis for ovarian tumors is high, often exceeding 90%, although diagnostic performance may vary according to tumor type, borderline histology, tumor size, and sampling adequacy [ 16 , 17 ]. In the present case, frozen section analysis indicated an epithelial malignancy in the ovarian tumor and showed no evidence of malignancy in the mesenteric lesion. These findings supported the decision to complete comprehensive surgical staging rather than proceed with treatment based solely on the preoperative impression of advanced disease. Nevertheless, frozen section should be regarded as an adjunct to, rather than a substitute for, final histopathological evaluation. In conclusion, this case underscores the importance of tissue-based diagnosis when radiologic findings suggest advanced-stage ovarian malignancy, particularly in OCCC [ 6 , 16 , 18 ]. Elevated tumor markers and mesenteric abnormalities may raise suspicion for peritoneal dissemination, but benign mimickers—such as hemorrhagic mesenteric cysts or endometriosis-related lesions—should also be considered [ 5 , 19 ]. This case is limited by its single-case nature and relatively short follow-up period; therefore, the findings should not be overgeneralized. Nevertheless, it illustrates a clinically relevant diagnostic and staging pitfall. Careful integration of imaging findings, intraoperative assessment, frozen section analysis, and final histopathological confirmation remains essential for accurate staging and appropriate clinical decision-making [ 7 , 16 ].