Pseudo-peritoneal carcinomatosis: extensive endosalpingiosis mimicking metastatic dissemination in gastric cancer : a case report and literature review.

We report the case of a 57-year-old female patient with no significant personal medical history. Her family history is notable for gastric cancer in her mother. She presented with epigastric pain evolving over three months without other associated symptoms. The epigastric pain was considered clinically related to the gastric lesion. Clinical examination was unremarkable. Laboratory tests were within normal limits. The patient did not report weight loss, vomiting, or gastrointestinal bleeding. She had no gynecological symptoms. Upper gastrointestinal endoscopy (EGD) revealed a large, ulcerated, infiltrative lesion involving the gastric body and extending toward the antrum. The lesion appeared as an irregular, raised mass with friable mucosa and spontaneous superficial bleeding. Central ulceration with fibrinous and necrotic debris was noted, surrounded by thickened, indurated gastric folds, consistent with a circumferential infiltrative growth pattern (Fig.  1 ). Biopsy confirmed a poorly differentiated gastric carcinoma with a component of signet ring cells. Fig. 1 A Baseline upper gastrointestinal endoscopy. Upper gastrointestinal endoscopy showing a large ulcerated and infiltrative lesion involving the gastric body and extending toward the antrum. The lesion appears as an irregular, raised mass with friable mucosa and spontaneous superficial bleeding. B Central ulceration with fibrinous and necrotic debris is present, associated with thickened and indurated gastric folds A Baseline upper gastrointestinal endoscopy. Upper gastrointestinal endoscopy showing a large ulcerated and infiltrative lesion involving the gastric body and extending toward the antrum. The lesion appears as an irregular, raised mass with friable mucosa and spontaneous superficial bleeding. B Central ulceration with fibrinous and necrotic debris is present, associated with thickened and indurated gastric folds For staging, a thoraco-abdominopelvic CT scan showed circumferential thickening of the antrum-pyloric region measuring 80 × 50 mm with surrounding fat stranding and a few suspicious satellite lymph nodes. In addition, diffuse peritoneal nodules consistent with peritoneal carcinomatosis were observed throughout the peritoneal cavity (Fig.  2 ). The uterus and both ovaries were of normal size and appearance. Osteosclerotic lesions of the right 5th and 7th ribs were also noted. A subsequent bone scintigraphy did not reveal any secondary osseous involvement. Tumor markers were: CEA :1.58 ng/mL, CA 19 − 9 : 30.87 U/mL. The tumor was initially classified as stage IV disease based on radiological findings. Fig. 2 CT scan axial ( A ) slices and coronal ( B ) reconstruction with contrast injection showing parietal thickening of the stomach (asterisk), micronodular thickening of the fat (arrow) and irregular thickening of the peritoneal layers at the level of the pelvis (arrowhead) suggestive of carcinomatosis CT scan axial ( A ) slices and coronal ( B ) reconstruction with contrast injection showing parietal thickening of the stomach (asterisk), micronodular thickening of the fat (arrow) and irregular thickening of the peritoneal layers at the level of the pelvis (arrowhead) suggestive of carcinomatosis The case was discussed in a multidisciplinary tumor board meeting, given the radiological suspicion of diffuse peritoneal carcinomatosis highly suggestive of metastatic disease, the decision was to start first-line systemic chemotherapy. The patient underwent four cycles of chemotherapy combining docetaxel, cisplatin, and 5-fluorouracil (5-FU). After chemotherapy, the patient reported a progressive improvement of epigastric pain. Post-chemotherapy CT scan showed a dissociated response. Indeed, the post-chemotherapy CT scan showed a significant radiological response, with only a small residual gastric mass measuring10 × 10 mm remaining in the antrum-pyloric region. Surrounding fat stranding and previously noted satellite lymph nodes decreased to millimetric size. However, the multiple peritoneal nodules previously identified showed no reduction in size, number, or extent, appearing completely stable despite the treatment. A follow-up EGD confirmed this major local response. Specifically, a small residual infiltrative lesion measuring approximately 15 mm in diameter persisted at the antral region, characterized by irregular but significantly flattened mucosal folds and reduced friability compared to baseline, consistent with a marked partial response to chemotherapy (Fig.  3 ). Fig. 3 Follow-up upper gastrointestinal endoscopy after chemotherapy. Follow-up upper gastrointestinal endoscopy showing a residual infiltrative ulcerated lesion measuring approximately 15 mm in the antrum. The lesion appears with irregular but flattened mucosal folds and reduced friability compared to baseline, without active bleeding Follow-up upper gastrointestinal endoscopy after chemotherapy. Follow-up upper gastrointestinal endoscopy showing a residual infiltrative ulcerated lesion measuring approximately 15 mm in the antrum. The lesion appears with irregular but flattened mucosal folds and reduced friability compared to baseline, without active bleeding The case was re-evaluated in a multidisciplinary tumor board ; due to this clinical discrepancy, the malignant nature of the peritoneal nodules was strongly questioned. An exploratory laparoscopy was therefore decided upon to obtain definitive histological characterization of these persistent lesions, as their nature would determine whether the patient remained a candidate for curative surgery or should be redirected toward a second-line systemic chemotherapy. Intraoperatively, the exploration confirmed the presence of multiple, diffuse, millimetric whitish nodules. These lesions were extensively distributed throughout the pelvic cavity, particularly involving the pouch of Douglas and the parietal peritoneum, mimicking a high-load peritoneal carcinomatosis (Fig.  4 ). Biopsies of the peritoneal nodules were performed. The frozen section revealed benign glandular inclusions lined by a single layer of regular epithelium, without nuclear atypia, mitoses, or desmoplastic stroma. Crucially, there was no evidence of malignant cells or signet-ring cells. This intraoperative finding strongly suggested a benign mimicker rather than metastatic spread. Reassured by the absence of true peritoneal carcinomatosis, the surgical team decided to proceed with the planned curative-intent gastric resection. Simultaneously, multiple peritoneal biopsies were collected for definitive permanent sectioning and immunohistochemistry to formally confirm the benign diagnosis of the nodules. Fig. 4 Laparoscopic view showing diffuse peritoneal carcinomatosis nodules ( A , B ) Laparoscopic view showing diffuse peritoneal carcinomatosis nodules ( A , B ) The patient underwent a total gastrectomy with a D2 lymph node dissection and a Roux-en-Y esophagojejunal anastomosis. The postoperative course was completely uneventful, and the patient was successfully discharged on postoperative day 8. The final histopathological examination confirmed the intraoperative suspicion : Microscopically all sampled peritoneal nodules consisted of benign glandular structures lined by a ciliated tubal-type epithelium (Fig.  5 ). Fig. 5 Hematoxylin & Eosine stained section; at low magnification (x2) ( A ) showing a fibrous tissue occupied by a cystic proliferation with papillary projections lined (at high magnification x40) ( B ) by a tubal epithelium made of regular ciliated cells Hematoxylin & Eosine stained section; at low magnification (x2) ( A ) showing a fibrous tissue occupied by a cystic proliferation with papillary projections lined (at high magnification x40) ( B ) by a tubal epithelium made of regular ciliated cells Immunohistochemical analysis showed strong positivity for epithelial membrane antigen (EMA), supporting the epithelial nature of the lesion, and PAX8, confirming a Müllerian origin. The lesional cells also exhibited a wild-type (non-mutated) p53 expression pattern (Fig.  6 ). Fig. 6 The immunohistochemical analysis of the peritoneal lesions shows: A Epithelial Membrane Antigen(EMA) immunostaining (×200) showing strong cytoplasmic positivity, consistent with epithelial differentiation of the glandular structures; ( B ) PAX8 immunostaining (×200) demonstrating diffuse nuclear positivity, supporting a Müllerian origin; ( C ) p53 immunostaining (×200) showing a wild-type (heterogeneous/patchy) expression pattern The immunohistochemical analysis of the peritoneal lesions shows: A Epithelial Membrane Antigen(EMA) immunostaining (×200) showing strong cytoplasmic positivity, consistent with epithelial differentiation of the glandular structures; ( B ) PAX8 immunostaining (×200) demonstrating diffuse nuclear positivity, supporting a Müllerian origin; ( C ) p53 immunostaining (×200) showing a wild-type (heterogeneous/patchy) expression pattern Simultaneously, the histopathological examination of the gastrectomy specimen revealed a 10-mm residual tumor focus composed of signet-ring cells, infiltrating the muscularis propria (ypT2). No lymph node metastases were identified among the 24 harvested lymph nodes (ypN0), and there was no evidence of lymphovascular or perineural invasion. The proximal, distal, and circumferential resection margins were all strictly negative (R0). Following surgery, the patient was referred for adjuvant chemotherapy to complete her multimodal treatment. At the 4-month follow-up, she remains in complete clinical and radiological remission, and during subsequent clinical surveillance, she reported complete disappearance of epigastric pain.

This case emphasizes that histological confirmation should be obtained in cases of suspected peritoneal carcinomatosis when diagnostic uncertainty persists. However, an individualized approach may be considered in selected clinically stable patients, particularly when a dissociated response between the primary tumor and peritoneal lesions raises the possibility of a benign mimicker. In such situations, short-interval reassessment and/or additional imaging may be useful, although diagnostic laparoscopy remains the most definitive method for establishing the diagnosis.

Gastric cancer is a heterogeneous malignant disease arising from the gastric mucosa, most commonly represented by adenocarcinoma. Although the incidence of gastric cancer has decreased, it remains one of the most common types of cancer and one of the leading causes of cancer-associated mortality worldwide [ 1 ]. Ranking as the fifth most common cancer globally and the third leading cause of cancer-related death, it continues to pose a major public health challenge [ 2 ]. Despite advances in diagnosis and treatment, gastric cancer is frequently diagnosed at an advanced stage, often with locoregional extension or distant metastases, particularly to the peritoneum. In the present case, the patient presented with isolated epigastric pain, which is a common symptom of gastric cancer. The patient’s epigastric pain showed progressive improvement following chemotherapy and completely resolved after surgical resection, supporting its direct association with the gastric primary tumor rather than with the peritoneal lesions. Peritoneal carcinomatosis in gastric cancer is considered a distant metastasis and is detected in ~ 60% of patients at diagnosis or following initial curative-intent treatments [ 1 ]. It is associated with a poor prognosis, with median survival ranging from 4 to 6 months, and limited therapeutic options [ 3 ]. As with any malignancy, initial management must rely on accurate assessment of disease extent, including locoregional and distant spread. Cross-sectional imaging modalities such as CT and Magnetic Resonance Imaging are standard ; however, they may underestimate peritoneal involvement, particularly in cases of small-volume disease or subtle peritoneal implants. Radiologically, peritoneal carcinomatosis typically presents as irregular enhancing peritoneal or omental nodules, often associated with ascites, omental caking, fat stranding with a desmoplastic reaction, and progressive disease on follow-up imaging [ 7 , 8 ]. In contrast, benign peritoneal conditions such as endosalpingiosis are usually non-specific on imaging and may present as small, well-defined or millimetric peritoneal nodules. Unlike metastatic disease, these lesions generally lack ascites, omental infiltration, or progressive peritoneal involvement over time [ 6 , 9 ]. However, significant radiological overlap exists, and imaging alone is insufficient for definitive differentiation. In the present case, a key suggestive feature was the dissociated response to systemic chemotherapy, characterized by marked regression of the primary gastric tumor while the peritoneal nodules remained completely stable. This radiological stability, in the context of an otherwise favorable tumor response and absence of ascites or progressive peritoneal changes, should raise suspicion for a non-malignant peritoneal process. Such a discordant pattern may warrant reconsideration of the initial staging and support histopathological confirmation before definitive therapeutic decisions. Consequently, diagnostic laparoscopy plays a pivotal role in improving staging accuracy by enabling direct visualization and biopsy of peritoneal lesions. This combined radiological–surgical approach is essential for guiding therapeutic strategy and avoiding misclassification caused by rare benign mimickers of peritoneal metastases. Diagnostic laparoscopy is widely used in gastrointestinal oncology and has demonstrated significant value in detecting occult peritoneal disease [ 4 ]. Ishigami et al. reported that peritoneal dissemination is frequently underestimated on imaging, leading to unnecessary laparotomies, while staging laparoscopy significantly improves detection of unsuspected metastases [ 5 ]. Differential diagnosis of peritoneal nodules remains a major clinical challenge. It is crucial to distinguish peritoneal carcinomatosis from other diffuse peritoneal diseases, as several benign or inflammatory conditions may mimic metastatic spread and lead to inappropriate management (Table  1 ). In practice, conditions such as lymphomatosis, tuberculosis, mesothelioma, and pseudomyxoma peritonei should always be considered [ 6 ]. Table 1 Selected reported cases of endosalpingiosis associated with malignancies Author – Year Malignancy Site of endosalpingiosis Clinical context Tsuchiya et al., 1999 [ 10 ] Gastric adenocarcinoma Perigastric lymph nodes Radical gastrectomy Yang et al., 2019 [ 11 ] Endometrial & cervical carcinoma Uterus (myometrium) Uterine mass surgery Niwa et al., 2020 [ 12 ] Colorectal surgical case (suspected malignancy) Mesocolic lymph nodes Oncologic resection Homsi et al., 2022 [ 13 ] Suspected peritoneal carcinomatosis Peritoneum Diffuse nodules Sabri et al., 2022 [ 14 ] Gynecologic malignancies (reviewed cases) Vaginal / pelvic sites Symptomatic lesions Bazmi et al., 2023 [ 15 ] Neuroendocrine tumor (GI origin suspected) Peritoneum, pelvis Abdominal pain Kobayashi2023 [ 16 ] Endometriosis-associated malignancy risk Pelvis Cystic pelvic mass Müller et al., 2025 [ 17 ] Incidental finding in abdominal surgery Appendix Surgical exploration Current case Suspected peritoneal carcinomatosis Diffuse peritoneal nodules Staging workup of gastric cancer Selected reported cases of endosalpingiosis associated with malignancies Among these mimickers, endosalpingiosis represents a particularly challenging entity. It is a benign Müllerian lesion characterized by ectopic tubal-type epithelium outside the fallopian tubes. Although its clinical relevance is limited by its benign nature, its ability to mimic peritoneal carcinomatosis both radiologically and intraoperatively makes it a significant diagnostic pitfall. Originally described by Sampson in 1930, endosalpingiosis was initially thought to result from post-surgical implantation but is now recognized as a developmental or metaplastic process involving ectopic benign tubal-type epithelium [ 12 ]. It is frequently identified as an incidental microscopic finding, with a reported prevalence of approximately 7.6% in women undergoing gynecologic laparoscopy [ 18 ]. Importantly, endosalpingiosis is typically identified in gynecological contexts; its occurrence in association with non-gynecological malignancies, particularly gastric cancer, is exceptional. Its clinical importance lies in its potential to mimic peritoneal spread rather than in its frequency. Macroscopically, endosalpingiosis appears as multiple small cystic lesions, often millimetric, whitish or translucent, giving a vesicular or glandular appearance [ 19 ]. Rarely, it may present as a mass-forming lesion [ 18 ]. It commonly involves the peritoneum, omentum, urinary bladder, and pelvic lymph nodes [ 12 ]. In our case, lesions were diffusely distributed across both supra- and inframesocolic compartments, mimicking extensive peritoneal carcinomatosis. This diffuse presentation, combined with a known gastric malignancy, created a major diagnostic dilemma. Indeed, peritoneal nodules in the context of gastric cancer are highly suggestive of stage IV disease. However, distinguishing benign conditions such as endosalpingiosis is essential, as misinterpretation may lead to inappropriate therapeutic de-escalation. In this setting, intraoperative frozen section analysis is crucial for rapid assessment, while definitive diagnosis relies on histopathology supported by immunohistochemistry. Endosalpingiosis typically expresses epithelial markers, including epithelial membrane antigen (EMA), together with Müllerian markers such as PAX8, and usually exhibits a wild-type p53 expression pattern [ 18 ]. In our case, these immunohistochemical findings supported the diagnosis of florid endosalpingiosis and excluded metastatic gastric adenocarcinoma. Several studies have suggested an association between endosalpingiosis and gynecological malignancies, particularly ovarian and uterine tumors, raising the hypothesis of a shared Müllerian origin. Florid forms may closely simulate diffuse peritoneal carcinomatosis [ 20 ]. Hermens et al. reported that 28.6% of patients with histologically confirmed endosalpingiosis had associated gynecological malignancies ( 18 ). Similarly, Bazmi et al. reported concurrent gynecologic malignancy in up to 42% of cases in a large cohort [ 15 ]. Endosalpingiosis may also mimic serous carcinoma, further complicating diagnosis [ 21 , 22 ]. Extra-gynecological presentations are rare but documented, including mesocolic lymph node involvement [ 12 ] and cutaneous umbilical lesions [ 20 ]. Regarding gastric cancer, only a single previous case has been reported, limited to microscopic lymph node involvement without peritoneal disease [ 20 ]. That case did not involve macroscopic carcinomatosis or staging confusion. In contrast, our case demonstrates a unique scenario of florid peritoneal endosalpingiosis mimicking diffuse metastatic carcinomatosis during gastric cancer staging. This represents a diagnostic pitfall rather than an exceptionally rare entity. Making this distinction is critical due to major therapeutic implications. Gastric cancer with true peritoneal carcinomatosis is classified as stage IV disease and is treated with palliative systemic chemotherapy, typically fluoropyrimidine- and platinum-based regimens. Conversely, the absence of true peritoneal metastasis, even in the presence of extensive benign endosalpingiosis, allows continuation of a curative-intent multimodal strategy. In this case, accurate pathological assessment directly impacted management and enabled curative surgery. Overall, this case emphasizes careful clinicopathological correlation in suspected peritoneal carcinomatosis to avoid misclassification and inappropriate management.

Peritoneal carcinomatosis is a frequent and prognostically adverse manifestation of advanced gastric cancer, representing stage IV disease and significantly influencing therapeutic strategy and survival outcomes. Therefore, accurate staging remains a cornerstone in the management of gastric cancer, directly determining eligibility for curative surgical resection versus systemic therapy [ 1 , 2 ]. Although cross-sectional imaging modalities such as computed tomography and magnetic resonance imaging are routinely used for staging, they may fail to detect small peritoneal implants or may overestimate disease burden. In this context, staging laparoscopy has emerged as a highly valuable tool, significantly improving the detection of occult peritoneal metastases and preventing unnecessary laparotomies [ 3 , 4 ]. However, despite its high diagnostic accuracy, staging laparoscopy is not immune to interpretative pitfalls, particularly when benign peritoneal conditions mimic metastatic dissemination. Among these rare mimickers, endosalpingiosis is an uncommon benign Müllerian-derived lesion characterized by ectopic tubal-type epithelium outside the fallopian tubes. Although typically incidental and asymptomatic, it may occasionally present as multiple peritoneal nodules closely resembling carcinomatosis both radiologically and intraoperatively. Its florid or extensive forms can create a striking macroscopic appearance indistinguishable from malignant peritoneal spread, thereby representing a significant diagnostic challenge for surgeons and pathologists [ 5 , 6 ]. However, cases in which extensive endosalpingiosis completely mimics peritoneal carcinomatosis in patients with an active non-gynecological malignancy are rarely reported in the literature and may lead to erroneous stage IV classification ; such cases remain exceedingly rare. This diagnostic overlap becomes particularly critical in gastric cancer, where the presence of peritoneal metastases determines prognosis and treatment strategy and may preclude potentially curative surgical approaches [ 3 , 6 ]. To our knowledge, the coexistence of gastric cancer with extensive peritoneal endosalpingiosis mimicking peritoneal carcinomatosis has rarely been reported, and this diagnostic confusion represents an underrecognized clinical pitfall. Very few cases have been described in the literature. Herein, we present a rare case of gastric cancer with extensive peritoneal endosalpingiosis entirely mimicking peritoneal carcinomatosis, managed at the Salah Azaïez Institute, highlighting the importance of histopathological confirmation to avoid overstaging and to guide appropriate therapeutic management. To our knowledge, this represents one of the very few reported cases of extensive peritoneal endosalpingiosis mimicking metastatic carcinomatosis in the setting of gastric cancer staging, highlighting a rare but clinically significant diagnostic pitfall.