MOC exhibits notable molecular and phenotypic similarities with upper gastrointestinal tumors, particularly adenocarcinomas at the gastroesophageal junction, in terms of histomorphology and molecular classification (19). The immunohistochemical expression profiles of CK7, CK20, CDX2, and Special AT-rich sequence-binding protein 2 (SATB2) bear striking resemblance between MOC and gastric-origin tumors. The differential expression of Paired Box Protein 8 (PAX8) and Carbohydrate Antigen 125 (CA125) aids in distinguishing MOC, with 35% and 24% of primary MOCs expressing these markers, respectively, a contrast to their absence in early gastric cancer (20). However, recent studies have shown that when mucinous gastric cancer metastasizes to the peritoneum, CA125 has a certain level of increase, so it is not easy to distinguish CA125 from MOC for mucinous gastric cancer that metastasizes to the peritoneum in the late stage (21, 22). In MOC, Estrogen Receptor (ER) and Progesterone Receptor (PR) are usually not expressed, and the expression in gastric cancer is related to the degree of cell differentiation and histological type. However, due to the low sensitivity of ER and PR, the diagnostic performance is not strong (23, 24).

Research by Heinzelmann-Schwarz et al. (25) has shown that in the tissues from 10 MOC and 38 gastrointestinal mucinous carcinoma cases, cytoplasmic galectin-4 displays consistent expression in both MOC and gastrointestinal mucinous carcinomas, albeit at significantly lower levels in MOC compared to gastrointestinal cancers. They also identified meprin A as a valuable adjunct marker for discerning primary ovarian mucinous adenocarcinomas from metastatic counterparts (25).Currently, CK7 and CK20 remain the standard immunohistochemical markers for ascertaining the tissue origin of mucinous ovarian adenocarcinomas.

Differentiating primary MOC from metastatic tumors originating from the pancreaticobiliary tract is a complex process, primarily due to the overlapping marker positivity rates between these two tumor types.PAX8, Deleted in Pancreatic Carcinoma 4 (DPC4), also known as Smad4, and Cytokeratin 17 (CK17) serve as critical differential markers. PAX8 is present in 36% of primary MOC cases, compared to a minimal 4% in pancreatobiliary malignancies (1). The lack of DPC4 expression, which occurs in 53% of pancreatic cancers and 5–10% of primary MOCs, is a significant diagnostic feature suggestive of pancreatobiliary tract metastasis (26, 27). Such absence is considered to be a landmark differential point for the confirmation of MMC of pancreaticobiliary origin (18). Meanwhile, CK17 is typically absent in mucinous ovarian tumors but is found in 27–83% of metastatic pancreatic cancers, marking its expression as an indicator of pancreatic origin (28). The study by Yang et al. (29) revealed that claudin-18 is a sensitive and specific marker for adenocarcinomas arising from the gastric and pancreaticobiliary ducts, boasting a sensitivity of 79% and a specificity of 93%. However, claudin-18 positivity was also noted in over half (more than 54%) of MOC cases, indicating that while useful, claudin-18 expression alone cannot definitively distinguish MOC from tumors of pancreaticobiliary duct origin. Mucin 6 (MUC6) is primarily expressed in the upper gastrointestinal tract and pancreas. In primary MOC, MUC6 expression is typically low. In contrast, mucinous carcinomas originating from the upper gastrointestinal tract exhibit high MUC6 expression, with a positivity rate of 76% (31/41 cases) (30).

The diagnostic distinction of primary MOC from lower gastrointestinal metastases (colorectal and appendiceal cancers) relies on the nuanced interpretation of immunohistochemical markers including CK7, CK20, CDX2, SATB2, and PAX8. CK7 and CK20 are crucial markers in this context. CK20, a marker with high tissue specificity, is naturally present in the gastrointestinal mucosa and is commonly upregulated in gastrointestinal tumors. CK7, although more broadly distributed, is typically absent in colorectal cancers and is present in approximately 90% of primary MOC cases, often diffusely expressed in approximately 85% of the tumor cells (1). In metastatic from colorectal and appendiceal cancers, CK7 is detected in a minority of cases—31% and 26% respectively—with only 6% and 13% showing diffuse expression (1). CK20 is detected in 65–70% of primary MOC cases and is diffusely present in around 40% of these cases. However, CK20 is detected in the 90% and 92% colorectal and appendiceal metastases, with high rates of diffuse expression (1). In terms of co-expression, primary MOC demonstrates CK7+/CK20+ in 67% of cases, CK7+/CK20- in 26% and CK7– CK20+ positive in 7% of cases. Interestingly, MOC of teratomatous origin often shows a CK7-/CK20+ profile in 50% of the cases (1). In comparison, only a fraction of appendiceal and colorectal cancer cases exhibit both markers (22% and 11%, respectively), 78% and 79% of these cases, respectively, are CK7– CK20+, 0% and 3% are CK7+ CK20–, and 0% and 6% are CK7– CK20– (1).

CDX2, a transcription factor crucial in gastrointestinal development, manifests in 49% of primary MOC cases with strong expression in 26%. However, its strong expression is much more prevalent in colorectal (93%) and appendiceal (97%) cancers (1). SATB2, another marker with high specificity for colorectal and appendiceal malignancies (31), is typically absent in primary MOC but diffusely expressed in teratoma-associated MOC (9). Meagher et al. (24) have established that a lack of CK7 coupled with any level of SATB2 expression is indicative of a primary gastrointestinal tumor. focal CK7 positivity with diffuse SATB2 positivity also reflects a primary gastrointestinal tumor, Conversely, diffuse CK7 expression with absent or negative SATB2 expression points to primary ovarian cancer, while diffuse positivity for both CK7 and SATB2 suggests a lower gastrointestinal origin. PAX8, noted for its limited expression in ovarian tissue, is weakly and focally expressed in about 35% of MOC cases, contrasting with its 5% expression in appendiceal cancers and absence in colorectal cancers (1).

IMP3 has been observed to vary significantly between metastatic and primary mucinous ovarian adenocarcinomas, with its expression correlating with tumor aggressiveness (32). While typically negative or weak positive in primary ovarian lesions, IMP3 tends to be moderate to strong in metastatic lesions, especially those affecting the uterus and greater momentum (32). The diagnostic potential of IMP3 in differentiating primary from metastatic MOC needs further investigation.

Mucin2 (MUC2) is a secretory mucin predominantly expressed in the lower gastrointestinal tract. In metastatic colorectal adenocarcinoma involving the ovary, MUC2 has an expression rate of approximately 51%, whereas it is not expressed in MOC (33). Conversely, Mucin5AC (MUC5AC) is almost not expressed in colorectal-origin mucinous carcinomas, but shows diffuse positive expression in primary MOC, with an expression rate of 86% (34).

A multi-marker immunohistochemical approach significantly enhances the accuracy of distinguishing primary from metastatic MOC. The concomitant assessment of CDX2, CK7, and Dipeptidase 1 (DPEP1) has demonstrated high diagnostic precision—97% for primary MOC (16/16 tumors) and 100% for metastatic rectal cancer (16/16 tumors). However, the accuracy for detecting upper gastrointestinal metastases remains lower at 56% (9/16 cases) (35). Given the distinct immunophenotype of teratoma-derived MOC, reliance solely on immunohistochemical results is inadequate, and genetic testing is recommended to provide a comprehensive diagnostic conclusion.

Tumors associated with human papillomavirus (HPV) of cervical adenocarcinomas origin, can present with histological similarities to primary MOC (36). However, the expression of the cell cycle regulatory protein p16 provides a valuable discriminative marker. Primary MOC is only diffusely positive for p16 in a minor fraction of cases—approximately 5.7% (37). In contrast, p16 has been identified as a highly sensitive (100%) and specific (98%) marker for the identification of ovarian metastases stemming from cervical adenocarcinoma (38). Non-HPV-associated cervical adenocarcinomas of gastrointestinal origin usually express CK7, with about 50% of these tumors also expressing CK20 and CDX-2. Typically, ER and PR are not expressed, while PAX8 may be positive in these cervical tumors. They may also exhibit limited positivity for gastric mucin differentiation markers, such as MUC6 and the Anti-Mucin antibody1083 (HIK1083), though these markers lack sensitivity and specificity for this tumor type (39).

Endometrioid ovarian carcinoma typically exhibits positive immunoreactivity for ER and PR, as well as for CA125 and vimentin, markers that are generally negative in primary MOC. Notably, with the confirmation of endometrial histology based on PR and waveform protein expression (40), approximately one-fifth (19.6%) of cases previously classified as MOC have been reclassified as endometrioid carcinomas (41). The expression of PAX8, a transcription factor, further aids in the distinction between these two entities. While endometrioid carcinoma commonly shows diffuse and strong PAX8 positivity, primary MOC is often negative or only weakly positive for this marker (24). The work of Wookbeck et al. (40) emphasizes the diagnostic precision of using a combined immunohistochemical profile of PR and vimentin. This approach has been reported to accurately differentiate ovarian endometrioid carcinoma from MOC with over 95% accuracy, underscoring the utility of a multimarker strategy in refining ovarian cancer subtyping.

The therapeutic management of primary versus metastatic ovarian tumors differs markedly, yet intraoperative challenges often impede definitive diagnosis. Limitations in rapid intraoperative pathology sampling for frozen pathological analysis and the inability to perform timely immunohistochemistry present significant obstacles. Consequently, the most efficacious approach for intraoperative diagnosis leverages a combination of macroscopic tumor assessment, histological examination, and careful review of the patient’s clinical history. Definitive pathological conclusions drawn from paraffin-embedded specimens are crucial for guiding subsequent treatment strategies.

Accurate differentiation is imperative, especially in distinguishing MOC from conditions with similar presentations. High-grade serous carcinoma (HGSC), more prevalent than MOC, is typically distinguishable through gross morphological inspection and immunohistochemical markers such as Wilms Tumor 1 protein (WT1) (42). A comprehensive summary of immunohistochemical markers employed in the identification of MOC and its differentiation from other tumor origins, such as MMC and HGSC, is detailed in Table 1 .

MOC is genetically distinct from other subtypes of epithelial ovarian cancer and shares several molecular characteristics with gastrointestinal tract tumors. The prevalence of Kirsten rat sarcoma viral oncogene (KRAS) mutations in MOC is slightly higher compared to mucinous and non-mucinous colorectal carcinomas, with mutations observed in 43–46% of MOC cases versus 30% of mucinous colorectal tumors. Furthermore, approximately 20% of mucinous colorectal carcinomas exhibit mutations in V-raf murine sarcoma viral oncogene homolog B1 (BRAF) (42).The amplification of Human Epidermal Growth Factor Receptor-2 (HER2), also known as ERBB2, is found in approximately 20–30% of invasive MOC cases and 6% of mucinous borderline tumors (MBTs), contrasting with its rarity in colorectal and non-mucinous carcinomas (42). Conversely, Tumor Protein P53 (TP53) mutations are emblematic of high-grade serous carcinoma (HGSC), identified in 96% of HGSC cases but less than 30% of MOC (43). Ohnishi et al. (45) discovered KRAS mutations in 43.8% of MOC cases (7/16cases) and 20% of MBTs(2/10cases), yet found no such mutations in benign mucinous ovarian tumors. BRAF mutations were present in four MBTs but absent in MOC and benign tumors. Notably, TP53 mutations were not observed in their study (45). These findings suggest a possible association between KRAS mutations in MBTs and the progression to invasive MOC, while BRAF mutations do not exhibit a similar correlation.

Genomic studies reveal significant mutational overlap among benign, borderlin, and malignant mucinous ovarian tumors. Cheasley et al. (46) identified copy number alterations as pivotal in the malignant transformation and metastatic potential of mucinous ovarian tumors, suggesting their value as prognostic indicators. Benign mucinous ovarian tumors frequently harbor mutations in KRAS or Cyclin-dependent kinase inhibitor 2A (CDKN2A). Mucinous borderline tumors, which often develop in the presence of these mutations, may also exhibit additional genomic copy number variations. A high prevalence of CDKN2A mutations has been documented in both MOC (91%) and mucinous borderline tumors (95%) (46). Concordantly, other research has correlated KRAS and CDKN2A pathway alterations withmucinous cystadenomas and borderline tumors (47). CDKN2A and KRAS occur in the early stages of tumor development (42). Common genomic features shared by benign, borderline, and malignant mucinous ovarian tumors include the overall mutational burden and the number of point mutations.TP53 pathogenic variants, which increase significantly in malignancy, suggest a driving role in the malignant evolution of these tumors (46, 48). The transition from borderline to malignant status is often marked by TP53 mutations or amplifications (49). Notably, the frequency of KRAS mutations increased from benign to borderline to malignant stages, underscoring its importance in the continuum of cancer developmen (50). In another study, BRAF mutations, while rarer, appear more frequently in malignant carcinomas compared to their benign and borderline counterparts (51).The intricate molecular landscape, detailing the mutational profiles of MOC, mucinous borderline tumors, HGSC, and colorectal cancer, is systematically presented in Table 2 .