By Jason Wasserman MD PhD FRCPC
April 16, 2026
Mucinous carcinoma of the ovary is a type of ovarian cancer that develops from cells that produce mucus. Under the microscope, the tumor cells resemble the cells that line parts of the digestive tract, such as the stomach or intestines, which is why pathologists describe this tumor as having “mucinous” or gastrointestinal-type features. Mucinous carcinoma is relatively uncommon, accounting for about 3–4% of ovarian carcinomas. Most cases are diagnosed at an early stage and are still confined to the ovary at the time of detection. This article will help you understand the findings in your pathology report — what each term means and why it matters for your care.
What are the symptoms?
Most patients develop symptoms related to a pelvic mass. These may include abdominal swelling or bloating, pelvic pressure, abdominal or pelvic pain, or a feeling of fullness. Because mucinous carcinomas can grow slowly and often reach a large size before causing symptoms, they are sometimes detected when imaging is performed for unrelated reasons. In many cases the tumor is very large at the time of diagnosis.
What causes mucinous carcinoma of the ovary?
The exact cause is not fully understood, but several genetic changes within tumor cells are known to play a role. Mutations in the KRAS gene — which normally helps regulate cell growth — are found in about two-thirds of cases and are thought to occur early in tumor development. Loss of the CDKN2A gene, which normally acts as a brake on cell division, is also found in about three-quarters of cases. Mutations in the TP53 gene, which controls cell growth and DNA repair, are present in roughly two-thirds of cases and appear to be associated with progression from a less aggressive tumor to invasive carcinoma. Amplification of the ERBB2 (HER2) gene, which drives cell growth signaling, is seen in approximately 15–25% of cases.
Relationship to noncancerous ovarian tumors
Many mucinous carcinomas of the ovary develop from pre-existing noncancerous or borderline ovarian tumors. In most cases, the cancer arises from a mucinous borderline tumor — a tumor whose cells are abnormal but have not yet grown into the surrounding tissue. Over time, additional genetic changes can allow the tumor to become invasive. Less commonly, mucinous carcinoma arises from other benign ovarian tumors such as mature cystic teratomas or Brenner tumors. When the pathologist examines the tumor under the microscope, it may contain a mixture of benign, borderline, and malignant areas side by side, which supports the idea that the carcinoma developed gradually from a pre-existing lesion.
Could a mucinous tumor in the ovary have come from another organ?
One of the most important questions in the evaluation of a mucinous ovarian tumor is whether it actually started in the ovary or whether it spread to the ovary from another organ. Cancers that begin in the appendix, colon, stomach, or pancreas can spread to the ovary and produce tumors that look almost identical to primary mucinous carcinoma of the ovary under the microscope — because they also produce mucus and contain intestinal-type cells.
Determining the site of origin is critical because the treatment and prognosis differ significantly depending on where the cancer started. Several features help the pathologist make this distinction. Primary mucinous carcinomas of the ovary tend to be unilateral (affecting only one ovary), are often very large (greater than 10–13 cm), and frequently show a mixture of benign, borderline, and malignant areas. Metastatic tumors from another organ are more likely to involve both ovaries, to be smaller, and to show a pattern of growth on the surface of the ovary or as separate nodules within the ovarian tissue rather than arising from within it.
For this reason, surgeons often remove the appendix during surgery for a mucinous ovarian tumor, even if it looks normal to the naked eye. The appendix is then examined under the microscope to look for a primary tumor that may have spread to the ovary. Additional immunohistochemistry tests are also used to help identify the site of origin.
How is the diagnosis made?
The diagnosis is usually made after a tissue sample is examined under the microscope by a pathologist. The sample is most often obtained when the tumor is removed at surgery. If surgery is performed, the pathologist also examines other tissues removed during the operation — including the fallopian tubes, uterus, appendix, lymph nodes, and any abdominal tissue samples — to help determine where the tumor started and how far it has spread.
Under the microscope, mucinous carcinoma of the ovary shows cells that produce large amounts of mucus, forming glands that resemble those of the gastrointestinal tract. The tumor may contain a mixture of benign, borderline, and malignant areas within the same specimen. Two main patterns of invasion can be seen, and identifying which pattern is present is important because it affects prognosis. In the expansile (confluent) pattern, the tumor glands are tightly crowded together, with very little supporting tissue between them, creating a labyrinth-like or maze-like appearance. This pattern is more common and associated with better outcomes. In the infiltrative (destructive) pattern, irregular glands, nests, or individual tumor cells push into the surrounding tissue and often cause a fibrous reaction called desmoplasia. This pattern is less common but is associated with a higher risk of spread and a worse prognosis.
To confirm the diagnosis and distinguish primary mucinous carcinoma of the ovary from a tumor that has spread from another organ, the pathologist uses immunohistochemistry (IHC) — a technique that uses antibodies to detect specific proteins in tumor cells. Most primary ovarian mucinous carcinomas stain strongly for CK7 and may also show variable staining for CK20, CEA, and CDX2, which are markers commonly expressed in gastrointestinal-type cells. Many tumors also stain for CA19-9. Markers that are typically negative include WT1, napsin A, estrogen receptor (ER), and progesterone receptor (PR), which help distinguish mucinous carcinoma from high-grade serous carcinoma and endometrioid carcinoma of the ovary. A small subset of tumors shows focal PAX8 staining, which may support an ovarian origin. Metastatic colorectal or appendiceal cancers, by contrast, often show the reverse pattern: strong CK20 and CDX2 with weak or absent CK7.
Once the cancer is confirmed as a primary ovarian tumor, imaging — typically CT of the abdomen and pelvis — is used to assess the extent of disease and guide staging and treatment planning.
Histologic grade
Mucinous carcinoma of the ovary is graded based on the FIGO (International Federation of Gynecology and Obstetrics) system, using the proportion of the tumor that grows as solid sheets of cells rather than as recognizable glands:
- FIGO grade 1 (well differentiated) — Less than 5% of the tumor grows as solid sheets. The tumor forms well-developed glands and tends to behave in a relatively predictable way.
- FIGO grade 2 (moderately differentiated) — Between 6% and 50% of the tumor grows as solid sheets.
- FIGO grade 3 (poorly differentiated) — More than 50% of the tumor grows as solid sheets. This is associated with more aggressive behavior and a higher risk of recurrence.
In addition to FIGO grade, the pattern of invasion — expansile versus infiltrative — is also an important prognostic factor specific to mucinous carcinoma. Tumors with infiltrative invasion behave more aggressively than those with expansile invasion, even at the same FIGO grade.
Tumor spread
The pathologist examines all tissue samples to determine whether the tumor has spread beyond the ovary. Tumor cells may involve nearby structures such as the fallopian tube, uterus, or other pelvic tissues. Tumor cells may also spread to the peritoneum (the thin lining of the abdominal cavity) or the omentum. Unlike most other types of ovarian carcinoma, the majority of mucinous carcinomas are confined to the ovary at the time of diagnosis. When spread beyond the ovary is found in a mucinous tumor, the possibility that it originated elsewhere in the body is considered carefully before concluding the spread represents true advanced-stage primary ovarian cancer.
Ovarian capsule status
The outer covering of the ovary is called the capsule. The pathologist will note whether the capsule is intact or ruptured, and whether tumor is present on the outer surface. These findings affect the stage:
- Intact capsule, no surface tumor — Suggests the cancer is still contained within the ovary, associated with an earlier stage and better prognosis.
- Ruptured capsule or tumor on the surface — Even if no other spread is identified, capsule rupture or surface involvement increases the stage.
- Intraoperative rupture — If the capsule ruptures during surgery rather than before, this is noted separately and also affects staging.
Lymphovascular invasion
Lymphovascular invasion means that tumor cells have been found inside small blood vessels or lymphatic channels within the tissue. This finding suggests that tumor cells may have had an opportunity to travel to lymph nodes or distant organs, and it can influence staging and treatment decisions.
Lymph nodes
Lymph nodes are small, bean-shaped structures that help filter the body’s lymphatic fluid and support the immune system. In ovarian cancer surgery, lymph nodes from the pelvis and along the major abdominal blood vessels (para-aortic nodes) may be removed and examined. If tumor cells are found in the lymph nodes, the cancer is considered to have spread beyond the ovary and the stage increases. Lymph node spread is less common in mucinous carcinoma than in high-grade serous carcinoma, but it still occurs and must be assessed.
The pathology report will describe:
- The total number of lymph nodes examined.
- The number of lymph nodes containing tumor cells.
- The size of the largest tumor deposit.
- The location of any involved nodes (pelvic vs. para-aortic).
Lymph node deposits are classified by size. Isolated tumor cells (measuring 0.2 mm or less) are recorded as pN0(i+) and are not counted as definitive metastases in all staging systems. Deposits between 0.2 mm and 10 mm are classified as pN1a (small metastases), and deposits larger than 10 mm are classified as pN1b (large metastases). These size distinctions affect the N stage.
Biomarker and molecular testing
Biomarker testing in mucinous carcinoma of the ovary examines specific proteins and genetic changes in tumor cells that may help guide treatment decisions. The biomarker landscape for this tumor type differs from that of other ovarian carcinomas — in particular, BRCA and HRD testing are not routinely performed because mucinous carcinoma does not respond to PARP inhibitors the way high-grade serous carcinoma does. The most clinically important biomarker in this tumor type is HER2.
HER2 (ERBB2)
HER2 is a protein found on the surface of some cancer cells that acts like a receiving antenna for growth signals, telling the cell to grow and divide. In a healthy cell, HER2 activity is tightly controlled. In some mucinous carcinomas of the ovary, the ERBB2 gene — which provides the instructions for making HER2 — is amplified, meaning extra copies of the gene are present. This leads to overproduction of HER2 protein, which drives uncontrolled cell growth. HER2 amplification is found in approximately 15–25% of mucinous ovarian carcinomas, making it the most important actionable biomarker in this tumor type.
HER2 testing follows a two-step process. First, immunohistochemistry (IHC) is performed to measure the amount of HER2 protein on the surface of tumor cells. Results are reported as 0, 1+, 2+, or 3+. A score of 3+ is considered positive, indicating high HER2 protein overexpression. A score of 0 or 1+ is negative. A score of 2+ is equivocal — meaning the result is borderline — and requires confirmatory testing by fluorescence in situ hybridization (FISH), which directly counts the number of HER2 gene copies in the tumor cells to determine whether the gene is amplified.
Patients with HER2-positive mucinous ovarian carcinoma may be eligible for HER2-targeted therapy. Agents such as trastuzumab (Herceptin) and trastuzumab-based combinations have shown activity in HER2-amplified mucinous ovarian carcinomas, particularly in advanced or recurrent disease. Your oncologist will advise whether HER2-targeted therapy is appropriate for your situation.
Mismatch repair proteins (MMR)
Mismatch repair (MMR) proteins — MLH1, PMS2, MSH2, and MSH6 — work together to fix small copying errors that arise when cells divide. When one or more of these proteins is absent, errors accumulate and the tumor is described as mismatch repair–deficient (dMMR) or microsatellite instability–high (MSI-H). Mismatch repair deficiency is uncommon in mucinous carcinoma of the ovary, occurring in a minority of cases.
When present, dMMR has two important implications. First, dMMR/MSI-H tumors may be eligible for immunotherapy with pembrolizumab (Keytruda), which has pan-tumor approval for dMMR/MSI-H solid tumors that have progressed after prior treatment. Second, dMMR may indicate Lynch syndrome — an inherited condition caused by a germline mutation in one of the MMR genes that significantly raises the lifetime risk of ovarian, uterine, colorectal, and other cancers. When MMR deficiency is identified, referral to a genetic counsellor is recommended to assess for Lynch syndrome, as the implications extend to blood relatives who may not yet know they are at risk.
Testing is performed by immunohistochemistry on tumor tissue. Results are reported as retained expression (normal) or loss of expression (abnormal) for each of the four MMR proteins. When MLH1 and PMS2 are both lost, additional testing for a BRAF V600E mutation or MLH1 promoter methylation helps distinguish sporadic, non-inherited loss from Lynch syndrome.
KRAS
KRAS is a gene that acts like an on/off switch for cell growth signals. Normally, the KRAS protein turns on briefly in response to growth signals, then turns off again. A mutation in KRAS can lock the switch in the “on” position, causing the cell to grow and divide continuously without waiting for the normal signal. KRAS mutations are found in about two-thirds of mucinous ovarian carcinomas and are thought to be an early event in tumor development.
KRAS mutation results are reported as mutated or wild-type (normal). At present, there is no approved KRAS-targeted therapy specifically for mucinous ovarian carcinoma, although KRAS G12C inhibitors (such as sotorasib) have been approved for other cancer types and may be considered in some clinical settings. KRAS testing is increasingly performed as part of broader molecular profiling to characterize the tumor and identify any potentially targetable alterations.
PD-L1
PD-L1 is a protein that some tumor cells use to evade the immune system. Testing is performed by immunohistochemistry and is typically reported as a Combined Positive Score (CPS). In ovarian cancer, PD-L1 testing is most relevant in the setting of advanced or recurrent disease, where immunotherapy may be considered. The role of PD-L1 as a predictive marker, specifically in mucinous ovarian carcinoma, remains under investigation. Your oncologist will consider PD-L1 results alongside other clinical and molecular findings.
For more information about biomarker testing in ovarian cancer, see the Biomarkers and Molecular Testing section.
Pathologic stage (pTNM)
Staging describes how far the cancer has spread. For ovarian cancer, the pathologic stage is based on the AJCC TNM system, which closely corresponds to the FIGO staging system used by gynecologic oncologists. The stage is made up of three components: T (how far the tumor has grown locally), N (whether it has spread to lymph nodes), and M (whether it has spread to distant organs). M stage is determined by imaging and is not typically assigned in the pathology report unless distant spread was sampled at surgery.
Tumor stage (pT)
- pT1 (FIGO Stage I) — Tumor is limited to one or both ovaries or fallopian tubes.
- pT1a — Tumor in one ovary or fallopian tube only; capsule intact; no tumor cells in abdominal fluid.
- pT1b — Tumor involves both ovaries or fallopian tubes; capsules intact; no tumor cells in abdominal fluid.
- pT1c — Tumor limited to ovary/fallopian tube but with capsule rupture, tumor on the outer surface, or cancer cells in abdominal fluid or washings.
- pT2 (FIGO Stage II) — Tumor has spread beyond the ovaries or fallopian tubes into the pelvis.
- pT2a — Spread to the uterus, the other fallopian tube, or the other ovary.
- pT2b — Spread to other pelvic tissues such as the bladder or rectum.
- pT3 (FIGO Stage III) — Tumor has spread beyond the pelvis to the peritoneum or regional lymph nodes.
- pT3a — Microscopic spread to the peritoneum outside the pelvis, with or without regional lymph node involvement.
- pT3b — Visible tumor deposits up to 2 cm on the peritoneum outside the pelvis, with or without lymph node involvement.
- pT3c — Visible tumor deposits larger than 2 cm outside the pelvis, or spread to the outer surface (capsule) of the liver or spleen, with or without lymph node involvement.
Note: Spread inside the substance of the liver or spleen (rather than only on their surface) is classified as M1 (Stage IVB).
Nodal stage (pN)
- pN0 — No cancer cells found in regional lymph nodes.
- pN0(i+) — Only isolated tumor cells (0.2 mm or less) found in lymph nodes; not counted as definitive metastases in all staging systems.
- pN1 — Cancer cells present in regional lymph nodes.
- pN1a — Tumor deposits up to 10 mm.
- pN1b — Tumor deposits larger than 10 mm.
What is the prognosis?
The prognosis for mucinous carcinoma of the ovary depends primarily on the stage at diagnosis and the pattern of invasion. Because the majority of cases are diagnosed at stage I — confined to one ovary — overall survival is more favorable than for high-grade serous carcinoma. However, advanced-stage mucinous ovarian carcinoma is difficult to treat because this tumor type responds poorly to the standard platinum-based chemotherapy that works well for serous carcinoma.
Approximate five-year survival rates are:
- Stage I — 85–95%. Most patients with mucinous ovarian carcinoma are diagnosed at this stage, and outcomes are excellent with surgery alone in many cases.
- Stage II — 55–65%.
- Stage III — 20–35%. Advanced-stage mucinous carcinoma has a significantly worse prognosis than advanced-stage serous carcinoma because it is less responsive to standard chemotherapy.
- Stage IV — Less than 15%.
Beyond stage, the following features influence prognosis:
- Pattern of invasion — Tumors with infiltrative (destructive) invasion behave more aggressively and have a worse prognosis than those with expansile (confluent) invasion, even when confined to the ovary.
- Histologic grade — Higher-grade tumors (grade 3) are associated with a greater risk of recurrence.
- Unilateral vs. bilateral involvement — A tumor confined to one ovary is associated with a better prognosis. Bilateral involvement raises the possibility of metastatic disease from another organ, which carries a different prognosis.
- HER2 status — HER2-positive tumors may respond to targeted therapy, which is an emerging treatment option in advanced disease.
- Capsule status — Capsule rupture before or during surgery increases the stage and is associated with a higher risk of recurrence.
What happens after the diagnosis?
Treatment is planned by a multidisciplinary team that typically includes a gynecologic oncologist, medical oncologist, pathologist, and radiologist. The approach depends on the stage, invasion pattern, and biomarker results.
Surgery is the cornerstone of treatment. For most patients, this involves removal of the affected ovary and fallopian tube (and the contralateral ovary and uterus in most postmenopausal patients), the omentum, and the appendix. The appendix is routinely removed both to rule out an appendiceal primary tumor and because the appendix is a common site of origin for mucinous tumors that spread to the ovary. For early-stage, low-grade tumors with expansile invasion confined to one ovary, surgery alone may be sufficient, and fertility-sparing surgery may be possible in younger patients after careful discussion.
For patients with higher-stage disease or infiltrative invasion, chemotherapy is typically recommended after surgery. However, standard platinum-based chemotherapy (carboplatin and paclitaxel) is less effective in mucinous ovarian carcinoma than in serous carcinoma. For this reason, alternative chemotherapy regimens modeled on gastrointestinal cancers — such as FOLFOX or XELOX (oxaliplatin-based regimens) — are used in some cases, reflecting the gastrointestinal-like biology of mucinous ovarian carcinoma.
In patients with HER2-positive advanced or recurrent mucinous carcinoma, HER2-targeted therapy with trastuzumab or trastuzumab-based combinations may be considered. This is an area of active clinical investigation. Patients whose tumors show mismatch repair deficiency may be eligible for immunotherapy with pembrolizumab. Your oncologist will discuss which treatment approach is most appropriate for your specific situation.
Follow-up after treatment typically involves regular clinical assessments and monitoring with CA-125 (and CA19-9 in some cases), with imaging performed when there are signs of potential recurrence.
Questions to ask your doctor
- Has my tumor been confirmed as a primary mucinous carcinoma of the ovary, and was the appendix examined to rule out an appendiceal primary?
- What stage is my ovarian cancer, and what does that mean for my treatment and prognosis?
- Does my tumor show expansile or infiltrative invasion, and how does that affect my prognosis?
- What is the histologic grade of my tumor?
- Was the ovarian capsule intact, or did it rupture before or during surgery?
- Was HER2 testing performed, and is my tumor HER2-positive?
- If my tumor is HER2-positive, am I eligible for HER2-targeted therapy?
- Was mismatch repair testing performed, and were any MMR proteins lost?
- If MMR deficiency was found, do I need testing for Lynch syndrome, and should my family members be tested?
- What chemotherapy regimen do you recommend, given that mucinous carcinoma responds differently to treatment than other ovarian cancers?
- What follow-up schedule do you recommend after I complete treatment?
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