Mucinous Carcinoma of the Ovary: Understanding Your Pathology Report

By Jason Wasserman MD PhD FRCPC
March 8, 2026

Mucinous carcinoma of the ovary is a type of ovarian cancer that develops from cells that produce mucus. Under the microscope, the tumour cells resemble the cells that line parts of the digestive tract, such as the stomach or intestines. Because of this appearance, pathologists describe the tumour as having “mucinous” or gastrointestinal-type features.

This tumour type is relatively uncommon and accounts for about 3–4% of ovarian carcinomas in North America. Most mucinous carcinomas are diagnosed at an early stage and are confined to the ovary when they are first detected.

Because mucinous carcinoma behaves differently from other ovarian cancers, it is important for the pathologist to carefully evaluate the tumour to confirm that it originated in the ovary and not from another organ.

What are the symptoms of mucinous carcinoma of the ovary?

Most patients develop symptoms related to a pelvic mass. These symptoms may include abdominal swelling or bloating, pelvic pressure, abdominal pain, or a feeling of fullness. Because these tumours can grow slowly, they may become very large before they cause symptoms.

In many cases, mucinous carcinomas are detected when imaging is performed for abdominal or pelvic symptoms.

What causes mucinous carcinoma of the ovary?

The exact cause of mucinous carcinoma of the ovary is not fully understood. Genetic changes within tumour cells play an important role in the development of this cancer. Early molecular changes commonly involve genes that regulate cell growth, particularly KRAS, which is mutated in many mucinous tumours. Changes affecting the CDKN2A gene are also common. Additional mutations, including TP53 alterations, may occur as the tumour becomes more aggressive.

Relationship to noncancerous ovarian tumors

Many mucinous carcinomas of the ovary develop from pre-existing noncancerous or borderline ovarian tumours.

In many cases, the tumour develops from a mucinous borderline tumour, which is a tumour with abnormal cells but without clear invasion into surrounding tissue. Over time, additional genetic changes can allow the tumour to become invasive.

Less commonly, mucinous carcinoma can arise from other benign ovarian tumours such as mature cystic teratomas or Brenner tumours.

Under the microscope, the tumour may show a mixture of benign, borderline, and malignant areas, supporting the idea that the carcinoma developed from a pre-existing lesion.

Could a mucinous carcinoma in the ovary come from another organ?

One important feature of mucinous tumours of the ovary is that some tumours found in the ovary may actually have started somewhere else in the body.

In particular, cancers that begin in the appendix, colon, or, occasionally, the stomach can spread to the ovary and form a tumour that can look very similar to mucinous carcinoma of the ovary. Because these tumours produce mucus and resemble intestinal cells under the microscope, it can sometimes be difficult to determine where the tumour started.

To help answer this question, surgeons often remove the appendix during surgery for a mucinous ovarian tumour. The appendix is then examined under the microscope to determine whether a tumour is present. If a tumour is found in the appendix or another part of the digestive tract, the ovarian tumour may represent spread (metastasis) from that site rather than a primary ovarian cancer.

Determining where the tumour started is important because treatment and prognosis may differ depending on whether the cancer began in the ovary or in another organ.

How is this diagnosis made?

The diagnosis of mucinous carcinoma of the ovary usually begins when a mass in the ovary is identified by imaging or during surgery.

The tumour is removed and examined under the microscope by a pathologist. The pathologist studies the appearance of the tumour cells and their growth pattern to determine the type of ovarian cancer.

If surgery is performed, the pathologist also examines other tissues removed during the operation, such as the fallopian tubes, uterus, lymph nodes, and abdominal tissues. This examination helps determine how far the tumour has spread and is important for staging.

Because mucinous tumours can also spread to the ovary from other organs, especially the gastrointestinal tract, the pathologist carefully evaluates the tumour to determine whether it is a primary ovarian tumour or a metastatic tumour from another site.

Microscopic features

Under the microscope, mucinous carcinoma of the ovary shows tumour cells that produce large amounts of mucus.

The tumour may contain a mixture of benign, borderline, and malignant areas. The invasive carcinoma is identified when tumour cells invade the surrounding ovarian tissue.

Two main patterns of invasion can be seen:

  • In the expansile (confluent) pattern, the tumour glands grow closely together with little intervening supporting tissue. This creates a crowded, labyrinth-like appearance.
  • In the infiltrative (destructive) pattern, irregular glands, nests, or individual tumour cells invade the surrounding tissue, often accompanied by a fibrous reaction called desmoplasia.

The expansile pattern is more common and is associated with a better prognosis than the infiltrative pattern.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside tumour cells. These tests help confirm the diagnosis and distinguish mucinous carcinoma of the ovary from other tumours.

Most ovarian mucinous carcinomas show strong CK7 staining. They may also show variable staining for CK20, CEA, and CDX2, markers often expressed in gastrointestinal-type cells.

Many tumours also show staining for CA19-9, a protein associated with mucin-producing tumours.

Markers that are usually negative include WT1, napsin A, vimentin, estrogen receptor (ER), and progesterone receptor (PR). These findings help distinguish mucinous carcinoma from other ovarian tumour types, such as serous or endometrioid carcinoma.

A small subset of tumours shows weak or focal PAX8 staining, which may support an ovarian origin.

Biomarkers

Biomarker testing examines proteins or genetic changes in tumour cells that may help guide treatment decisions. Some tests use immunohistochemistry to detect proteins in tumour cells, while others evaluate changes in tumour DNA.

HER2 (ERBB2)

HER2 is a protein involved in cell growth signaling. Some mucinous carcinomas of the ovary produce too much HER2 because the ERBB2 (HER2) gene is amplified.

HER2 testing is usually performed using immunohistochemistry and may be followed by additional molecular testing if the result is unclear. Results are commonly reported as 0, 1+, 2+, or 3+. A result of 3+ is considered positive. A result of 2+ is considered equivocal and may require additional testing.

HER2 is important because patients with HER2-positive mucinous carcinoma may be eligible for HER2-targeted therapy in advanced disease.

KRAS

KRAS is a gene involved in pathways that regulate cell growth. Mutations in KRAS are among the most common genetic changes in mucinous carcinoma of the ovary and are thought to occur early in tumour development.

Results are usually reported as mutated or wild-type (normal).

p53

p53 is a protein that helps control cell growth and repair damaged DNA.

Results are usually reported using immunohistochemistry as either wild-type (normal pattern) or abnormal (mutant-type pattern). In mucinous carcinoma, p53 may show either pattern. TP53 mutations are more common in mucinous carcinoma than in mucinous borderline tumours and may be associated with progression to invasive carcinoma.

Mismatch repair proteins (MMR)

Mismatch repair proteins help correct errors that arise during DNA replication. The most commonly tested proteins are MLH1, PMS2, MSH2, and MSH6.

Results are reported as retained expression (normal) or loss of expression (abnormal).

Mismatch repair deficiency is uncommon in mucinous carcinoma of the ovary, but it may be evaluated when Lynch syndrome is suspected or when broader molecular testing is performed.

Estrogen receptor (ER) and progesterone receptor (PR)

ER and PR are proteins that allow tumour cells to respond to the hormones estrogen and progesterone.

These markers are tested using immunohistochemistry and are usually reported as positive or negative, sometimes with a percentage showing how many tumour cells express the receptor.

Most ovarian mucinous carcinomas are negative for ER and PR, which helps distinguish them from ovarian endometrioid carcinomas, which are often hormone receptor-positive.

Other features to look for in your pathology report

Tumor spread

Pathologists examine the tumour to determine whether it has spread beyond the ovary.

The tumour may involve nearby structures, such as the fallopian tube, uterus, or abdominal tissues. However, unlike many other ovarian cancers, most mucinous carcinomas are confined to the ovary when they are diagnosed.

The presence of tumour cells outside the ovary increases the stage of the tumour.

Intact versus ruptured ovary

Whether the ovary was intact or ruptured at the time of surgery is important for staging.

If the tumour is confined to the ovary and the capsule is intact, the cancer may be stage I. If the capsule is ruptured or tumour cells are found on the surface of the ovary, the stage may be higher.

Lymphatic and vascular invasion

Lymphatic and vascular invasion means tumour cells are seen inside small lymphatic channels or blood vessels.

This finding increases the risk that tumour cells could spread to lymph nodes or distant organs.

Lymph nodes

Lymph nodes are small, bean-shaped structures in the lymphatic system. They help filter harmful substances from the body and play an important role in the immune system.

In ovarian cancer surgery, lymph nodes from the pelvis and abdomen may be removed and examined under the microscope. These are called regional lymph nodes. They include the pelvic and para-aortic lymph nodes.

If tumour cells are found in these lymph nodes, the cancer is considered to have spread beyond the ovary, and the tumour stage increases. Lymph node involvement may also influence treatment decisions, such as the use of chemotherapy or other systemic therapies.

When tumour cells are found in lymph nodes, the pathology report often describes the size of the tumour deposits. The size helps doctors determine the extent of lymph node involvement.

Three main categories may be described:

Isolated tumour cells (ITCs)

These are very small clusters of tumour cells measuring 0.2 mm or less. When only isolated tumour cells are present, the lymph nodes are often reported as N0(i+), meaning that only very small deposits of tumour cells were found.

Small lymph node metastases

These tumour deposits measure more than 0.2 mm but 10 mm or less. These are considered true lymph node metastases, indicating that the cancer has spread to the lymph nodes.

Large lymph node metastases

These tumour deposits measure more than 10 mm. Larger tumour deposits generally indicate greater tumour involvement of the lymph node.

Your pathology report may also describe:

  • the number of lymph nodes examined

  • the number of lymph nodes containing tumour cells

  • the location of the involved lymph nodes

  • the size of the largest tumour deposit

These findings are important because they help determine the pathologic stage of the tumour, which guides treatment decisions and helps estimate prognosis.

How do doctors stage ovarian cancer?

Staging describes how far a cancer has spread in the body. For ovarian cancer, two main systems are used: the TNM and FIGO systems. Both are internationally accepted and provide important information about prognosis (the expected outcome) and treatment planning.

The TNM system

The TNM system was developed by the American Joint Committee on Cancer (AJCC). It looks at three main factors:

  • T (tumour): Describes the size of the tumour and how far it has spread in or around the ovary or fallopian tube.

  • N (lymph nodes): Describes whether cancer cells have spread to nearby lymph nodes.

  • M (metastasis): Describes whether cancer has spread to distant parts of the body.

Breakdown of the T stage:

  • T1: The tumour is limited to one or both ovaries or fallopian tubes.

    • T1a: Tumour is inside one ovary or fallopian tube, with the outer surface intact and no cancer cells in fluid taken from the abdomen.

    • T1b: Tumour is inside both ovaries or fallopian tubes, but the outer surfaces are intact, and no cancer cells are found in the fluid.

    • T1c: The tumour is limited to one or both ovaries or tubes, but there has been a rupture, tumour on the outer surface, or cancer cells found in abdominal fluid.

  • T2: The tumour has grown into tissues in the pelvis, such as the uterus or bladder.

    • T2a: Spread to the uterus or other fallopian tube or ovary.

    • T2b: Spread to other pelvic tissues.

  • T3: The tumour has spread beyond the pelvis into the abdomen or to regional lymph nodes.

    • T3a: Cancer cells are found microscopically outside the pelvis or in nearby lymph nodes.

    • T3b: Visible tumour deposits up to 2 cm outside the pelvis or in nearby lymph nodes.

    • T3c: Visible tumour deposits larger than 2 cm outside the pelvis or involving the capsule of the liver or spleen (without entering the organ itself).

Breakdown of the N stage:

  • N0: No cancer cells are seen in regional lymph nodes.

  • N0(i+): Only isolated tumour cells smaller than 0.2 mm are seen in the lymph nodes.

  • N1: Cancer cells are found in regional lymph nodes.

    • N1a: Deposits up to 10 mm.

    • N1b: Deposits larger than 10 mm.

The FIGO system

The FIGO (International Federation of Gynecology and Obstetrics) system is specifically designed for gynecologic cancers like ovarian cancer. It uses similar criteria to the TNM system but is grouped into broader stages that are easier to interpret clinically.

Breakdown of FIGO stages:

  • Stage I: Cancer is limited to the ovaries or fallopian tubes.

    • IA: In one ovary or fallopian tube only.

    • IB: In both ovaries or fallopian tubes.

    • IC: Cancer is still limited to the ovaries or tubes, but there has been a rupture, tumour on the surface, or cancer cells found in fluid.

  • Stage II: Cancer involves one or both ovaries or tubes with spread to pelvic organs such as the uterus, bladder, or rectum.

    • IIA: Spread to the uterus or other ovary/tube.

    • IIB: Spread to other pelvic tissues.

  • Stage III: Cancer has spread outside the pelvis into the abdominal cavity or to regional lymph nodes.

    • IIIA1: Cancer in lymph nodes only.

    • IIIA2: Microscopic spread outside the pelvis.

    • IIIB: Visible spread outside the pelvis up to 2 cm.

    • IIIC: Visible spread larger than 2 cm or spread to the capsule of the liver or spleen.

  • Stage IV: Cancer has spread to distant organs outside the abdomen.

    • IVA: Cancer cells are found in the fluid around the lungs.

    • IVB: Cancer has spread to organs such as the liver, spleen, or lymph nodes outside the abdomen.

Why staging is important

Both TNM and FIGO staging systems provide doctors with essential information about how far the cancer has spread. This helps guide treatment decisions, such as whether surgery alone is sufficient or whether chemotherapy or other treatments are needed.

Staging also helps predict prognosis. Early-stage disease (stage I) has a much better survival rate compared to advanced-stage disease (stage III or IV). By using staging information, doctors can personalize care and discuss treatment options and expectations with patients.

Questions to ask your doctor

  • What stage is my ovarian cancer?

  • Was the tumour confined to the ovary or had it spread beyond the ovary?

  • Was the ovarian capsule intact or ruptured?

  • Were lymph nodes involved?

  • Were biomarker tests performed, and what do the results mean?

  • Do my biomarker results affect treatment options?

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