Malignant Brenner Tumor of the Ovary: Understanding Your Pathology Report

by Jason Wasserman MD PhD FRCPC
March 11, 2026

---

Malignant Brenner tumor of the ovary is a rare type of ovarian cancer. It belongs to a group of tumors called Brenner tumors, which develop from cells that resemble the lining of the urinary tract.

Most Brenner tumors are benign (noncancerous), but in rare cases, they become malignant. When this happens, the tumor cells grow in an invasive way and behave like cancer.

Malignant Brenner tumors are uncommon, accounting for less than 5% of all Brenner tumors. They usually occur in people over the age of 50.

What are the symptoms of a malignant Brenner tumor of the ovary?

Many patients with a malignant Brenner tumor develop symptoms related to a pelvic mass. Symptoms may include abdominal swelling, pelvic pain, abdominal pressure, or a feeling of fullness. Some patients may also experience abnormal vaginal bleeding.

Because ovarian tumors may grow slowly before causing symptoms, they can become relatively large before they are detected.

What causes malignant Brenner tumor of the ovary?

The exact cause of malignant Brenner tumor of the ovary is not fully understood. Most malignant Brenner tumors are thought to develop from pre-existing benign or borderline Brenner tumors. Over time, additional genetic changes may allow tumor cells to become invasive and behave like cancer cells.

Some genetic alterations have been reported in malignant Brenner tumors, including PIK3CA mutations and MDM2 amplification, both of which regulate cell growth. However, the exact sequence of molecular changes that leads to malignant transformation is still being studied.

Relationship to benign and borderline Brenner tumors

Malignant Brenner tumors usually develop from benign or borderline Brenner tumors.

Under the microscope, the malignant tumor is often found next to areas of benign or borderline Brenner tumor. These areas contain cell nests that resemble the lining of the urinary tract.

The presence of a benign or borderline Brenner tumor component helps confirm the diagnosis. If this background component is not identified, pathologists may consider other types of ovarian cancer that can resemble malignant Brenner tumor.

How is this diagnosis made?

The diagnosis of a malignant Brenner tumor of the ovary usually begins when a mass is identified on imaging or at surgery.

The tumor is removed and examined under the microscope by a pathologist. The pathologist studies the appearance of the tumor 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, including the fallopian tubes, uterus, lymph nodes, and abdominal tissues. This examination helps determine how far the tumor has spread and is important for staging.

Microscopic features

Under the microscope, a a malignant Brenner tumor is composed of irregular nests and sheets of tumor cells that resemble those lining the urinary tract.

These tumor cells typically have dark, irregular nuclei, visible nucleoli, and pink cytoplasm. The tumor may form solid areas or cystic spaces lined by multiple layers of abnormal cells.

The tumor often grows within a dense fibrous tissue background. Because of this fibrous background, invasion can sometimes be difficult to identify. A reaction in the surrounding tissue, called desmoplasia, may help pathologists identify areas where the tumor has invaded.

Some tumors may also show squamous differentiation, where the tumor cells resemble squamous cells.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside tumor cells. These tests help confirm the diagnosis and distinguish malignant Brenner tumor from other ovarian cancers.

Malignant Brenner tumors are usually negative for WT1, which helps distinguish them from high-grade serous carcinoma.

Most tumors show wild-type (normal) p53 staining, whereas p16 staining is usually focal rather than diffuse.

Hormone receptors such as estrogen receptor (ER) and progesterone receptor (PR) are usually negative or only weakly positive.

Markers associated with urothelial differentiation, such as p63, may also be positive.

Biomarkers

Biomarker testing examines proteins or genetic changes in tumor cells that may help guide treatment decisions.

PIK3CA

Mutations in the PIK3CA gene have been reported in some malignant Brenner tumors. This gene plays an important role in signaling pathways that regulate cell growth and survival.

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

MDM2

Amplification of the MDM2 gene has been identified in some malignant Brenner tumors.

MDM2 regulates the activity of the p53 protein. Increased MDM2 activity may allow tumor cells to grow by interfering with normal cell growth control.

p53

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

Results are usually reported using immunohistochemistry as wild-type (normal pattern) or abnormal (mutant-type pattern). Most malignant Brenner tumors show a wild-type p53 pattern, which helps distinguish them from high-grade serous carcinoma.

Estrogen receptor (ER) and progesterone receptor (PR)

ER and PR are proteins that allow tumor 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 tumor cells express the receptor.

Most malignant Brenner tumors show negative or weak ER and PR expression.

Other features to look for in your pathology report

Tumor spread

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

Malignant Brenner tumors are often confined to the ovary when they are first diagnosed. However, in some cases, the tumor may spread to nearby organs or tissues within the abdomen.

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

Intact versus ruptured ovary

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

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

Lymphatic and vascular invasion

Lymphatic and vascular invasion means tumor cells are seen inside small lymphatic channels or blood vessels. This finding increases the risk that tumor cells may 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.

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 tumor 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?