by Jason Wasserman MD PhD FRCPC
April 19, 2026
Malignant Brenner tumor of the ovary is a rare type of ovarian cancer that develops from cells resembling those that line the urinary tract. It belongs to a family of ovarian tumors called Brenner tumors, which range from entirely benign to malignant. Most Brenner tumors are noncancerous, and malignant Brenner tumor is the rarest type, accounting for less than 5% of all Brenner tumors. It typically occurs in people over the age of 50. Although it is a cancer, most malignant Brenner tumors are diagnosed at an early stage when the tumor is still confined to the ovary. 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?
Many patients develop symptoms related to a pelvic mass. These may include abdominal swelling or bloating, pelvic pressure or pain, or a feeling of fullness. Some patients experience abnormal vaginal bleeding. Because ovarian tumors can grow slowly before causing noticeable symptoms, they can become relatively large before they are detected. Some tumors are found incidentally during imaging or surgery performed for another reason.
What causes malignant Brenner tumor of the ovary?
The exact cause is not fully understood. Most malignant Brenner tumors are thought to develop from pre-existing benign or borderline Brenner tumors through the gradual accumulation of additional genetic changes that allow the cells to become invasive. Some genetic alterations have been identified in malignant Brenner tumors — including mutations in PIK3CA and amplification of MDM2, both of which affect cell growth regulation — but the full sequence of molecular events leading to malignant transformation remains under investigation. Malignant Brenner tumor is not associated with BRCA mutations, Lynch syndrome, or other hereditary risk factors relevant to epithelial ovarian carcinomas.
Relationship to benign and borderline Brenner tumors
Malignant Brenner tumors almost always develop from a pre-existing benign or borderline Brenner tumor, and the two components are frequently found side by side in the same specimen under the microscope. The benign or borderline component consists of small nests of uniform cells that resemble the lining of the urinary tract, sitting within a dense fibrous background. The malignant component shows the same urothelial-type cells but with features of invasion — the cells break through the fibrous tissue and grow in an uncontrolled way.
Finding the benign or borderline Brenner component alongside the malignant component is important because it helps confirm the diagnosis. When no benign or borderline component can be identified, pathologists must carefully consider whether the tumor may be a different type of ovarian cancer that resembles malignant Brenner tumor under the microscope, particularly high-grade carcinoma with urothelial features.
How is the diagnosis made?
The diagnosis is usually made after the tumor is removed at surgery and examined under the microscope by a pathologist. If surgery is performed, the pathologist also examines other tissues removed at the same time — including the fallopian tubes, uterus, lymph nodes, omentum, and any peritoneal biopsies — to determine whether the tumor has spread.
Under the microscope, a malignant Brenner tumor is composed of irregular nests and sheets of tumor cells that resemble the cells lining the urinary tract (urothelial cells). The tumor cells have dark, irregular nuclei with visible nucleoli and pink cytoplasm. The tumor may form solid areas or cystic spaces lined by multiple layers of abnormal cells. It grows within a dense fibrous tissue background, and a scarring reaction in the surrounding tissue called desmoplasia may help the pathologist identify where invasion is occurring. Some tumors also show squamous differentiation — areas in which the cells take on features resembling those of squamous cells. The number of mitotic figures (dividing cells) is variable.
To confirm the diagnosis and distinguish malignant Brenner tumor from other ovarian cancers that can look similar, the pathologist uses immunohistochemistry (IHC) — a technique that uses antibodies to detect specific proteins in tumor cells. Malignant Brenner tumors are typically negative for WT1, which helps distinguish them from high-grade serous carcinoma. Most tumors show wild-type (normal) p53 staining, in contrast to high-grade serous carcinoma, where abnormal p53 is almost universal. p16 staining is usually focal rather than diffuse. Estrogen receptor (ER) and progesterone receptor (PR) are usually negative or only weakly positive. Markers associated with urothelial (urinary tract-type) differentiation — such as p63, GATA3, and uroplakin — may be positive and support the diagnosis. The benign or borderline component shows the same urothelial marker profile as benign Brenner tumors.
Once the diagnosis is confirmed, imaging — typically CT of the abdomen and pelvis — is performed to determine the extent of disease and guide staging and treatment planning.
Histologic grade
Malignant Brenner tumor of the ovary is considered high grade by definition and is not assigned a FIGO grade in the way that endometrioid carcinomas are graded. The grading systems used for endometrioid carcinoma — which are based on the proportion of solid growth — do not apply to this tumor type. The most important factors predicting outcomes are the stage at diagnosis and whether all visible tumor can be removed at surgery.
Tumor spread
The pathologist examines all submitted tissue to determine whether the tumor has spread beyond the ovary. Tumor cells may involve nearby structures such as the fallopian tube, uterus, peritoneum, or omentum. Because most malignant Brenner tumors are diagnosed at an early stage and are confined to the ovary, spread to other organs at the time of diagnosis is less common than in high-grade serous carcinoma. However, when spread is present it increases the stage and is associated with a higher risk of recurrence.
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 when no spread to other organs is found, 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 sites, and it can influence staging and treatment planning.
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.
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 looks for specific proteins or genetic changes — called mutations — in tumor cells that may help guide treatment decisions. In the most common types of ovarian cancer, such as high-grade serous carcinoma, biomarker tests like BRCA gene mutation testing and HRD (homologous recombination deficiency) testing play a central role in selecting targeted therapies. These tests are not routinely performed for malignant Brenner tumor because this cancer develops through a different set of genetic changes and is not known to respond to the drugs that those tests identify.
The genetic changes most often found in malignant Brenner tumors — mutations in PIK3CA (a gene that regulates cell growth) and amplification of MDM2 (a gene that controls a key tumor-suppressing protein) — are currently of interest to researchers but are not yet linked to specific approved targeted therapies. In advanced or recurrent disease, broader molecular profiling — a type of testing that surveys many genes at once — may be performed to identify genetic changes that could guide treatment decisions.
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. Malignant Brenner tumor is staged using 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. The majority of malignant Brenner tumors are diagnosed at stage I.
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 malignant Brenner tumor is generally favorable compared to more common high-grade ovarian cancers, primarily because most cases are diagnosed at stage I when the tumor is still confined to the ovary. Five-year survival rates for stage I disease are reported at approximately 80–90% in published series, though the rarity of this tumor means survival estimates are based on relatively small numbers of patients. Advanced-stage disease is associated with a substantially worse prognosis.
Factors associated with outcomes include:
- Stage — Stage at diagnosis is the most important prognostic factor. The large majority of patients are diagnosed at stage I.
- Residual disease after surgery — Complete removal of all visible tumor is associated with better outcomes.
- Capsule status — Capsule rupture before or during surgery increases the stage and is associated with a higher risk of recurrence.
- Presence of benign/borderline component — Confirmation of a co-existing benign or borderline Brenner tumor component supports the diagnosis and is associated with the more favorable prognosis of true malignant Brenner tumor compared to other high-grade ovarian carcinomas.
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 and individual clinical circumstances.
Surgery is the cornerstone of treatment. Standard surgery typically involves the removal of both ovaries, both fallopian tubes, and the uterus, along with assessment of the peritoneum, omentum, and lymph nodes. For younger patients with early-stage unilateral disease who wish to preserve fertility, conservative surgery may be discussed with the gynecologic oncologist, though the evidence base for this approach in malignant Brenner tumor is limited given the rarity of the tumor.
For patients with early-stage disease (stage IA, intact capsule), surgery alone is generally considered sufficient treatment. For patients with higher-stage disease, capsule rupture, or other high-risk features, adjuvant chemotherapy is typically recommended. Because malignant Brenner tumor is rare, there is no dedicated chemotherapy trial evidence for this specific diagnosis. In practice, regimens used for urothelial (bladder) carcinoma — such as gemcitabine and cisplatin — or standard ovarian carcinoma regimens with carboplatin and paclitaxel are used depending on the clinical situation. Your oncologist will advise on the most appropriate approach.
Patients with advanced or recurrent disease may be candidates for broader molecular profiling to identify any targetable alterations. Clinical trial participation may also be worth exploring, given the rarity of this tumor type.
Follow-up after treatment involves regular clinical assessments and imaging, with the schedule determined by the stage and treatment received.
Questions to ask your doctor
- What stage is my ovarian cancer, and what does that mean for my prognosis?
- Was a benign or borderline Brenner tumor component identified alongside the cancer, confirming the diagnosis?
- Was the tumor confined to one ovary, and was the capsule intact?
- Was the tumor completely removed, or was any tumor left behind?
- Has the tumor spread to the lymph nodes, peritoneum, or other organs?
- Is adjuvant chemotherapy recommended, and if so, which regimen do you recommend for this tumor type?
- Given that malignant Brenner tumor is rare, is referral to a specialist center or a clinical trial appropriate for my situation?
- What follow-up schedule do you recommend after treatment?
- What symptoms should I watch for that might suggest recurrence?
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