Low Grade Serous Carcinoma of the Ovary: Understanding Your Pathology Report

by Emily Goebel, MD FRCPC
March 10, 2026

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Low-grade serous carcinoma of the ovary is a type of ovarian cancer that develops from cells similar to those lining the fallopian tube. It is called “low grade” because the tumour cells look less abnormal under the microscope and tend to grow more slowly than those seen in high-grade serous carcinoma.

Low-grade serous carcinoma is relatively uncommon, accounting for about 5% of ovarian carcinomas. This tumour usually affects people at a younger age than high-grade serous carcinoma, with a typical diagnosis in the early to mid-40s.

Low-grade serous carcinoma often develops from a precursor tumour called a serous borderline tumour. Because of this relationship, both tumours may sometimes be present at the same time.

What are the symptoms of low-grade serous carcinoma of the ovary?

Some patients have symptoms related to a pelvic mass, while others have no symptoms at the time of diagnosis.

Symptoms may include abdominal swelling or bloating, pelvic pain or pressure, and a feeling of fullness. In some patients, fluid may build up in the abdomen, a condition called ascites, which can cause abdominal discomfort or swelling.

Because ovarian tumours can grow slowly before causing symptoms, they may be discovered during imaging or surgery performed for another reason.

What causes low-grade serous carcinoma of the ovary?

The exact cause is not fully understood. Low-grade serous carcinoma often arises from a serous borderline tumour, a tumour with abnormal cells but limited invasive growth. Over time, additional genetic changes may allow the tumour to become invasive.

Several genetic alterations are commonly found in low-grade serous carcinomas, including mutations in KRAS, NRAS, and BRAF. These mutations affect pathways that control cell growth and division.

Other changes affecting genes involved in cell cycle regulation, such as the CDKN2A/2B locus, have also been reported.

Relationship to serous borderline tumors

Low-grade serous carcinoma is closely related to serous borderline tumours of the ovary.

In many cases, the carcinoma develops from a pre-existing borderline tumour. Under the microscope, areas of borderline tumour and invasive carcinoma may be seen in the same tumour.

This relationship helps explain why low-grade serous carcinoma tends to develop more slowly than high-grade serous carcinoma.

How is this diagnosis made?

The diagnosis of low-grade serous carcinoma 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.

Microscopic features

Under the microscope, low-grade serous carcinoma shows several characteristic growth patterns.

The tumour may form small nests of cells, glands, papillae, or micropapillae. In some cases, larger papillary structures are present that appear to float within clear spaces.

The tumour cells show mild to moderate nuclear atypia, meaning the nuclei are somewhat abnormal but not as strikingly abnormal as in high-grade serous carcinoma. The nuclei typically vary less than threefold in size.

Mitotic figures, which represent dividing cells, are usually uncommon. Psammoma bodies, small, round calcifications, are frequently present.

Necrosis is uncommon. Many tumours are associated with a coexisting serous borderline tumour.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside tumour cells. These tests help confirm the diagnosis and distinguish low-grade serous carcinoma from other types of ovarian cancer.

Low-grade serous carcinomas usually show strong staining for CK7, PAX8, WT1, and estrogen receptor (ER).

The tumour typically shows wild-type (normal) p53 staining, which helps distinguish it from high-grade serous carcinoma, where abnormal p53 staining is almost always present.

Staining for p16 is usually patchy rather than diffuse.

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.

KRAS

Mutations in the KRAS gene are common in low-grade serous carcinoma. KRAS plays an important role in signalling pathways that regulate cell growth.

Results are usually reported as mutated or wild-type (normal). KRAS mutations have been associated with tumour recurrence in some studies.

BRAF

Mutations in the BRAF gene are present in some low-grade serous carcinomas and are more commonly seen in earlier-stage tumours or tumours associated with serous borderline tumours.

These mutations affect the same signalling pathway as KRAS mutations and may influence tumour behaviour.

NRAS

The NRAS gene is another gene involved in cell growth signalling pathways. Mutations in NRAS are found in a smaller proportion of low-grade serous carcinomas.

p53

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

Low-grade serous carcinomas usually show wild-type p53 staining, meaning the p53 protein appears normal. This finding helps distinguish these tumours from high-grade serous carcinoma.

Estrogen receptor (ER)

Most low-grade serous carcinomas are ER-positive. ER is detected using immunohistochemistry, and results are usually reported as positive or negative, sometimes with a percentage showing how many tumour cells express the receptor.

Hormone receptor expression may influence treatment decisions in some patients.

Other features to look for in your pathology report

Tumor spread

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

Low-grade serous carcinoma often spreads to the peritoneum, which is the lining of the abdominal cavity. Tumour cells may also involve nearby organs or tissues.

Although these tumours grow more slowly than high-grade serous carcinomas, they can still spread widely within the abdomen.

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 present 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.

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?