Borderline ovarian tumors (BOTs) are abnormal cells or growths that develop in the tissue encapsulating an ovary. While BOTs are not cancerous, they have the potential to develop into cancer. As such, doctors typically recommend surgery to remove these tumors.

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This article describes how doctors classify BOTs, including their different types and stages.

We also outline how doctors diagnose and treat BOTs and provide information on survival rates and causes.

BOTs are noninvasive abnormal cells or growths that form in the tissue covering the ovaries.

Cancer Research UK states that because these tumors are slow-growing and noninvasive, doctors deem them noncancerous. However, it reports that BOT cells may develop into cancerous cells in very rare instances. Medical professionals may also refer to BOTs as atypical proliferative tumors or tumors of low malignant potential.

According to the National Cancer Institute (NCI), around 75% of borderline ovarian tumors are stage 1 at the point of diagnosis, meaning tumors are confined to the ovaries or fallopian tubes.

A doctor will need to properly diagnose a BOT since treatment for his type of tumor differs from treatment for ovarian tumors that are definitively malignant.

A 2019 study reports that the incidence rate for BOTs in the Netherlands peaked at around 4.2 per 100,000 in 2011, but this rate has slightly declined in recent years.

Another study from 2021 notes that BOTs account for 10–15% of ovarian epithelial tumors. Epithelial cancers grow on or within the thin tissues that line body parts.

There are several types of BOT, and these differ according to the cells affected. The most common types are serous and mucinous. Serous BOTs account for around 50% of all BOTs, while mucinous BOTs account for around 45%.

Other, less common types include:

  • borderline brenner
  • endometrioid
  • seromucinous
  • clear cell

Medical professionals classify BOTs into four stages, according to size and location. The four stages are:

  • Stage 1: The earliest stage, in which the tumor is located within one or both ovaries.
  • Stage 2: May involve tumor cells spreading to nearby tissues, such as the:
    • fallopian tubes
    • uterus
    • bladder
    • rectum
  • Stage 3: Involves the tumor spreading to the abdominal cavity.
  • Stage 4: The most advanced stage, in which tumor cells have spread to distant organs.

Borderline ovarian tumors that have advanced beyond stage 1 can grow in size and spread to nearby tissues and lymph nodes, as well as distant areas of the body.

However, most BOTs have not advanced beyond stage 1 at the point of diagnosis.

Diagnosis of a BOT may involve several steps.

Typically, the first step involves a general physical examination and a review of a person’s medical history to check for unusual signs and symptoms. A doctor will then perform a pelvic exam to examine the following areas:

  • ovaries
  • fallopian tubes
  • vagina
  • cervix
  • uterus
  • rectum

A doctor will also likely perform a Pap smear, a test that helps assess the cervix’s health.

In order to definitively diagnose a BOT, a doctor will likely order several diagnostic tests. These may include the following:

The treatment for a BOT varies according to its stage at diagnosis.

Treatment will also depend on the person’s fertility preferences. If desired, treatment will aim to preserve fertility, where this is possible and safe.

Stage 1 or 2

Doctors will likely recommend surgery to remove a BOT at stage 1 or 2. The surgical options vary based on a person’s fertility preferences.

People who want to have children may have a unilateral salpingo-oophorectomy, which involves removing the affected ovary and its adjoining fallopian tube. Alternatively, they may have a partial oophorectomy, which involves the removal of part of an ovary.

If a person does not wish to have children, doctors may recommend a bilateral salpingo-oophorectomy, in which a surgeon removes both ovaries and fallopian tubes. A doctor may also recommend a hysterectomy, which involves the removal of the uterus. A hysterectomy may also entail the removal of other organs, such as the cervix, ovaries, and fallopian tubes.

Stage 3 or 4

According to the NCI, a person with a stage 3 or 4 BOT will need similar surgical treatment, which may include the following:

  • bilateral salpingo-oophorectomy
  • hysterectomy
  • omentectomy, which is surgical removal of the omentum, the fatty tissue inside the abdomen that surrounds the abdominal organs

If a stage 3 or 4 BOT recurs, a doctor may recommend a combination of surgery and chemotherapy.

According to a 2019 study using data from the Netherlands, the 5-year relative survival rate (RSR) for BOT is high. Between 2011 and 2016, the 5-year RSR was 98%.

Researchers continue to investigate additional treatment options for borderline tumors. A person can talk to their doctor about clinical trials in their area that they may be eligible to participate in.

According to Cancer Research UK, around 15% of ovarian tumors are BOTs. These tumors usually affect people who are 20–40 years old.

In rare instances, a stage 1 BOT may become cancerous and spread to other parts of the body. According to a 2012 article, 20–30% of BOTs transform into cancer, based on several factors.

The exact cause of ovarian cancer is unknown. The American Cancer Society states that the following factors may play a role in whether a person develops ovarian tumors or cancer:

  • levels of hormones called androgens
  • an increased number of ovulations, for example due to starting to menstruate early or experiencing menopause late
  • genetic factors

Are borderline ovarian tumors hereditary?

An older study from 2010 noted that genetic factors play a role in the development of BOTs. No current studies dispute the claim.

Ovarian cancer has a hereditary component that can increase a person’s risk of developing the disease. About 5–10% of cases of ovarian cancer develop in people who have had a close family member develop the disease.

Borderline ovarian tumors are abnormal cells or growths that develop in the tissue encapsulating an ovary. As these growths are slow-growing and have a low incidence of spreading to other body tissues, doctors typically consider them noncancerous.

However, while most BOTs are noncancerous, they have the potential to become cancerous in rare instances. A cancerous BOT grows in size and spreads to nearby or distant body tissues.

Treatment for a BOT depends on its stage at diagnosis. A Stage 1 or 2 BOT may require surgical treatment alone, while a stage 3 or 4 BOT may require more aggressive surgical treatment in combination with chemotherapy. People can talk with their doctor about their diagnosis and treatment options.