What is a borderline tumor?

A borderline tumor is a type of growth that shows features between benign (noncancerous) and malignant (cancerous). These tumors usually grow slowly and do not spread to distant parts of the body, but they may invade nearby tissue or come back (recur) after removal. The term borderline in this situation is used to describe the uncertainty around the behavior of the tumor—whether it will continue to behave in a noncancerous way or develop features more typical of cancer.

Borderline tumors are sometimes called tumors of uncertain malignant potential or intermediate-grade tumors. This means that while most behave in a noncancerous way, a small number may act more aggressively.

How do pathologists diagnose a borderline tumor?

A pathologist makes the diagnosis of a borderline tumor after examining a tissue sample under the microscope. Pathologists assess the shape, size, and color of the tumor cells, their arrangement, the rate of cell division, and whether they are invading the surrounding tissue.

Borderline tumors often exhibit some abnormal cellular features, known as cytologic atypia, which means the cells appear somewhat irregular in shape, size, or nuclear appearance compared to normal cells. They may also show mitotic activity, which refers to visible evidence of cells dividing under the microscope. However, invasion—the movement of tumor cells into surrounding normal tissue—is typically absent, and other features seen in malignant tumors, such as tissue destruction or widespread necrosis (dead tissue), are also lacking.

Where are borderline tumors found?

Borderline tumors can develop anywhere in the body, but they are most common in the female reproductive tract, particularly the ovary. Other common sites include the urinary tract, breast, thyroid gland, and soft tissue.

Examples of borderline tumors include:

• Ovary: Serous borderline tumor, mucinous borderline tumor, endometrioid borderline tumor, clear cell borderline tumor, seromucinous borderline tumor, borderline Brenner tumor, serous borderline tumor of the peritoneum, and serous borderline tumor of the fallopian tube.

• Urinary tract: Urothelial tumor of uncertain malignant potential and inverted urothelial papilloma with atypical features.

• Breast: Borderline phyllodes tumor, which falls between benign and malignant phyllodes tumors.

• Thyroid gland: Follicular tumor of uncertain malignant potential (FT-UMP) and well-differentiated tumor of uncertain malignant potential (WD-UMP).

• Soft tissue and sinonasal tract: Glomangiopericytoma, which typically behaves in a noncancerous way but may rarely invade surrounding bone or recur locally.

In each of these areas, the term borderline indicates that the tumor has features suggesting limited or unpredictable growth but does not show the destructive behavior of an invasive cancer.

Why is the term borderline tumor important in a pathology report?

When a pathology report describes a tumor as borderline, it helps doctors and patients understand that the tumor’s behavior is uncertain but often favorable. Most borderline tumors can be treated successfully with surgery alone. However, because a small number may recur or develop more aggressive features over time, regular follow-up with physical examinations or imaging is recommended.

Questions to ask your doctor

• What type of borderline tumor do I have, and where did it start?

• What features made it borderline rather than benign or malignant?

• Was the tumor completely removed?

• What kind of follow-up or monitoring will I need?

• What are the chances that the tumor could come back or change over time?