Tumor of Uncertain Malignant Potential: Definition

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A tumor of uncertain malignant potential is a tumor that a pathologist cannot clearly call benign or malignant. It has some worrisome features, but not enough to diagnose cancer. Most of these tumors behave like benign tumors. A small number come back after removal or, rarely, spread.

This article explains what this diagnosis means, which tumors receive it, how doctors treat them, and what your report may say.

What does uncertain malignant potential mean?

Uncertain malignant potential means the pathologist cannot confidently predict how the tumor will behave. A benign tumor stays in one place and does not spread. A malignant tumor, or cancer, can grow into nearby tissue and spread. A tumor of uncertain malignant potential has features of both. The term describes this uncertainty honestly. It is not a vague diagnosis.

Why does a pathologist use this term?

The pathologist decides whether a tumor is benign or malignant by looking at specific features under the microscope. Each tumor type requires certain features. A tumor receives this label when it shows some of them, but not enough. The key features include:

Abnormal cells. The cells look somewhat abnormal. Pathologists call this atypia.
Dividing cells. The tumor has more dividing cells than expected. See mitotic rate.
Dead tumor cells. The tumor may show areas of tumor necrosis.
Growth into nearby tissue. The pathologist may not be able to tell whether the tumor has grown into surrounding tissue or vessels. See invasion.

Which tumors can have uncertain malignant potential?

Doctors use this term for specific tumor types. Each has its own rules. Examples include:

Smooth muscle tumor of the uterus. A smooth muscle tumor of uncertain malignant potential (STUMP) falls between a fibroid and leiomyosarcoma. The pathologist weighs atypia, the number of dividing cells, and tumor cell necrosis.
Thyroid tumors. Follicular tumor of uncertain malignant potential (FT-UMP) and well-differentiated tumor of uncertain malignant potential (WDT-UMP) occur in the thyroid. In these tumors, the pathologist cannot be sure whether the tumor has grown through its capsule. The 2022 WHO classification groups them with NIFTP as low-risk tumors.
Prostate stromal tumor. A prostatic stromal tumor of uncertain malignant potential starts in the supporting tissue of the prostate.
Salivary gland tumor. Reports may use salivary gland neoplasm of uncertain malignant potential (SUMP) for a salivary gland tumor.

How is uncertain malignant potential different from borderline?

The two terms describe a similar idea. Both mean a tumor between benign and malignant. The term used depends on the organ. In the ovary and for phyllodes tumors of the breast, pathologists say borderline tumor. Examples include serous borderline tumor of the ovary and borderline phyllodes tumor. For the tumors listed above, pathologists describe them as having uncertain malignant potential.

How do doctors treat these tumors?

Surgery to remove the whole tumor is the main treatment. For most of these tumors, surgery alone is enough. Chemotherapy and radiation are not usually needed. Because a small risk remains, doctors usually recommend regular follow-up.

Uterine STUMP shows how this works. About 1 in 10 come back, and a recurrence is more often another STUMP or a fibroid than a cancer. Follow-up often means an exam and an ultrasound every six months for five years, then once a year. Teams differ in their exact plans.

What might my report say?

Reports describe these tumors in a few standard ways. Each laboratory phrases them a little differently:

The full name. For example, “smooth muscle tumor of uncertain malignant potential.”
An abbreviation. For example, STUMP, FT-UMP, WDT-UMP, or SUMP.
The features seen. The report may list the atypia, the mitotic count, or necrosis.
The margins. The report states whether the tumor reached the edge of the tissue removed.
A comment. The pathologist may explain why the tumor does not fit a benign or malignant diagnosis.

Questions to ask your doctor

What type of tumor do I have, and where did it start?
Which features made it “of uncertain malignant potential”?
Was the whole tumor removed, and were the margins clear?
What is the chance it could come back or spread?
Do I need any more treatment?
What follow-up will I need, and for how long?
Should another pathologist review my slides?