A tumor of uncertain malignant potential is a type of growth that cannot be clearly classified as either benign (noncancerous) or malignant (cancerous) after microscopic examination. In most cases, these tumors behave in a noncancerous way, but because they show some worrisome features, doctors cannot rule out the possibility that they might act more aggressively.
The term reflects the uncertainty about how the tumor will behave over time—whether it will stay localized or develop the ability to invade or spread. Pathologists use this diagnosis when the microscopic and molecular features of a tumor do not fit neatly into either benign or malignant categories.
A pathologist makes the diagnosis after carefully examining a tissue sample under the microscope. To determine whether a tumor should be classified as benign, malignant, or uncertain, the pathologist examines several features, including the shape, size, and color of the tumor cells, the arrangement of the cells, and whether they have invaded surrounding tissues.
Tumors of uncertain malignant potential may show mild to moderate cytologic atypia, meaning that some cells appear abnormal in size, shape, or nuclear detail compared to normal cells. They may also show increased mitotic activity, which is visible evidence of cell division. However, the key malignant features—such as definite invasion into surrounding tissue, a high mitotic rate, or tumor necrosis (areas of dead tumor cells)—are typically absent or incomplete, making it difficult to assign a precise malignant diagnosis to the tumor.
Tumors of uncertain malignant potential can occur in many parts of the body. Still, they are most often diagnosed in the thyroid gland, female reproductive tract, urinary tract, adrenal gland, and soft tissue.
Examples of tumors of uncertain malignant potential include:
Thyroid gland: Follicular tumor of uncertain malignant potential (FT-UMP) and well-differentiated tumor of uncertain malignant potential (WD-UMP). These tumors are composed of follicular cells that resemble normal thyroid tissue but exhibit minimal invasion, making it challenging to determine whether they are benign adenomas or carcinomas.
Female reproductive tract: Endometrial stromal tumor of uncertain malignant potential, in which the tumor resembles normal endometrial stroma but shows limited invasion into surrounding tissue.
Urinary tract: Urothelial proliferation of uncertain malignant potential is used when abnormal cell growth is seen on the bladder lining, but the degree of invasion cannot be confirmed.
Adrenal gland: Adrenocortical tumor of uncertain malignant potential, used when an adrenal cortical tumor shows mild atypia or limited invasion but does not meet the full criteria for carcinoma.
Soft tissue: Certain rare tumors, such as myxoid spindle cell tumors or smooth muscle tumors of uncertain malignant potential (STUMP), may occur in the uterus or other soft tissues when the degree of cellular atypia or mitotic activity falls between benign and malignant categories.
In each of these situations, the phrase ‘of uncertain malignant potential’ communicates that the tumor’s future behavior cannot be predicted with certainty, even though it is likely to behave in a noncancerous manner.
When a tumor is described as having uncertain malignant potential, it alerts both patients and doctors that the tumor may have a low but not zero risk of recurrence or progression. Most of these tumors are treated with surgical removal, and no further therapy is needed. However, long-term follow-up is usually recommended to monitor for any signs of recurrence or change in behavior.
This term also helps prevent the overtreatment of tumors that are unlikely to behave aggressively while ensuring that appropriate monitoring is in place.
What organ or tissue did this tumor start in?
What features made it “of uncertain malignant potential”?
Was the tumor completely removed?
What is the chance that it could come back or spread?
What kind of follow-up will I need?