Smooth Muscle Tumour of Uncertain Malignant Potential (STUMP) of the Uterus: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
September 2, 2026

A smooth muscle tumor of uncertain malignant potential, usually shortened to STUMP, is an uncommon tumor of the uterus. It grows from the smooth muscle cells that make up the wall of the uterus, the same cells that form an ordinary fibroid.

The name reflects the limits of what the microscope can tell us. Pathologists separate a benign leiomyoma, or fibroid, from a malignant leiomyosarcoma using a set of established features. A small number of tumors have a combination of those features that fits neither category. STUMP is the term for that situation.

STUMP is not a diagnosis of cancer. It accounts for only a few in every hundred smooth muscle tumors of the uterus, and it is most often diagnosed between the ages of 40 and 50. Most people diagnosed with one do well. This article will help you understand what this diagnosis means on your pathology report, what each term means, and why it matters for your care.

What does “uncertain malignant potential” mean?

It means the pathologist cannot predict how the tumor will behave based on its appearance, and is saying so rather than guessing. That honesty is the point of the category. Before it existed, tumors like these were forced into one of the two other groups. Some people were told they had a cancer they did not have, and others were reassured when caution was warranted.

In practical terms, a STUMP sits closer to a fibroid than to a cancer. It is not treated as a cancer, and it is not given a cancer stage. It does carry a small chance of coming back, which is why follow-up is recommended and why the diagnosis is taken seriously, even though it is not malignant.

What causes a STUMP?

The cause is not well understood. Like other smooth muscle tumors of the uterus, a STUMP grows from a single smooth muscle cell in the wall of the uterus. That cell acquires genetic changes and begins growing more than it should. Researchers have not fully identified the changes that produce this in-between appearance, and research continues.

Hormones and age appear to play a role, as they do for ordinary fibroids, but no established list of risk factors is specific to this diagnosis.

What are the symptoms?

The symptoms are the same as those of an ordinary fibroid, and nothing about them points to this diagnosis in advance.

Heavy or irregular bleeding — The most common symptom.
Pelvic pain or discomfort — Often a dull ache or a feeling of pressure.
Bloating or fullness in the lower abdomen — Caused by the bulk of the tumor.
Needing to urinate more often — If the tumor presses on the bladder.

Some people have no symptoms, and the tumor is found during imaging or surgery performed for another reason.

How is the diagnosis made?

A STUMP is diagnosed only after the tumor is removed and examined under a microscope. Imaging cannot make this diagnosis, and neither can a naked-eye examination of the tumor. Almost everyone with this diagnosis went into surgery expecting to be treated for a fibroid.

Examination during surgery is also unreliable here. A frozen section examines a piece of tissue while the patient is still in the operating room. It is accurate for many diagnoses but performs poorly for this one. The features that matter require careful assessment of the whole tumor.

The pathologist assesses three things and weighs them together: how the cells look, how often they are dividing, and whether a particular type of cell death is present. STUMP is assigned when the combination does not fit either the benign or the malignant category.

What does a STUMP look like under the microscope?

A STUMP is a smooth muscle tumor of the uterus with a mixture of features that does not fit an established category. Three findings drive the assessment.

Nuclear atypia — Whether the cells look abnormal. Nuclear atypia means the nuclei, the part of the cell holding the genetic material, vary in size and shape more than they should.
Mitotic count — How many cells are caught in the act of dividing. These are called mitotic figures, and the pathologist counts them over a defined area.
Tumor cell necrosis — Whether tumor cells are dying in the pattern associated with cancer. Tumor cell necrosis is not the same as infarct-type necrosis, which is common and harmless in fibroids and happens when part of a tumor outgrows its blood supply.

A benign fibroid has none of these. A leiomyosarcoma usually has a clear combination of them. A STUMP falls in between, and the common patterns are:

Abnormal cells but few dividing cells — The cells look abnormal, but division is infrequent, and there is no tumor cell necrosis.
Necrosis without anything else — Tumor cell necrosis appears to be present, but the cells look normal,l and division is infrequent. Separating tumor cell necrosis from infarct-type necrosis can be difficult. A tumor with necrosis that is not clearly benign may therefore be placed here.
Frequent division without anything else — The cells are dividing often, but they look normal, and there is no tumor cell necrosis.
Abnormal cells with a count that cannot be measured — The cells look abnormal throughout, but dividing cells cannot be counted reliably because the nuclei are breaking apart.

Why is a second pathology opinion often recommended?

Because this diagnosis sits between two others, it relies more on judgment than most. When a second pathologist reviews STUMP cases, the original diagnosis is confirmed roughly three times out of four. In the remaining cases, the tumor is reclassified, sometimes as a benign fibroid and sometimes as a leiomyosarcoma.

For that reason, many centers send these cases to a pathologist who specializes in gynecologic pathology for review. If your report carries this diagnosis, it is entirely reasonable to ask whether that review has been done, or whether it should be. This is a normal part of managing an uncommon diagnosis, not a sign that anything went wrong.

Immunohistochemistry

Immunohistochemistry is a laboratory test that uses antibodies to detect specific proteins inside cells. It does not make this diagnosis, but several proteins have been studied as possible clues to how a tumor will behave, and your report may list them.

p16 and p53. Strong or widespread staining for either has been associated with a higher chance of the tumor returning.
Ki-67. An estimate of how many cells are preparing to divide. Higher results have also been linked with recurrence.
Progesterone receptor. Low levels have been reported more often in tumors that later came back.

None of these has been validated well enough to predict what will happen in an individual case. They are used to decide how closely to follow someone, not to reclassify the tumor.

What is the outlook after a STUMP diagnosis?

Good, for most people. Survival five years after diagnosis is reported at 90 to 100 out of every 100 people, and deaths from this diagnosis are uncommon.

The main concern is recurrence. Across published series, roughly 1 in 10 STUMPs recur, though reported figures vary widely between centers. When one does return, it more often returns as another STUMP or as a fibroid than as a cancer. Progression to leiomyosarcoma happens in a small minority.

Two features of recurrence shape follow-up planning. It can happen late, sometimes several years after the original surgery, with an average of around four years in one series. And there is no reliable way to predict in advance which tumors will do it. Together, these are why follow-up continues for years rather than months.

How is a STUMP treated and followed?

Complete surgical removal is the treatment. Chemotherapy and radiation are not used, because this is not a cancer. What varies is how much is removed and what happens afterward.

Hysterectomy — Removal of the uterus is the usual approach for those who have completed their family or have gone through menopause.
Myomectomy — Removing the tumor alone, leaving the uterus in place. This is a reasonable option for those who wish to preserve fertility. Published series report similar recurrence rates to hysterectomy, and many patients have gone on to conceive afterward.
Avoiding morcellation — Morcellation cuts a tumor into pieces so it can be removed through small incisions. It is associated with a higher rate of recurrence in these tumors and is generally avoided when a STUMP is suspected.
Removing the ovaries — Not routinely necessary, particularly before menopause. This is decided individually.
Long-term follow-up — Typically an examination and pelvic ultrasound every six months for the first five years, then annually. Some teams continue longer, given that recurrences can be delayed.

No single protocol is agreed upon for this diagnosis, so your plan may differ from what someone else was offered. Asking your team to explain the reasoning behind yours is worthwhile.

What other findings may be described in the report?

Your report will describe the tumor and the rest of the uterus. Findings commonly listed include:

Tumor size — Measured in centimeters, and relevant because larger tumors have been associated with recurrence in some series.
The mitotic count and degree of atypia — Often stated explicitly, since the diagnosis rests on these findings.
Whether the tumor was removed intact — The report may note whether the specimen arrived whole or in fragments.
Other fibroids — Ordinary fibroids are frequently present alongside a STUMP and are reported separately.
The lining of the uterus — If a hysterectomy was performed, the lining is examined and reported separately.

Questions to ask your doctor

What features in my tumor led to this diagnosis rather than a fibroid or a cancer?
Has a specialist gynecologic pathologist reviewed my case?
How many dividing cells were counted, and was necrosis present?
Was the tumor removed whole, or was morcellation used?
How large was the tumor?
Was the whole uterus removed, or only the tumor?
What is my chance of this coming back?
What follow-up do you recommend, and for how long?
What symptoms should make me contact you between appointments?
Were p16, p53, or Ki-67 tested, and what did they show?
Do I need any additional treatment?
If I want to become pregnant, is that safe, and when?
Should I be referred to a gynecologic oncologist?