Smooth muscle tumour of uncertain malignant potential (STUMP) of the uterus

by Jason Wasserman MD PhD FRCPC
September 5, 2024


A smooth muscle tumour of uncertain malignant potential (STUMP) is a rare type of tumour that arises in the smooth muscle of the uterus. These tumours are considered “uncertain” because it is difficult to determine whether they are benign (non-cancerous) or malignant (cancerous) based on their appearance under the microscope. STUMP falls somewhere between a benign leiomyoma (fibroid) and a malignant leiomyosarcoma. While many STUMPs behave in a non-aggressive manner, some can recur or progress to a more aggressive form.

Is smooth muscle tumour of uncertain malignant potential of the uterus cancerous?

STUMP of the uterus is not considered cancerous, but its behaviour is unpredictable. It falls between a benign tumour, like a leiomyoma (fibroid), and a malignant tumour, like leiomyosarcoma. While many STUMPs behave indolently, there is a risk that some may recur or, in rare cases, progress to a more aggressive form.

Because of this uncertainty, patients diagnosed with STUMP are carefully monitored with follow-up exams and imaging to check for signs of recurrence or progression. Treatment decisions depend on the tumour’s features and the patient’s overall health.

What are the symptoms of smooth muscle tumour of uncertain malignant potential?

The symptoms of STUMP are often similar to those of other uterine tumours, such as fibroids. They may include:

  • Abnormal uterine bleeding (heavy or irregular periods)
  • Pelvic pain or discomfort
  • Pressure or bloating in the lower abdomen
  • Frequent urination if the tumour presses on the bladder

However, some women with STUMP may not have any noticeable symptoms, and the tumour may be found during imaging or surgery for another condition.

What causes smooth muscle tumour of uncertain malignant potential?

The exact cause of STUMP is not well understood. Like other smooth muscle tumours of the uterus, STUMP likely arises from genetic mutations in the smooth muscle cells of the uterine wall. These mutations lead to abnormal growth, but not enough to clearly classify the tumour as benign or malignant. Factors such as hormonal changes, age, and genetic predispositions may play a role in the development of STUMP, but more research is needed to fully understand the causes.

How is this diagnosis made?

The diagnosis of STUMP is made through the examination of tumour tissue under a microscope. The tissue is typically obtained through a biopsy or surgery. Pathologists look for specific features to determine whether the tumour should be classified as STUMP rather than a benign leiomyoma or a malignant leiomyosarcoma. The diagnosis is based on a combination of factors, including the appearance of the cells, the number of mitotic figures (dividing cells), and the presence or absence of necrosis (dead cells).

What are the microscopic features of smooth muscle tumour of uncertain malignant potential in the uterus?

A smooth muscle tumour of uncertain malignant potential (STUMP) is made up of smooth muscle cells, which are normally found in the wall of the uterus. These cells help the uterus contract during menstruation and childbirth. While there are no strict rules for diagnosing STUMP, pathologists classify a tumour as STUMP if it fits into one of the following groups. Each of these groups is based on specific characteristics seen under the microscope:

  1. Tumours with nuclear atypia and low mitotic activity: In this group, the tumour cells show nuclear atypia, which means the nuclei (the part of the cell that controls growth and division) look different from normal cells, indicating they might not be functioning correctly. There are 2–4 mitotic figures per square millimetre, which refers to the number of cells dividing to create new cells. A low number of dividing cells suggests the tumour is growing slowly. These tumours do not show tumour cell necrosis (dead cells in the tumour), which is a feature that can suggest more aggressive behaviour.
  2. Tumours with tumour cell necrosis but no other worrisome features: This group includes tumours with areas of tumour cell necrosis (dead cells within the tumour) but without other concerning features like abnormal-looking nuclei or a high number of dividing cells. Necrosis can be a sign of aggressive behaviour in some tumours. However, it can sometimes be difficult to tell the difference between tumour cell necrosis and infarct-type necrosis, which occurs when part of the tumour dies due to a lack of blood supply. Because of this, any tumour with necrosis that is not clearly benign may be classified as STUMP.
  3. Tumours with high mitotic activity but no other worrisome features: These tumours lack nuclear atypia (their nuclei look relatively normal) and tumour cell necrosis. However, they have high mitotic activity, meaning there are more than 6 mitotic figures (dividing cells) per square millimetre (or 15 mitoses in 10 high-powered fields under the microscope).
  4. Tumours with diffuse nuclear atypia and uncertain mitotic counts: In this group, the tumour cells show diffuse (widespread) nuclear atypia (many cells have abnormal-looking nuclei). However, it may be difficult to accurately count the number of dividing cells because of a process called karyorrhexis, where the nuclei break apart.

These four groups help pathologists determine how the tumour might behave in the future and whether additional treatment or close follow-up is needed.

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