Prostatic Stromal Tumour of Uncertain Malignant Potential (STUMP): Understanding Your Pathology Report

Section Editor: Trevor Flood MD FRCPC
October 26, 2026

Prostatic stromal tumor of uncertain malignant potential, or STUMP, is a rare tumor that starts in the stroma of the prostate gland. The stroma is the supporting tissue that surrounds the prostate gland. STUMP is different from the common types of prostate cancer, which start in the glands. Most STUMPs grow slowly and do not spread to other parts of the body. However, STUMP often comes back after incomplete removal, and a small number turn into a cancer called prostatic stromal sarcoma.

The phrase “uncertain malignant potential” means pathologists cannot reliably predict how this tumor will behave from the microscope. Our article on tumors of uncertain malignant potential explains this category in more detail. This article explains how pathologists diagnose prostatic STUMP, what the findings in your pathology report mean, and the treatment and follow-up your care team may discuss.

Who gets prostatic STUMP?

Prostatic STUMP is rare, and much of what we know comes from a small number of reported cases. It usually affects men between 50 and 70 years old, with an average age at diagnosis of about 59. It has been reported in men as young as their 20s and as old as their 80s.

What causes prostatic STUMP?

The cause of prostatic STUMP is not known. Prostate stromal cells respond to hormones, and most STUMPs express the progesterone receptor. This suggests that hormones may play a role in their growth. No inherited condition or lifestyle factor has been linked with prostatic STUMP.

What are the symptoms of prostatic STUMP?

The symptoms of prostatic STUMP are caused by the tumor pressing on the urethra, the tube that carries urine out of the body. Common symptoms include a weak urine stream, needing to urinate often, especially at night, and difficulty starting urination. Some men are unable to urinate at all, a condition called urinary retention. Some STUMPs cause no symptoms and are found during a rectal examination or imaging done for another reason.

Because STUMP starts in the stroma rather than the glands, it usually does not raise the prostate-specific antigen (PSA) level much. A raised PSA may be caused by another condition in the prostate, such as an enlarged prostate or prostate cancer.

How is the diagnosis made?

Prostatic STUMP is often diagnosed in tissue removed during a transurethral resection of the prostate (TURP). This is an operation done through the urethra to remove prostate tissue that is blocking urine flow. STUMP may also be found on a needle biopsy of the prostate or after the whole prostate is removed. An MRI of the pelvis is often used to see the size of the tumor and whether it has grown beyond the prostate.

A pathologist examines the tissue under the microscope. In STUMP, the stroma contains more cells than normal, and some of the cells may look abnormal. The tumor cells are usually long, thin spindle cells mixed with normal prostate glands. The pathologist carefully checks for features of a sarcoma, such as many dividing cells (mitotic figures) or areas of dead tumor tissue (necrosis).

Patterns of prostatic STUMP under the microscope

Prostatic STUMP can grow in several patterns, and some tumors show more than one. Your report may describe one of these patterns:

Degenerative atypia. Scattered stromal cells have large, dark, irregular nuclei and are mixed with normal prostate glands. This is the most common pattern. The abnormal nuclei are thought to reflect cellular aging rather than aggressive growth.
Hypercellular stroma. The stroma is packed with more cells than normal, with little or no atypia.
Myxoid stroma. The stromal cells sit in a loose, gel-like background.
Phyllodes-like. The stroma pushes the prostate glands into leaf-like shapes. This pattern resembles a breast tumor called a phyllodes tumor.

Immunohistochemistry

Immunohistochemistry uses antibodies to show which proteins cells make. In prostatic STUMP, pathologists use immunohistochemistry to confirm that the tumor started from prostatic stroma and to rule out other spindle cell tumors. The stains used vary between laboratories, but typical results include:

CD34. Usually positive. CD34 is found in the specialized stromal cells of the prostate.
Progesterone receptor (PR). Usually positive, which supports a tumor that started in the hormone-responsive stroma of the prostate.
Estrogen receptor (ER). Less often positive than PR.
Smooth muscle actin (SMA) and desmin. Sometimes positive. These proteins are found in muscle cells.
CD117 and S100. Usually negative. A positive result suggests a different tumor type, such as a gastrointestinal stromal tumor or a nerve sheath tumor.

Not every tumor needs all of these stains, and the exact panel depends on the laboratory.

STUMP and prostatic stromal sarcoma

Prostatic stromal sarcoma is a cancer that also starts in the prostatic stroma. It differs from STUMP because the tumor cells are more crowded, more abnormal, divide more often, and the tumor may contain necrosis. Stromal sarcoma can spread to other parts of the body, most often the lungs and bones. In older studies, about 1 in 4 men with stromal sarcoma developed distant spread. Distant spread has not been reported in STUMP.

Distinguishing STUMP from stromal sarcoma can be difficult, especially on a small biopsy. Your report may suggest examining more tissue, or your doctor may ask for a second opinion from a specialist pathologist.

Surgical margins

A margin is the cut edge of tissue removed during surgery. Margins are only reported when the tumor has been removed as one piece, such as after removal of the whole prostate. They cannot be assessed in tissue removed by TURP, which comes out in many small fragments. Your report will describe the margins in one of these ways:

Negative margin. No tumor cells are seen at the cut edge, which means the tumor was completely removed.
Positive margin. Tumor cells are present at the cut edge. This means some tumor may remain, which raises the chance that it will come back.

What is the prognosis?

The prognosis for most men with prostatic STUMP is good, because the tumor rarely spreads beyond the prostate. The main concern is that the tumor can come back if it is not completely removed. A 2025 review of 84 men with STUMP found that about 22 in 100 had the tumor come back or grow, on average about 3 years after diagnosis. The other 78 in 100 had no further growth during follow-up. In men treated without removal of the prostate, about 35 in 100 had the tumor come back. None of the men who had the prostate or the tumor area surgically removed had the tumor come back during follow-up.

A small number of STUMPs turn into prostatic stromal sarcoma over time. In one review of published cases, this happened in about 5 in 100 men. No feature seen under the microscope reliably predicts which STUMPs will come back or progress.

What happens after the diagnosis?

After a prostatic STUMP diagnosis, the next steps depend mainly on your age, general health, and the size and extent of the tumor. Because STUMP is rare, no single treatment plan is agreed upon, and care is often planned at a specialized center. Options your care team may discuss include:

Removal of the prostate. Removing the whole prostate, or the part containing the tumor, gives the lowest chance of the tumor coming back. It is often considered for younger men and for larger or growing tumors.
Transurethral resection of the prostate (TURP). This operation relieves urinary symptoms, but the tumor is more likely to come back afterward, and repeat procedures may be needed.
Active surveillance. Regular checkups and imaging, with treatment only if the tumor grows or causes symptoms. This may be considered for some older men or men without symptoms.

Whatever treatment you choose, regular follow-up imaging is usually recommended because STUMP can come back several years after treatment. Removal of the prostate can affect urinary control and erections, and your care team can discuss these effects with you.

Questions to ask your doctor

Which pattern of STUMP was seen in my tissue?
Was there any sign of prostatic stromal sarcoma?
Which special stains were used, and what did they show?
Should a specialist pathologist review my tissue?
How large is the tumor, and has it grown beyond the prostate?
Was the tumor completely removed, and were the margins negative?
What are my treatment options, and what are the benefits and risks of each?
What is my chance of the tumor coming back?
How often will I need imaging and checkups, and for how long?
How might treatment affect my urinary control and sexual function?