Pseudosarcomatous Myofibroblastic Proliferation: Understanding Your Pathology Report

Section Editor: Trevor Flood MD FRCPC
October 12, 2026

Pseudosarcomatous myofibroblastic proliferation is a noncancerous tumor made of long, thin cells called myofibroblasts. It most often occurs in the bladder, but it can also occur elsewhere in the urinary tract and in the female genital tract. The word “pseudosarcomatous” means the tumor can look like a type of cancer called a sarcoma under the microscope, even though it is not one.

Pseudosarcomatous myofibroblastic proliferation can grow back in the same place after removal, but it does not spread to other parts of the body. Many pathologists now consider it part of a larger group of tumors called inflammatory myofibroblastic tumor. Your report may use either name, or a related term such as “pseudosarcomatous myofibroblastic neoplasm.”

This article explains how pathologists diagnose pseudosarcomatous myofibroblastic proliferation, how it differs from cancer and similar growths, and what your pathology report means for your care.

Who gets pseudosarcomatous myofibroblastic proliferation?

Pseudosarcomatous myofibroblastic proliferation is rare. In the bladder, it is most often found in young and middle-aged adults, and it has been reported more often in women than in men. It can also occur in teenagers and, less often, in older adults.

What causes pseudosarcomatous myofibroblastic proliferation?

The cause of pseudosarcomatous myofibroblastic proliferation is not fully understood. Some of these tumors form without any obvious trigger, while others appear in tissue that was injured during an earlier procedure. In many cases, the tumor cells carry a change in a gene called ALK, in which part of the ALK gene has joined with another gene. This change, called a fusion, occurs only in the tumor cells and is not inherited.

The ALK fusion is one reason pathologists now consider pseudosarcomatous myofibroblastic proliferation a true tumor rather than a healing reaction. A similar-looking growth that forms only as part of healing after surgery is called a postoperative spindle cell nodule.

What are the symptoms of pseudosarcomatous myofibroblastic proliferation?

The most common symptom of pseudosarcomatous myofibroblastic proliferation in the bladder is blood in the urine, which may be visible or found only on a urine test. A larger tumor can cause pain, a need to urinate often, or difficulty emptying the bladder. Some tumors cause no symptoms and are found on a scan done for another reason.

How is the diagnosis made?

A pathologist makes the diagnosis of pseudosarcomatous myofibroblastic proliferation after the tumor is removed or sampled with a biopsy. In the bladder, the tumor is usually seen during cystoscopy and removed through the urethra in a procedure called a TURBT. On scans and during cystoscopy, it can look like bladder cancer, so the pathologist needs to examine it under a microscope.

Under the microscope, the pathologist sees the following features:

Spindle cells. Long, thin cells with tapering ends, spread out in loose tissue that often looks watery or bubbly. The cells can look large, but they do not show the severe abnormalities seen in a sarcoma.
Dividing cells. Some cells may be dividing, which pathologists see as mitotic figures. Atypical mitotic figures are not seen.
Immune cells and blood vessels. Scattered immune cells and many small blood vessels run through the tumor. Compared with other inflammatory myofibroblastic tumors, there are usually fewer plasma cells.
Growth into the bladder wall. The tumor can extend into the thick muscle layer of the bladder wall, called the muscularis propria. For this tumor, this finding does not mean it is a cancer.

Because pseudosarcomatous myofibroblastic proliferation can resemble several cancers, the pathologist usually performs additional tissue tests. These tests are described in the next section.

Immunohistochemistry

Immunohistochemistry uses special stains to show which proteins cells in pseudosarcomatous myofibroblastic proliferation make. The results help the pathologist confirm that the cells are myofibroblasts and rule out similar-looking cancers. The stains used and how results are worded vary between laboratories. Results you may see in your report include:

Smooth muscle actin (SMA). This stain is usually positive in the tumor cells. Myofibroblasts make the protein.
Desmin. This stain is positive in some tumors and negative in others. The protein is found in muscle cells and in some myofibroblasts.
Cytokeratins. This stain is often positive in at least some of the tumor cells. In this tumor, a positive result does not mean the growth is a carcinoma.
ALK. This stain is positive in most bladder tumors. A positive result strongly supports the diagnosis because most similar-looking cancers are ALK negative. A negative result does not rule out the diagnosis.

Not every case needs every stain. The pathologist chooses the stains based on how the tumor looks and where it was found.

How is it different from other growths?

Pseudosarcomatous myofibroblastic proliferation belongs to a group of growths made of myofibroblasts, sometimes called myofibroblastic proliferations. The pathologist uses the history, cell appearance, and test results to determine which diagnosis fits best. Your report may mention one of these related diagnoses:

Inflammatory myofibroblastic tumor. The World Health Organization (WHO) classification, 5th edition (2022), groups pseudosarcomatous myofibroblastic proliferation of the urinary tract with this tumor. Some reports use the two names for the same growth.
Postoperative spindle cell nodule. A healing reaction that forms within a few months after a procedure at the same site. It looks very similar under the microscope, and a recent procedure is the main clue.
Leiomyosarcoma. A cancer made of smooth muscle cells. Its cells look much more abnormal, and it often shows atypical mitotic figures and areas of dead tissue.
Sarcomatoid carcinoma. A cancer in which some cells have become long and thin. In the bladder, it often occurs together with urothelial carcinoma, and its cells look very abnormal.

Margins

When the pathologist removes pseudosarcomatous myofibroblastic proliferation, they may comment on whether the tumor reaches the edges of the tissue, called the margins. For a tumor removed through the urethra, the tissue comes out in small pieces, so margins usually cannot be assessed. If part of the bladder is removed, the report may describe the margins as follows:

Negative margin. No tumor cells are seen at the cut edge, which suggests the tumor was completely removed.
Positive margin. Tumor cells are seen at the cut edge. Because the tumor can grow back in the same place, your team may recommend closer follow-up.

Biomarker and molecular testing

Biomarker testing for pseudosarcomatous myofibroblastic proliferation looks for the ALK gene change that drives the growth of many of these tumors. This testing isn’t needed in every case, because the diagnosis can usually be made from microscopy and stains. It is most useful when the diagnosis is uncertain or when the tumor cannot be completely removed.

ALK

ALK is a gene that makes a protein involved in cell growth. In pseudosarcomatous myofibroblastic proliferation of the bladder, part of the ALK gene is often fused to a gene called FN1, written as FN1::ALK. This fusion is tested with molecular methods such as fluorescence in situ hybridization (FISH) or next-generation sequencing (NGS).

ALK fusion detected. A fusion involving the ALK gene was found. This confirms that the growth is a true tumor of this type. For the rare tumor that cannot be removed or keeps coming back, the FDA approved the ALK-targeted drug crizotinib in July 2022 for ALK-positive inflammatory myofibroblastic tumors.
ALK fusion not detected. No ALK fusion was found. This does not rule out the diagnosis because some of these tumors have other gene changes, or none that can be detected. Surgery remains the standard treatment.

For more information on tumor testing, visit our Biomarkers and Genetic Testing section.

Can pseudosarcomatous myofibroblastic proliferation come back?

Pseudosarcomatous myofibroblastic proliferation can recur in the same place, especially if it wasn’t completely removed. Studies differ, but this happens in about 4 to 25 in 100 people. No spread to lymph nodes or other organs has been reported for this tumor. If a tumor recurs, doctors usually treat it with another procedure to remove it.

What happens after the diagnosis?

After a diagnosis of pseudosarcomatous myofibroblastic proliferation, the main treatment is complete removal of the tumor. Chemotherapy and radiation are usually not needed. Your doctor will decide on next steps based on the size of the tumor, where it is, and whether it was removed completely.

TURBT. Most bladder tumors are removed through the urethra. A second procedure may be done if some of the tumor remains.
Partial removal of the bladder. A larger tumor, or one that grows deeply into the bladder wall, may be removed by taking out the part of the bladder that contains it.
Follow-up. Because the tumor can grow back, doctors usually repeat cystoscopy or imaging for a period after treatment.
ALK-targeted treatment. For rare tumor that cannot be removed safely, your doctor may consider an ALK-targeted drug if the ALK test is positive.

Approval and access to these drugs differ by country, and your care team can explain what applies where you live.

Questions to ask your doctor

Does my report call this a pseudosarcomatous myofibroblastic proliferation or an inflammatory myofibroblastic tumor?
How sure is the pathologist that this isn’t cancer?
Was the ALK stain positive or negative?
Was an ALK fusion test done, and what did it show?
Did the tumor grow into the muscle layer of my bladder?
Was the tumor removed completely?
Do I need a second procedure or partial removal of my bladder?
What is the chance that the tumor will grow back?
How often will I need cystoscopy or scans?
What symptoms should make me contact you?