What is a myofibroblastic lesion/proliferation?

Reviewed by Pathologists on:
January 8, 2026

A myofibroblastic lesion (also called a myofibroblastic proliferation) is a descriptive term used when a tissue sample contains an increased number of myofibroblasts. Myofibroblasts are specialized cells that share features of fibroblasts (cells that make connective tissue) and muscle cells (cells that can contract).

Myofibroblasts normally appear during wound healing and tissue repair, where they help close wounds and produce scar tissue. In some situations, however, these cells grow or accumulate more than expected, forming a visible lesion.

Importantly, the term myofibroblastic lesion describes what the cells look like, not whether the process is benign or malignant. For this reason, it is often used when more information is needed before a final diagnosis can be made.

Why might my pathology report use this term?

Pathologists use the term myofibroblastic lesion when the tissue shows features of myofibroblasts but does not clearly fit into a single, well-defined diagnosis.

This commonly happens when:

• The tissue sample is small, such as a biopsy.

• The lesion has overlapping features seen in several different conditions.

• Additional tests are still pending.

Using this descriptive term allows your healthcare team to continue evaluation while further studies are performed or additional tissue is obtained.

Are myofibroblastic lesions cancer?

Not all myofibroblastic lesions are cancerous. In fact, many are benign and represent a reactive or self-limited process. Examples include myofibroblastic growth after injury or surgery, as well as benign lesions such as nodular fasciitis, which often resolve on their own.

Some myofibroblastic lesions, however, can be locally aggressive or malignant, meaning they can invade nearby tissue and, in rare cases, spread to other parts of the body. Determining where a lesion falls along this spectrum requires careful evaluation and often additional testing.

What do myofibroblastic lesions look like under the microscope?

Under the microscope, myofibroblastic lesions are usually composed of spindle-shaped cells, long and thin, arranged in bundles or in loose patterns. These cells are often mixed with collagen, the protein that gives connective tissue strength.

In benign or reactive lesions, the cells tend to look fairly uniform and organized. In more aggressive or malignant lesions, the cells may appear more irregular, crowded, or disorganized. Pathologists also look for mitotic figures, which are cells in the process of dividing. While mitotic figures can be seen in both benign and malignant lesions, a high number may raise concern for a more aggressive process.

How do pathologists determine the exact diagnosis?

To better classify a myofibroblastic lesion, pathologists combine several types of information.

They carefully assess the microscopic appearance, including cell shape, growth pattern, and whether the lesion appears to invade surrounding tissue. They also consider the location of the lesion, the patient’s age, and the clinical history.

In most cases, immunohistochemistry is performed. This is a laboratory technique that uses special stains to detect proteins made by the cells. These stains help confirm that the cells are myofibroblastic and help distinguish them from other tumours that can look similar.

In some cases, molecular testing, such as next-generation sequencing, is used to identify genetic changes that support a specific diagnosis.

Why is identifying the specific tumour type necessary?

Different myofibroblastic conditions behave very differently. Some require no treatment or only local removal, while others may require surgery, medication, or close long-term follow-up.

Identifying the exact diagnosis helps your doctor:

• Understand how the lesion is likely to behave.

• Decide whether treatment is needed and, if so, what type.

• Avoid unnecessary treatment for benign or reactive conditions.

• Recognize rare malignant tumours that require more aggressive care.

What conditions can be described as myofibroblastic?

Several different conditions may fall under the broad category of myofibroblastic lesions. Examples include:

• Reactive myofibroblastic proliferation, a non-cancerous response to injury, inflammation, or surgery, often resolves once the underlying cause is determined.
• Nodular fasciitis, a benign and rapidly growing lesion that can look alarming but usually resolves with minimal treatment.
• Desmoid fibromatosis, a benign but locally aggressive growth that does not spread but may recur.
• Inflammatory myofibroblastic tumour, a lesion characterised by myofibroblasts and inflammation, sometimes associated with changes in the ALK gene.
• Malignant tumours, such as certain sarcomas, which show aggressive features and require specialized treatment.

Because these conditions can appear similar under the microscope, additional tests are often required to distinguish them.

Why might immunohistochemistry be mentioned in my report?

Immunohistochemistry helps confirm the nature of the cells in a myofibroblastic lesion. Common findings include proteins that support a myofibroblastic origin and help exclude smooth muscle, nerve, or other tumour types.

The pattern of staining, rather than any single test result, helps the pathologist narrow down the diagnosis.

Questions to ask your doctor

• Do the findings suggest a benign, reactive, or malignant process?

• Were special stains or molecular tests performed?

• Do I need an additional biopsy or surgery to clarify the diagnosis?

• What follow-up or treatment is recommended?