Myofibroblastic lesion/proliferation

A myofibroblastic lesion/proliferation is a growth or area of abnormal tissue composed of myofibroblasts, specialized cells that share features of muscle cells (myo-) and fibroblasts. These cells are typically involved in wound healing and tissue repair, but in some cases, they can form a lesion due to abnormal growth or accumulation.

The term myofibroblastic lesion/proliferation is descriptive and can refer to a wide variety of conditions. These conditions can be benign (non-cancerous) or malignant (cancerous). Pathologists use this term to describe what they see under the microscope, and additional tests, such as immunohistochemistry, may be needed to determine the exact diagnosis.

Is a myofibroblastic lesion a type of cancer?

Not all myofibroblastic lesions/proliferations are cancerous. Many are benign and do not pose significant health risks, such as nodular fasciitis or a reactive myofibroblastic proliferation after injury or surgery. These benign lesions often resolve on their own or with minimal treatment. However, some myofibroblastic lesions can be malignant. Malignant lesions, such as specific subtypes of sarcoma, may invade nearby tissues or spread to other body parts.

How is this diagnosis made?

The diagnosis of a myofibroblastic lesion is made by a pathologist who examines a tissue sample under the microscope. This sample is usually obtained through a biopsy or surgery. The pathologist looks for features such as the cells’ shape, size, and arrangement. These features help the pathologist determine whether the lesion is benign or malignant.

In addition to microscopic examination, pathologists often use immunohistochemistry and molecular tests such as next-generation sequencing (NGS) to characterize the lesion further. These tests help distinguish myofibroblastic lesions from other types of growths that may look similar under the microscope.

What does a myofibroblastic lesion look like under the microscope?

Under the microscope, myofibroblastic lesions typically show a mixture of spindle-shaped cells (long, thin cells) and collagen, the protein that gives tissues strength and structure. In benign lesions, the cells look uniform and are arranged in a way that suggests a non-aggressive process. In malignant lesions, the cells may appear more disorganized, with irregular shapes and sizes, and may invade surrounding tissues. Mitotic figures (dividing cells) are more common in malignant lesions. However, they can also be seen in benign growths.

Immunohistochemistry

Immunohistochemistry (IHC) is a laboratory technique that uses special stains to detect specific proteins in cells. It helps pathologists confirm the diagnosis and identify the type of lesion. The following markers are commonly used in the evaluation of myofibroblastic lesions:

• Smooth muscle actin (SMA): Frequently positive, highlighting the myofibroblastic cells.
• Vimentin: A general marker for connective tissue cells, often positive in these lesions.
• Desmin: May be positive in lesions with muscle-like features.
• ALK (anaplastic lymphoma kinase): Specifically associated with inflammatory myofibroblastic tumours, a subtype of myofibroblastic lesion.
• S100: Can be positive in lesions with neural or melanocytic features, helping exclude some differential diagnoses.
• CD34: Often highlights vascular components but can also be seen in some soft tissue tumours.
• Caldesmon: This protein helps distinguish myofibroblastic lesions from smooth muscle tumours, as it is more strongly expressed in smooth muscle.
• STAT6: Positive in solitary fibrous tumours, which may mimic some myofibroblastic lesions.

The combination of these markers helps pathologists narrow down the diagnosis and differentiate between similar conditions.

Differential diagnosis for a myofibroblastic lesion

Myofibroblastic lesions can resemble other types of growths under the microscope. The differential diagnosis includes:

• Reactive myofibroblastic growth: A benign proliferation of myofibroblasts often seen in response to injury, surgery, or inflammation. These growths are non-cancerous and resolve once the underlying cause is addressed.
• Nodular fasciitis: A benign, rapidly growing lesion often seen in younger individuals; it usually resolves on its own.
• Desmoid fibromatosis: A benign but locally aggressive growth of fibroblasts and myofibroblasts, often found in the abdominal wall or extremities.
• Inflammatory myofibroblastic tumour (IMT): A lesion with myofibroblasts and prominent inflammation, often associated with ALK positivity.
• Leiomyosarcoma: A malignant tumour of smooth muscle, distinguished by strong positivity for smooth muscle markers like desmin and caldesmon.
• Solitary fibrous tumour: A tumour that can mimic myofibroblastic lesions, often positive for STAT6.
• Fibrosarcoma: A malignant tumour of fibroblasts with a herringbone pattern under the microscope.
• Neurofibroma or schwannoma: Benign tumours of nerve tissue, often distinguished by S100 positivity.
• Malignant peripheral nerve sheath tumour (MPNST): A malignant tumour of nerve sheath origin, usually positive for S100 and sometimes overlapping with myofibroblastic lesions.

The pathologist considers the location, patient’s history, and results of specialized tests like immunohistochemistry to determine the exact diagnosis.