Nodular fasciitis is a benign, self-limiting mesenchymal neoplasm that typically occurs in subcutaneous tissue. It most commonly presents as a rapidly growing nodule, usually measuring between 1-3 cm in diameter. The lesion often affects adults between the ages of 20 and 40, with common sites being the flexor forearm, chest, and back.

The exact etiology of nodular fasciitis is not fully understood. It is hypothesized to arise as a reactive process to injury or infection, or due to genetic alterations. The presence of a USP6 gene rearrangement, specifically the t(17;22) (p13.1;q12.3) translocation, has been identified in many cases, supporting the notion of a genetic component in its pathogenesis.^1,3

Imaging plays a crucial role in the diagnosis and differentiation of nodular fasciitis from other soft tissue lesions. On Ultrasound (US) Nodular fasciitis typically appears as well-defined isoechoic to hypoechoic nodules within the subcutaneous tissue. There is often mildly increased vascular flow within the lesion, they are commonly found adjacent to or within the fascia. On computed Tomography (CT), nodular fasciitis presents as a small, well-circumscribed mass, usually less than 4 cm in diameter. The lesion shows moderate to marked diffuse enhancement after contrast administration. Solid lesions have isointense signal intensity on T1-weighted images and hyperintense signal intensity on T2-weighted images compared to muscle. Magnetic Resonance Imaging (MRI) findings vary with the histologic composition of the lesion, appearing nearly isointense to skeletal muscle on T1-weighted images and hyperintense on T2-weighted images, though highly collagenous lesions are hypointense on all sequences, with contrast enhancement typically diffuse but sometimes peripheral.^3,4,5

Nodular fasciitis exhibits characteristic histopathological features. It usually presents well-circumscribed clusters of spindle cells with a high mitotic index and multinuclear giant cells. The tumor often shows inflammatory infiltrates and erythrocyte extravasation. The lesion consists of plump, uniform reactive proliferation of fibroblasts and myofibroblasts in a myxoid stroma, giving it a culture-like appearance. Vascular proliferation is frequently observed within the lesion. Immunohistochemistry (IHC) plays a crucial role in the diagnosis of nodular fasciitis and helps differentiate it from other spindle cell lesions. The lesional cells of nodular fasciitis typically show immunoreactivity for:

• Smooth Muscle Actin (SMA): Positive in myofibroblasts.
• Muscle-Specific Actin (MSA): Positive in myofibroblasts.
• CD68: Positive in some spindle cells and macrophages.
• S-100: Negative, helping to rule out neural origin.
• Desmin: Rarely positive, distinguishing it from other myogenic tumors.
• Beta-Catenin: Negative, differentiating it from fibromatoses.
• Cytokeratin: Negative, ruling out epithelial origin.^1,3,4

Clinically, the rapid growth of the nodule is one of the key clues to diagnosis. Despite its rapid growth, nodular fasciitis is typically painless and has a benign course. It generally arises from superficial fascia but can occasionally be found within intramuscular or intravascular locations. Due to its rapid growth and pseudosarcomatous morphology, it can be challenging to diagnose and is often mistakenly identified as a malignant tumor, such as a sarcoma. Therefore, it is essential to correlate histopathological findings with the clinical presentation to avoid misdiagnosis. Immunohistochemical staining and molecular analysis for USP6 gene rearrangement can aid in distinguishing nodular fasciitis from other spindle cell lesions and malignant tumors. ^5,6