Nodular fasciitis – A rare soft tissue tumour

A 34-year-old woman presented with progressive swelling in the region of her left shoulder for 8 months. Ultrasound and Magnetic Resonance Imaging (MRI) of the left shoulder were requested (Figures 1 to 5).

Soft tissue lesion with irregular margins in the left posterior suprascapular region involving the skin, subcutaneous plane and extending/infiltrating the underlying superficial fibres of the trapezius muscle (Figures 1 and 4a). The lesion appears T1 isointense and T2 hyperintense (as compared to skeletal muscles) with significant diffusion restriction and moderate enhancement. Small central fluid collection is seen, consistent with post-biopsy changes (Figures 2, 3, 4a and 4b). No significant necrosis/macroscopic fat is seen within the lesion. The rest of the adjacent fat planes appear normal.

Background

Nodular fasciitis, also known as pseudosarcomatous fasciitis, is a benign, rapidly growing soft tissue tumour due to the abnormal proliferation of fibroblasts and myofibroblasts involving subcutaneous tissues, muscles, and fascia. It is subclassified under the fibroblastic and myofibroblastic category in the 2020 WHO classification of soft tissue tumours [1]. It is most commonly seen in the upper extremities, predominantly in the forearm, followed by the trunk. Due to their rapid growth, aggressive imaging features such as transcompartmental spread, osseous and intraarticular extensions, high mitotic activity and cellularity on histopathology, these lesions were often mistaken for sarcomatous lesions, prompting unnecessarily aggressive surgical interventions.

Clinical Perspective

Nodular fasciitis most commonly occurs in individuals aged between 20 and 40, with rare instances in children. It typically manifests as a rapidly enlarging, rubbery mass that is frequently tender upon palpation [2].

Imaging Perspective

Magnetic resonance imaging (MRI) and ultrasonography (USG) are employed for lesion characterisation. While MRI findings for nodular fasciitis are typically nonspecific, the presence of indicators such as the fascial tail sign, inverted target sign, and transcompartmental spread, along with the relation to the fascia, can support a diagnosis of nodular fasciitis [3,4].

On ultrasound, it typically appears as a hypoechoic or mixed echogenicity mass. At times, it exhibits posterior acoustic enhancement. It is frequently located superficially along the fascial plane but can also be observed in the intramuscular plane. Previous studies have not documented calcifications within the lesion. Characterising intraosseous or intraarticular nodular fasciitis using ultrasound is challenging [3,4].

On MRI, its appearance varies depending on the amount of fibrous tissue, myxoid component, and cellularity within the lesion. It is suggested that early lesions tend to be cellular or myxoid, while mature lesions are fibrous. A lesion with high cellularity and myxoid component exhibits a T2 hyperintense signal compared with skeletal muscles, while in later stages, as the fibrous component increases, the T2 signal decreases. Lesions showing homogeneous enhancement may consist of cellular tissue, whereas those with minimal or no enhancement could be attributed to myxoid changes [2,3]. Typically, nodular fasciitis present as hypo- or iso-intense signals on T1-weighted images and hyperintense signals on T2-weighted images. They exhibit diffusion restriction and show homogeneous or heterogeneous post-contrast enhancement following gadolinium administration. An irregular shape, transcompartmental spread, and intraosseous extension can potentially result in misdiagnosis as aggressive sarcomatous lesions [3]. Nodular fasciitis and other soft tissue tumours exhibit four distinctive imaging signs, including the inverted target sign, fascia tail sign, solar halo sign, and cloud sign. The fascia tail sign and cloud sign can aid in distinguishing nodular fasciitis from other soft tissue lesions.