Nodular Fasciitis

Nodular fasciitis, formerly known as pseudosarcomatous fasciitis, is a mass-forming, rapidly growing proliferation of fibroblasts and myofibroblasts. It is most commonly found in the subcutaneous tissue, but can also be intramuscular, fascial, or even dermal (rare) locations. In about 50% of cases, it is found in the upper extremity (especially the volar aspect of the forearm). Other locations include the trunk, head and neck, and lower extremities.

Histologically, nodular fasciitis may be mistaken for a sarcoma because of abundant spindle-shaped cells and mitotic activity. Unlike sarcomas, however, atypical mitotic figures should not be seen. Intravascular fasciitis and cranial fasciitis are histologically similar lesions. Intravascular fasciitis extends into the lumens of small to medium-sized vessels, most commonly veins, and is also usually subcutaneous in location. Cranial fasciitis characteristically involves the outer table of the skull and adjacent soft tissues of the scalp. Intracranial extension to the meninges can also occur.

Some have suggested a traumatic etiology, especially in the case of cranial fasciitis (birth trauma). Patients are most commonly between 20 and 40 years of age and usually present with a rapidly growing mass, usually smaller than 5 cm. Tenderness and pain may be present.

The signal characteristics on MRI depend on the histology of the lesion. Hypercellular lesions (case shown above) are nearly isointense to skeletal muscle on T1-weighted images and hyperintense to fat on T2-weighted images. Collagenous lesions are hypointense on T1- and T2-weighted images. Post-contrast images usually reveal diffuse enhancement, but peripheral enhancement can also be seen.

Differential considerations include:

• Extra-abdominal desmoid tumor:
• Neurofibroma:
• Soft-tissue sarcoma:
• Early myositis ossificans: Intramuscular lesions.

References

• Dinauer PA, Brixey CJ, Moncur JT, Fanburg-Smith JC, Murphey MD. Pathologic and MR imaging features of benign fibrous soft-tissue tumors in adults. Radiographics. 2007 Jan-Feb;27(1):173-87.
• Evans HL and Bridge JA. Nodular fasciitis. in Pathology and Genetics of Tumours of Soft Tissue and Bone. Fletcher CDM, Unni KK, Mertens F (eds). IARCPress Lyon, 2002. pp 48-49.