Nodular Fasciitis

A small, rapidly growing nodule in the forearm could be nodular fasciitis, which is a benign, self-limiting reactive proliferation of myofibroblasts and the classic pseudosarcoma, meaning a benign lesion that is frequently mistaken for a sarcoma because of its rapid growth and cellular, mitotically active histology. It typically presents as a solitary nodule, usually under three to four centimetres, sometimes tender, classically on the volar forearm, trunk or, in children, the head and neck. It matters because if it is misdiagnosed as a sarcoma the patient may be overtreated with wide excision or radiotherapy, which is unnecessary and harmful. To make the diagnosis I would not assume it is benign on clinical grounds alone, because a rapidly growing soft-tissue mass must be taken seriously; instead I would image it and obtain a biopsy, and the key is the pathology, which shows plump uniform spindle cells in a loose tissue-culture pattern with myxoid stroma, extravasated red cells and mitoses but no nuclear atypia, with myofibroblastic immunohistochemistry, and molecular testing demonstrating a USP6 rearrangement, most commonly the MYH9-USP6 fusion, which confirms the benign diagnosis. Once confirmed, because the lesion is self-limiting I would manage it conservatively, by observation allowing spontaneous resolution, or a simple marginal excision if symptomatic or diagnostically uncertain, and I would avoid aggressive sarcoma-type treatment, since recurrence is rare even after incomplete excision.

Nodular fasciitis is called a pseudosarcoma because, although it is entirely benign and self-limiting, it behaves and looks like a sarcoma in two ways: clinically it grows rapidly, which is an alarming feature usually associated with malignancy, and histologically it is cellular and mitotically active, so an unwary pathologist or clinician can mistake it for a sarcoma. The way to avoid overtreating it is to make an accurate pathological diagnosis before any aggressive treatment: the histology shows the characteristic tissue-culture spindle-cell pattern with myxoid stroma and mitoses but, crucially, no nuclear atypia and no atypical mitoses, and molecular testing demonstrating a USP6 rearrangement, typically the MYH9-USP6 fusion, confirms the benign nature. Once confirmed, management is conservative, with observation for spontaneous resolution or a simple marginal excision, and not wide excision or radiotherapy. At the same time I must not swing too far and assume every rapidly growing lump is benign, because that risks missing a genuine sarcoma; so I follow the principle that any soft-tissue mass that is large, over about five centimetres, deep to fascia, or enlarging is referred and worked up through a sarcoma pathway with appropriate imaging and biopsy, and the diagnosis of nodular fasciitis is confirmed on biopsy and USP6 testing rather than presumed. This balance, confirm before conservative management but never presume benign, is the safe approach.