Nodular fasciitis (NF) is a benign, soft tissue tumor composed of myofibroblasts (i.e. immature cells that contain features of myocytes and fibroblasts) that typically occurs in subcutaneous tissue (i.e. lowermost layer of the skin), fascia (i.e. band of connective tissue just beneath the skin), and/or muscles. The literature sometimes titles rare NF variants according to their tissue locations. The most frequently used and important of these are: cranial fasciitis (NF variants that occur in the soft and hard tissues of the skull) and intravascular fasciitis (NF variants that involve arteries and/or veins). In 2020, the World Health Organization classified nodular fasciitis as in the category of benign fibroblastic/myofibroblastic tumors. NF is the most common of the benign fibroblastic proliferative tumors of soft tissue and exceeds in frequency any other tumor or tumor-like lesion in this group of tumors.

Nodular fasciitis is a rapidly growing, usually self-limiting neoplasm that occurs primarily but not exclusively in adults. Due to its rapid growth, NF has often been misdiagnosed as a malignant tumor, usually a sarcoma. Indeed, NF was originally termed subcutaneous pseudosarcomatous fibromatosis when first described in 1955 by Konwaler et al. The correct diagnosis of a tumor as NF is pivotal to prevent its overtreatment as a more aggressive and/or malignant growth.

NF tumors have long been mentioned as local reactions to traumatic injuries in at least some cases. While it still may be precipitated by such injuries, recent studies indicate that NFs are true neoplasms (i.e. abnormal proliferations of cells even after any precipitating event is removed). This is because up to 92% of NF cases have a specific type of fusion gene in their tumor cells. Fusion genes are abnormal and potentially tumor-inducing genes formed by mergers between parts of two different genes. The fusion gene involved in NF merges part of the USP6 gene to a part of any one of numerous other genes. The USP6 gene is responsible for producing ubiquitin carboxyl-terminal hydrolase 6a, a deubiquitinating enzyme that indirectly regulates various signaling pathways which regulate cell growth and death. All of the USP6-containing fusion genes overproduce a chimeric protein (i.e. an abnormal protein consisting of parts derived from different genes) with uncontrolled deubiquitinating enzyme activity that is thought to contribute to the development of NF.