Physicians are often called upon to do excisional biopsies in the diagnosis of subcutaneous tumors. Benign fibrous tumors represent a group of clinical entities that are often difficult to diagnose. NF is one such benign fibroblastic proliferation whose rapid growth and rich cellularity frequently cause the lesion to be misdiagnosed as sarcoma.

NF was first described by Konwaler in 1955 and was termed pseudosarcomatous fibromatosis. Other terms, such as pseudosarcomatous fasciitis, infiltrative fasciitis, and proliferative fasciitis, have also been used synonymously.

The incidence of nodular fasciitis is unknown. It is possible that the lesion's true incidence has been obscured by prior misdiagnosis as malignancy.

Nodular fasciitis is most commonly seen in young adults between 30 and 40 years of age. Approximately 10 percent of the lesions occur in children. Men and women appear equally affected, although in childhood the lesions may occur predominantly in boys. Most patients with this reactive proliferation have a rapidly growing mass. In several reports, nearly half the patients had noted the growth for less than a month. More than one third of patients report pain or tenderness associated with the lesion.

Diagnosis of NF requires histologic confirmation, and both diagnosis and treatment are accomplished by excisional biopsy.

Differential diagnosis:                
A.  Benign tumours:
1.Benign fibrous histiocytoma- Classical -Epidermal hyperplasia, peripheral collagen bundles, foamy macrophages and Touton giant cells . Cellular variant- Fascicular spindle cell architecture.
2.Neurofibroma- Architecture is different, S100 protein is positive
3.Spindle cell lipoma - Fat, ropy collagen, absence of markers
4.Fibromatosis- More infiltrative growth pattern, slender spindle shaped fibroblasts arranged in sweeping fascicles and separated by abundant intercellular collagen.

B. Malignant  tumours:
1. Leiomyosarcoma- The cells in fasciitis are tapered and the nuclei are tapered rather than blunt ended.  Atypical mitotic figures are prominent.Immunohistochemistry reveals h-caldesmon and desmin positivity.
2. Low grade myofibrosarcoma (myofibroblastic sarcoma) shows focal nuclear atypia,less inflammation,  more uniformly cellular, reaches a larger size and infiltrates muscle.
3. Inflammatory myofibroblastic tumour has fasciitis-like,fascicular and fibrous areas and a marked plasma cell infiltrate. Immunohistochemistry reveals that some cases are cytokeratin and ALK-1 positive.
4. Myxoid malignant fibrous histiocytoma is multinodular ,has vacuolated fibroblasts and shows nuclear pleomorphism, abnormal mitosis, distinct vascular pattern and is usually actin negative (some are CD34 positive).
5. Malignant peripheral nerve sheath tumour has alternating cellular and myxoid fascicles, is more uniform and has wavy buckled and bullet shaped nuclei. Better differentiated case are at least focally
S100 protein positive and myoid markers are negative.

The following features rule out malignant tumour:
1. Absence of atypia  2. Absence of atypical mitotic figures  3.Small size  4.Short history  5.Superficial location in young adults.

Variants:
1. Ossifying fasciitis: Nodular fasciitis like fibroblastic proliferations with metaplastic bone formation.
2. Intravascular fasciitis: Involve small or medium-sized veins or arteries. Histologically the features are similar to nodular fasciitis, however there are  greater number of multinucleate giant cells and less prominent mucoid matrix.
3. Cranial fasciitis: The lesion involves the soft tissue of the scalp land is usually present in infants. Histologically this is well circumscribed lesion showing NF like fibroblastic proliferation in a prominent myxoid stroma.

  These lesions rarely recur, do not develop metastases, and are readily cured by local excision. Spontaneous regression of incompletely excised lesions of NF has also been reported. Once the diagnosis is made by excisional biopsy, no further treatment appears necessary.