Proliferative fasciitis and proliferative myositis (PF/PM) are rare benign soft tissue lesions (i.e. a damaged or unspecified abnormal change in a tissue) that increase in size over several weeks and often regress over the ensuing 1–3 months. The lesions in PF/PM are typically obvious tumors or swellings. Historically, many studies had grouped the two descriptive forms of PF/PM as similar disorders with the exception that proliferative fasciitis occurs in subcutaneous tissues while proliferative myositis occurs in muscle tissues. In 2020, the World Health Organization agreed with this view and defined these lesions as virtually identical disorders termed proliferative fasciitis/proliferative myositis or proliferative fasciitis and proliferative myositis. The Organization also classified them as one of the various forms of the fibroblastic and myofibroblastic tumors. PF/PM lesions have been regarded as a tissue's self-limiting reaction to an injury or unidentified insult rather than an abnormal growth of a clone of neoplastic cells, that is, as a group of cells which share a common ancestry, have similar abnormalities in the expression and/or content of their genetic material, and often grow in a continuous and unrestrained manner. However, a recent study has found a common genetic abnormality in some of the cells in most PF/FM tumors. This suggests that PF/PM are, in at least most cases, neoplastic but nonetheless self-limiting and/or spontaneously reversing disorders. That is, they are examples of "transient neoplasms." In all events, PF/PM lesions are benign tumor growths that do not metastasize. PF/PM lesions may grow at alarming rates, exhibit abnormal histopathologies (e.g. high numbers and overcrowding of cells), and have other elements that are suggestive of a malignancy. Consequently, they have been mistakenly diagnosed as undifferentiated pleomorphic sarcoma (also termed malignant fibrous histiocytoma), rhabdomyosarcoma, or other types of sarcoma and treated unnecessarily with aggressive measures used for such malignancies, e.g. wide surgical resection, radiation therapy, and chemotherapy. The majority of PF/PM lesions are successfully treated with strictly conservative and supportive measures.
Presentation PF/PM lesions occur primarily in middle-aged and older adults (peak age of onset 50 to 55 years/old) with no appreciable differences in their incidences between males and females. Only very rare cases have been reported in children and adolescents. In up to 20% to 30% of cases, these lesions are apparently preceded by some sort of mechanical injury. Individuals commonly present within 1–3 weeks or, rarely, longer times (e.g. 3 months) of noticing a rapidly growing, small (<5 cm. in size) mass or swelling in the subcutaneous tissues or muscles of an extremity or, less commonly, the trunk wall, head, or neck areas. Uncommonly, the lesions are ulcerated. In rare cases, the lesions are extensive and highly disruptive, e.g. PF/PM has presented with lockjaw, i.e. a reduced ability to open the jaw due to a PF/PM lesion infiltrating and disrupting the function of the muscles of mastication (i.e. jaw-opening muscles). PF/PM lesions may be associated with tenderness, pain, and/or very rarely fever of unknown cause. The lesions may be regressing at the time of diagnose or, in rare instances, spontaneously regress beginning immediately after being biopsied. Very rarely, these lesions have evolved rapidly, compromised local blood flow, and/or recurred at the site where they were removed by conservative local surgical excision. However, PF/MF lesions do not metastasize to distant tissues.
Pathology PF/PM lesions are poorly circumscribed masses which on histopathological microscopic analyses consist of bland fibroblastic and myofibroblastic spindle-shaped cells mixed with variable proportions of giant epithelioid ganglion cell-like cells. These cells are in a myxoid (i.e. a clear, mucus-like substance which when prepared using a standard H&E staining method appears more blue or purple than the red color of normal tissues) to fibrous (i.e. high collagen fiber content) background which may contain areas of necrosis (i.e. sites of dead cells). Overall, the cells in these lesions are amphophilic or basophilic, may have vacuole-laden cytoplasm, are slowly multiplying based on their proliferative index, and lack atypical mitosis figures that might be suggestive of a malignancy. The presence of at least some giant epithelioid ganglion-like cells within this histopathological background are necessary and definitive evidence that a swelling or tumor is a PF/PM. Compared to adult cases, pediatric cases of PF/PM lesions are often better delineated from normal tissue, are more cellar, have a greater frequency of necrosis sites, contain diffuse sheets of epitheliod ganglion-like cell cells but lack a spindle-shaped cell component, and are lobulated. The spindle-shaped cells, but not epithelioid ganglion-like cell cells, in proliferative myositis lesions express smooth muscle actin proteins.
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