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.