Proliferative fasciitis and proliferative myositis

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.