• Aaron Auerbach, Julie C Fanburg-Smith, Guanghua Wang, and Elisabeth J Rushing.
• Department of Hematopathology, Armed Forces Institute of Pathology, Washington, DC, USA. auerbacha@afip.osd.mil
• Am. J. Surg. Pathol. 2009 Jul 1;33(7):1016-24.

BackgroundFocal myositis is an uncommon inflammatory pseudotumor of skeletal muscle that can be confused with a variety of neoplastic and inflammatory diseases. It is often misunderstood because it presents as a tumor-like mass, but histologically resembles a skeletal muscle myopathy or dystrophy. We wanted to discuss the detailed morphologic and immunophenotypic features of the largest reported group of focal myositis patients.DesignTwo hundred and six cases coded as "focal myositis" were culled from our files. Only 115 cases with adequate material, a solitary lesion, and correct diagnosis were included. A variety of immunohistochemical studies were performed, as were polymerase chain reaction for T cell receptor gene rearrangement and immunoglobulin heavy chain rearrangement.ResultsAge ranged from 7 to 94 years (mean 41, median 36 y). Most patients were otherwise healthy, and with the exception of 10 cases, lacked antecedent trauma. Masses that ranged in size from 1.0 to 20.0 cm (median 3.0 cm, mean 3.9 cm) were reported in specific muscles of the lower extremities (including vastus lateralis, adductor muscle, and groin muscles, n=39; gastrocnemius, n=22), followed by the trunk, neck (mentalis, n=8; sternocleidomastoid muscle, n=8), and upper extremity. Histologically, these were solitary intramuscular processes composed of variable myopathic (93%) and focal neurogenic (89%) changes, fibrosis, and inflammation (97%), occasionally accompanied by prominent eosinophils (n=20). By immunohistochemistry, most cases had CD163-positive macrophages that were negative for S100 protein and CD1a. Lymphocytes were mostly CD3, CD4-positive lymphocytes that were negative for cytotoxic markers, TIA-1 and granzyme-B. Polymerase chain reaction did not show B cell or T cell rearrangement. In situ studies for Epstein-Barr-encoded receptor were negative, as was ALK-1 immunohistochemistry. Major histocompatibility complex-1 and weak IgG4 were focally positive in skeletal muscle. Cases with severe inflammation had increased numbers of CD20-positive B cells and CD123-positive plasmacytic dendritic cells. S100 was strongest in skeletal muscle fibers with vacuolar change. Clinical diagnostic considerations ranged from benign entities such as rhabdomyoma, intramuscular lipoma, fibromatosis, myositis ossificans, proliferative myositis, inflammatory myofibroblastic tumor, and inflammatory myopathy to malignant entities such as rhabdomyosarcoma, leiomyosarcoma, liposarcoma, and lymphoma. Available follow-up revealed spontaneous regression.ConclusionsFocal myositis occurs in specific muscle groups of young adults of both sexes without significant trauma. It is a largely unrecognized entity with specific histology including myopathic, focal neurogenic, fibrosis, and inflammatory features. It can be easily mistaken for an inflammatory myopathy, dystrophy, alternate reactive, or even neoplastic process. Focal myositis seems to be a macrophage and T-cell-rich lesion that changes to B cell and dendritic plasmacytoid cells when markedly inflamed, but does not seem to have a known viral or molecular etiology. IgG4 presence may be linked to the fibrosis in these lesions; a possible transient autoimmune etiology cannot be excluded. Careful attention to reproducible clinicopathologic features can aid diagnosis and spare patients from excessive surgery or adverse therapy.

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