Focal Myositis with CD8+T-Cell predominance: an Inflammatory Myositis Mimicking a Soft Tissue Neoplasm

Abstract Introduction/Objective FM is a rare self-limiting T-cell rich lesion arising within muscles of young adults as a solitary lesion and can be confused with a variety of neoplastic/inflammatory conditions or lymphoma. Here we describe a rare T-cell rich variant of FM with CD8 predominance. Methods/Case Report A 51-year-old female presented with two-month history of left trapezius swelling. MRI showed an enhancing tumor within the muscle, suspicious of sarcoma, less likely myeloma. Results (if a Case Study enter NA) Biopsy showed diffuse infiltration of small lymphocytes in a fibrotic background and atrophic skeletal muscle. These lymphocytes stained positive for CD2, CD3, CD7, CD8, CD5 (partial), TIA1 (partial), CD43, CD57 and negative for CD4, CD30, CD56, CD57, granzyme B, perforin, BCL-6, BCL-2, Pax5, TCL1, and CD56. Ki-67 was (<10%) with few background CD20+ B cells. By flow cytometry, the CD8+ T-cells co-express CD2, CD3, CD7, with dim CD5 expression. TCR gene rearrangement study by PCR showed no clonal TCR Gamma gene rearrangement. Conclusion FM is extremely rare with only 22 cases well-described in the literature, all predominantly composed of CD4+ T-cells, clinically concerning for low-grade sarcomas, as inflammatory myofibroblastic tumor, inflammatory leiomyosarcoma, or liposarcoma. Careful analysis is essential for correct diagnosis. Mimics include other causes of myositis, such as polymyositis and inclusion body myositis. Depending on the extent of lymphocytic infiltrate, gene rearrangement studies might be necessary to rule out clonality. Recognition of this rare entity with excellent prognosis is crucial to provide appropriate management and avoid unnecessary, aggressive procedures.