3. DISCUSSION
IMM is a rare benign soft tissue tumor that probably occurs due to incomplete differentiation of mesenchymal cells into fibroblasts. Of course, IMM can be a part of McCune-Albright syndrome and Mazabraud’s syndrome, which in this case is caused by mutations in the GNAS gene (8). For the first time, the term myxoma was applied by Virchow to a soft tissue tumor of mesenchymal cell origin in 1863. Also, Stout in 1948 described the histopathological criteria for the diagnosis of myxoma and its differentiation from other lesions (9). Capsular mass with hypocellular tissue in a mucoid background with scattered reticular fibers in the skeletal muscles, which, if not diagnosed/treated in time, can cause movement limitation in the joint, interfere with the action of the involved muscle, and compress the nerves in the area. IMM has a good prognosis and usually the involved muscle/joint follows its anatomical function following supportive measures-physiotherapy (3).
This lesion is seen in T1-weighted images as an ovoid-shaped mass with low signal intensity and in T2-weighted images as high signal intensity. In the present case, a hyperintense egg-shaped mass was observed in T2-weighted sections inside the supinator muscle (10). Adamonis et al. (2019) also reported the presence of IMM in the deltoid muscle in a 34-year-old man, whose tumor lesion was observed in T2-weighted images with hyperintense appearance. In histopathological evaluation, after observing spindle-stellate fibroblast cells in mucoid background with scattered collagen fibers of deltoid IMM was confirmed (5). In the present case, the mass has a thin capsule around the homogenous hypocellular mucoid tissue, in which the spindle-shaped cells (fibroblasts) are located in scattered collagen fibers. The reports of the upper extremity IMM cases show that the lesions in the muscles are removed subcutaneously after surgery and cause few complications (11). Movement limitations caused by IMM are also quickly resolved with physiotherapy if the nerves of the brachial plexus are not compressed.