Intramuscular myxoma (IMM) are benign soft tissue that account for 0.1 to 0.13 per 100,000 populations (1). Various theories have been described regarding the mechanism of IMM occurrence. Some researchers suggest that the reason is fibroblasts (unable to synthesize collagen fibers) that are not well differentiated from mesenchymal stem cells, which cause the synthesis of myxoid stroma without reticular fibers. Others consider the etiology of IMM to be caused by traumatic mechanisms or the growth of polysaccharide-producing cells in the neoplastic process (2). IMM is rare and can occur in the buttocks, thigh, upper extremities and shoulder muscles. Epidemiologically, its occurrence rate is higher in women (70%), increases with age (6th and 7th decade of life) and the most common sites of IMM is upper extremities muscles (50%-60%) (3). According to the location of the masses, soft-tissue myxomas are classified into superficial angiomyxoma, intramuscular myxoma, nerve sheath myxoma and aggressive angiomyxoma. From the clinical point of view, IMM is a palpable mass, painless, without inflammatory secretions and symptoms, which has no contractile properties and no stretch-contraction changes during flexion-extension of the adjacent muscles (4).

From a diagnostic point of view, IMM is observed as a non-calcified mass in plain radiograph, which is seen in the supplementary findings with the help of ultrasonography as echogenic cystic lesions among the muscle tissue. The most important diagnostic method of IMM from other soft tissue lesions is magnetic resonance imaging (MRI), which can be seen as hypointense homogeneous mass in T1-weighted sections and hyperintense in T2-weighted sections (5). In case of edema with IMM in MRI sections (T1-weighted sections), it should be differentiated from other fluid-containing lesions (such as cystic teratoma, hematoma, myxoid sarcoma, cystic hygroma and even normal lymph nodes). Also, IMM should be differentiated from proliferative lesions, other myxoid neoplasms, myxochondroma, myxochondroma and myxoid liposarcoma (6). Cytology-histopathology findings with the help of intraoperative frozen section and needle biopsy help the information of MRI sections in the diagnosis of IMM. Density and ratio of cells/ collagen fibers, mucoid material secretion, nodular-vesicular pattern and fat density in histopathological sections contribute to IMM (7). In the present case, forearm intramuscular myxoma was observed inter-supinator muscle.