Intramuscular granular cell tumor of the forearm: a rare case and diagnostic challenge.

Granular cell tumors (GCT) are rare neoplasms of neuroectodermal origin that can arise at various anatomical sites. Intramuscular GCTs in the extremities, particularly the upper limbs, are uncommon. MRI may suggest a GCT; however, variable signal intensity patterns complicate the diagnosis, necessitating histopathological confirmation. A 55-year-old woman presented with a gradually enlarging painless mass on the ulnar side of the left forearm. MRI revealed a poorly defined intramuscular lesion within the flexor carpi ulnaris, showing peripheral high signal intensity on T2-weighted and fat-suppressed images, with rim-like enhancement. Gadolinium-enhanced images revealed internal linear structures, interpreted as the stripe sign, within the tumor. Differential diagnoses included undifferentiated pleomorphic sarcoma, myxofibrosarcoma, inflammatory myositis, and granular cell tumors. Needle biopsy suggested the presence of a GCT or inflammatory lesion. Surgical resection revealed large epithelioid-to-polygonal cells with eosinophilic granular cytoplasm, perineural invasion, and lymphocytic infiltration. Immunohistochemical analysis showed positivity for S-100, SOX10, Inhibin, NSE, CD68, and TFE3. The Ki-67 index was 3%, supporting the diagnosis of benign GCT. Although the surgical margins were positive, no recurrence was observed at the two-year follow-up. Peripheral T2 hyperintensity and the stripe sign have been proposed as MRI findings of GCT. However, peripheral T2 hyperintensity is considered to have low specificity, whereas the stripe sign may be more specific to GCT. Although histopathological examination remains essential for the definitive diagnosis, further radiologic evaluation of the utility of the stripe sign is warranted.