Intramuscular Hybrid Nerve Sheath Tumor of the Thigh: Case Report and Literature Review

Background/Aim: Hybrid nerve sheath tumor (HNST) is a benign peripheral nerve sheath tumor with combined features of more than one histological type, such as schwannoma, neurofibroma, and perineurioma. It remains under -recognized in routine clinical practice. Herein, we describe an unusual case of intramuscular HNST of the thigh. Case Report: The patient was a 41 -year -old man with no history of trauma who presented with a 3 -month history of a palpable mass in the right thigh. Physical examination revealed a 4 -cm, elastic hard, mobile, nontender mass. Magnetic resonance imaging exhibited a well -circumscribed intramuscular mass with low -to -intermediate signal intensity on T1 -weighted sequences and higher signal intensity peripherally and lower signal intensity centrally, representing a target sign, on T2 -weighted sequences. Complete surgical excision of the tumor was carried out. Microscopically, the tumor showed dual histological components of both schwannoma and neurofibroma. Immunohistochemically, the schwannomatous component was strongly and diffusely positive for S-100 protein and negative for CD34, while the neurofibromatous component contained CD34-positive fibroblasts and S-100 protein -positive Schwann cells. Epithelial membrane antigen was negative for both components. These findings were consistent with a diagnosis of HNST (hybrid schwannoma/neurofibroma). The patient had no evidence of local recurrence and no neurological deficit at the final follow-up. Conclusion: Although extremely rare, HNST should be included in the extended differential diagnosis of a well -circumscribed, intramuscular soft -tissue mass in the extremities, particularly in young and early middle-aged adults.