[Schwannoma developing in the psoas major muscle: a case report].

Schwannoma can occur wherever peripheral nerve Schwann cells are found. However, retroperitoneal schwannoma is extremely rare in that only 16 cases have been reported, including that of the present patient. A 51-year-old male complained of chronic lower back pain and paresthesia in the lower left region. Lower back pain increased with leftward bending. Magnetic resonance imaging (MRI) revealed a neoplastic lesion about 6 cm in diameter in the left psoas major muscle, and the lesion was in contact with the L4 and L5 vertebral bodies. The tumor did not continue to the lumbar foramen. The symptom had become disabling, therefore tumor removal was performed using Wiltse's approach. From intraoperative findings, the origin of the tumor was suspected to be the L4 nerve root. The ventral surface of the tumor could not be observed with this posterior approach. Lumbar plexus might adhere closely to the ventral surface of the tumor, therefore the capsule of the tumor was left to avoid neurological deterioration. Schwannoma was diagnosed by pathological analysis, and no malignancy was evident. After the operation, symptoms were completely relieved, and the residual capsule of the tumor diminished in size on MRI after 12 months. The posterior Wiltse's approach can be useful to treat mass lesions in the psoas major muscle. The 16 reported cases of retroperitoneal schwannoma, including that of the present patient, are reviewed.