Benign schwannoma is a tumor arising from Schwann cells (forming the neural sheath of peripheral nerves). The retroperitoneal location is unusual (0.5–5% of cases). The most common locations are cranial nerves (especially the vestibular nerve) and, in the peripheral nervous system the neck, mediastinum, and extremities [1].

Primary multiple obturator nerve schwannomas originate from Schwann cells and are extremely rare. Patients with schwannomas are asymptomatic and a retroperitoneal schwannoma is often misdiagnosed as an adnexal mass [2]. An association between obturator nerve injury and iliopsoas muscle is also poorly reported.

Pelvic computed tomography (CT) or magnetic resonance (MR) imaging helps to limit diagnostic hypotheses. The resection of this tumor is the appropriate treatment, even though it is really a complex one. Prognosis is quite good since post-surgical recurrences are unusual. Complete resection is the best treatment for retroperitoneal pelvic schwannoma and nowadays it can be performed also by laparoscopy. Partial resection can be used when the mass is strongly connected to essential organs in order to prevent iatrogenic harms (neural deficit, vessel lesions); this may occur in 10% of cases [1].

Differential diagnosis must always be considered; in view of this, biopsy and histopathological report are necessary [3]. We report the case of a patient with two schwannomas in the pelvic region and neuromuscular involvement. Clinical control was not efficient in managing neuropathic pain and in slowing muscle weakness in the iliopsoas muscle.

A 51-year-old female patient was under investigation for lumbosacral plexus injuries. The clinical picture was marked by pain in the topography of the iliopsoas muscle on the left and involvement of the obturator nerve (mild paresis in the adduction of the thigh and allodynia in the anterolateral region) in the same limb. A PET-CT study was performed to assess the lesions. Despite the possibility of false positive result, the tomographic images of the whole body were acquired in a hybrid PET-CT device with a 16-channel multislice CT, after the administration of venous contrast.

Metabolic findings: The fusion of metabolic images (PET) to anatomical images (CT) showed increased contrast enhancement in elongated and nodular lesions, with soft-tissue attenuation: two lesions located laterally to the left iliopsoas muscle, the largest measuring 10 × 8 mm; medially to the left iliopsoas muscle, measuring 24 × 23 mm (Figure 1A and Figure 1B). It presents heterogeneous enhancement by the iodinated contrast medium. There was also a small lesion in the intraperitoneal fat on the left flank, in the plane of the iliac crest, measuring about 20 × 12 mm (Figure 1C).

Impression: Glycolytic hypermetabolism on elongated and nodular images, most adjacent to the left iliopsoas muscle, that may correspond to neurofibromas or schwannomas.

She was treated the use of duloxetine 60 mg and gabapentin 400 mg each 8 hours. Occasionally, it is necessary to use anti-inflammatory drugs due to the clinical condition of “psoitis” and, consequently, difficulties in performing flexion of the thigh over the hip. She also has neuropathic pain due to the involvement of the obturator nerve.

Schwannomas are usually common benign neoplastic lesions that occur due to the proliferation of Schwann cells in the cranial nerve sheath, spinal roots, and peripheral nerves. Pelvic location in the region of the obturator nerve and iliopsoas muscle is uncommon [4]. We present the case of a patient with associated findings of peripheral nerve and muscle involvement and, unfortunately, refractory to conservative treatment.

Differential diagnosis includes fibromatosis, spindle cell sarcomas, and eventually ependymoma. However, the histological picture combined with the immunohistochemical study is essential for diagnostic elucidation. Immunohistochemical assay of our patient confirmed the schwannomas to be positive for Sox10 [3],[4].

Reports in the literature have presented the feasibility of a minimally invasive resection of retroperitoneal or pelvic schwannomas. However, there are only a few reports in the literature about a robot-assisted nerve-sparing approach toward obturator schwannomas [5]. The patient in this case was instructed to perform this type of surgical intervention, although we have other techniques that are no less satisfactory.

The presence of schwannoma is an uncommon retroperitoneal pelvic tumor, and it can be misdiagnosed as an adnexal mass. To our knowledge, this is a rare case of two schwannomas arising from the obturator nerve and iliopsoas muscle. It usually does not have any neurological deficit, unlike the present clinical case (marked by difficulties in flexing the thigh due to injury and adduction of the hip).