A 44 y.o. woman presented with a soft elastic mass in the distal antero-lateral region of the leg, near to peroneal distal metaphisis; the mass had been slowly but steadily enlarging in the last 12 months. The Patients referred neither history of trauma nor pain at clinical examination
A 44 y.o. woman presented with a soft but consisting mass in the antero-lateral distal region of the leg. Radiography showed the presence of a soft-tissue mass. The MRI study was performed using an MR scanner (1,5T). The examination consisted of T1-weighted spin-echo (SE), PD-weighted turbo-spin-echo (TSE), T2-weighted gradient echo (GE) and GE-STIR sequences, performed in the axial, coronal and sagittal planes. On T1-weighted the mass was homogeneous and hyperintense relative to skeletal muscle; on T2-weighted MR images revealed the lesion to be hyperintense and mildly heterogeneous. Histological examination described that the tumor consisted of cellular Antoni A regions with wavy nuclei in bands, separated by eosinophilic cytoplasm and hypocellular Antoni B areas less organized, with vacuolar degeneration.
Schwannoma is a solitary benign peripheral nerve sheath tumor, slightly less common than neurofibroma, although it constitutes about 5% of all benign soft-tissue tumors. Most frequently it affects patients aged 20-30 years, and men and women equally. Another characteristic of schwannoma is that it has a true capsule composed of epineurium. In fact, the growth of schwannoma within the epineurium creates encapsulation, which moreover allows successful resection. On the contrary neurofibroma, which arises from the nerve fascicle, is rarely encapsulated, and cannot be separated from the involved nerve. Furthermore, when schwannoma affects large nerves, the mass is characteristically eccentric related to the affected nerve, with the nerve displaced to the periphery of the mass. This feature is usually not seen in small nerves: similar to neurofibromas, schwannomas of small nerves may obliterate the nerve of origin. In fact, neurofibroma is normally centrally located, but it can also obliterate the nerve. Moreover, neurofibroma can contain the spinal and sympathetic nerve roots of the head and neck, as well as nerves in the flexor surfaces of the upper and lower extremities (particularly ulnar and peroneal nerves). The histologic hallmark of schwannoma is represented by the presence of Antoni A and Antoni B regions. While Antoni A areas are more organized and composed of cellular spindle cells arranged in short bundles or interlacing fascicles, Antoni B regions are hypocellular, less organized, and contain more myxoid loosely arranged tissue with high water content. Furthermore, lesions are also S-100 protein positive at immunohistochemical analysis. At pathologic analysis, schwannomas are fusiform, representing the mass with the entering and exiting nerve. Lesions are regularly freely mobile to palpation except at the point of nerve attachment. In neoplasms of large nerves, this restrict movement is found often along the long axis of the affected nerve trunks. Schwannoma is usually solitary, and it has non aggressive features including slow growth and small size (<5cm). Moreover, pain and neurologic simptoms are unusual except in large tumors. The treatment of schwannoma is normally the surgical excision. The affected nerve is usually separable from the tumor intraoperatively after the incision of the epineurium, permitting the preservation of the native nerve and its function. Partial resection of the neoplasm may be performed when otherwise the complete removal would imply also the nerve resection. Recurrence is unusual, even after incomplete resection, and malignant transformation is extremely rare. The MR imaging appearance of schwannoma is an isointense signal relative to skeletal muscle on T1-weighted images, and an increased slightly heterogeneous signal intensity on T2-weighted images. Unfortunately, these patterns of signal intensity are not specific. Nevertheless, distinctive features that suggest a peripheral nerve sheath tumor include the presence of certain signs: split fat sign, fascicular sign, target sign. The target sign is characterised by low signal intensity centrally and high signal intensity peripherally, whereas the fascicular sign describe the presence of the fascicular bundles in neurogenic tumors and normal nerves. These are signs typically seen in benign lesions.
Schwannoma of the peroneal nerve
Ultrasound and MRI images indicate that the lesion is situated in the distal lower leg, near the distal metaphysis of the fibula in the anterolateral region, appearing as a nearly round or elliptical soft tissue mass. Based on the ultrasound findings, the lesion has well-defined margins and homogeneous or slightly heterogeneous internal echoes, presenting as a hypoechoic or slightly decreased echogenic area. On MRI, the lesion shows signal intensity similar to muscle (isointense or slightly hypointense) on T1-weighted images and high signal intensity on T2-weighted and fat-suppressed sequences, with clearly visible margins. It is well-demarcated from the surrounding soft tissues, displaying a rounded shape; in some sequences, mild internal signal heterogeneity can be observed. No obvious adhesion to surrounding muscle or fascial structures is noted, and in certain slices, the lesion is seen eccentrically adjacent to the course of the nerve. Overall, its morphology, signal characteristics, and location strongly suggest that it is a benign lesion originating from the nerve sheath or perineural region.
Considering the patient’s age (44 years), clinical presentation (slow growth, with no significant pain or other neurological symptoms), and imaging findings (T1 is isointense or slightly low signal, T2 is high signal, well-defined capsule, eccentric distribution, etc.), the following potential diagnoses can be taken into account:
Integrating the patient’s age, a clinically stable course (gradual enlargement but no significant pain or functional impairment), and imaging characteristics (well-defined margins, eccentric growth along the nerve, high signal on T2 indicating a benign pattern), the most likely diagnosis is Schwannoma. However, to further confirm the histological properties, intraoperative or preoperative biopsy could be performed, with pathology and immunohistochemistry (e.g., S-100 protein) used to finalize the diagnosis.
Treatment Strategy:
Postoperative or Conservative Rehabilitation Plan:
After surgery, develop a physical therapy program based on the local healing status. If surgery is not performed, maintain light functional training to avoid excessive local stress or trauma.
Example of an Exercise Prescription (FITT-VP principle):
Throughout rehabilitation, carefully watch for any unusual pain or neurological symptoms, including numbness, tingling, or muscle weakness. Should significant discomfort or abnormal findings appear, seek prompt evaluation.
Disclaimer:
This report provides a reference analysis based solely on the provided medical history and imaging data and cannot replace an in-person examination or professional medical advice. Patients should make final treatment decisions in consultation with their specialist physician, considering their actual conditions.
Schwannoma of the peroneal nerve
