Schwannoma, also termed neurilemmoma or neurinoma, is a benign nerve sheath tumour consisting of two components. Antoni A areas are more organised and are hypercellular, and are composed of spindle cells arranged in short bundles or interlacing fascicles. Antoni B regions are hypocellular, less organised and contain more myxoid, loosely arranged tissue, with a high water content. These components are intermixed within schwannomas and occur in varying amounts [4
]. Schwannoma is a slow-growing benign tumour and may be a large tumour with degeneration, especially when the tumour is situated in the deep regions such as the mediastinum and retroperitoneum [5
]. Schwannoma with pronounced degenerative changes is known as ancient schwannoma, which is a rare variant of schwannoma and is usually a deeply situated large mass of long duration [5
], representing 0.8% of all soft-tissue tumours [7
]. The term “ancient neurilemmoma” was first suggested by Ackerman and Taylor [8
] in a review of 48 neurogenic tumours of the thorax. The cited authors reported 10 patients with tumours showing features similar to those of typical neurilemmomas, but differing in that significant tumour portions contained only a few cells within hyalinised matrices. They found that these features occurred in schwannomas of long duration, and hence coined the term “ancient schwannoma”. Such tumours are characterised by diffuse hypocellular areas, relative loss of Antoni type A tissue, focal accumulations of hyaline material, calcification, cystic necrosis, haemorrhage and fatty degeneration [2
]. In addition, as the tumours are usually infiltrated by large numbers of siderophages and histiocytes, and display cellular degenerative changes, including nuclear atypia and pleomorphism, along with a tendency to nuclear palisading [5
], malignancy may be erroneously diagnosed [8
]. On histological tumour examination, intense immunostaining for S100 protein suggests a neural origin and is helpful in diagnosis, especially of a totally cystic degenerated mass [5
Because the tumour contains cystic areas, ancient schwannoma has been radiologically misdiagnosed as other tumour types, such as malignant fibrous histiocytoma, malignant peripheral nerve sheath tumour, liposarcoma, synovial sarcoma or haemangiopericytoma. However, only a few reports have dealt with the radiological features of ancient schwannoma in the thigh because the tumour is so rarely encountered [2
]. On MRI, the peripheries of ordinary schwannomas yield low-signal intensities on T1
weighted images and high-signal intensities on T2
weighted images, corresponding to the Antoni B region, and low-to-intermediate signal intensities in other areas on T1
weighted images with strong enhancement by gadolinium contrast media, a characteristic of the Antoni A regions [2
]. The ancient schwannoma, however, shows a well-circumscribed inhomogeneous signal intensity and a different enhancement pattern. The relative amounts of Antoni A and B areas vary, and these regions are randomly blended, reflecting such MRI findings [6
]. Isobe and colleagues [2
] reported imaging findings of seven ancient schwannomas situated mainly in the extremities as well-circumscribed complex cystic masses. The histological findings showed that the Antoni type B area occupied most of the tumours; the Antoni type A area had become smaller and was visible in the area around the degenerative lesions. The cited authors suspected that Antoni type A areas had degenerated to haematomas or cysts. In our cases, haemorrhagic changes were demonstrated on MRI. The first case showed a well-defined cystic mass with enhancement of the fibrous capsule on MRI, and histology showed an Antoni type B area in most of the tumours. The second case revealed a well-defined inhomogeneous signal intensity solid mass with random cystic areas.
The target sign of a peripherally hyperintense rim and central low intensity on T2
weighted imaging has been reported in peripheral nerve sheath tumours [4
]. This MRI finding corresponds histologically to peripheral myxomatous tissue and central fibrocollagenous tissue [4
] and is absent in lesions with cystic, haemorrhagic or necrotic degeneration. In our cases, the target sign was not seen.
The fibrous capsule, consisting of epineurium and residual nerve fibres, appears as a hypointense rim on T1
weighted images and shows contrast enhancement on post-contrast T1
weighted images [6
]. According to the findings of Isobe and colleagues [2
], the circumferences of the degenerated area and fibrous tumour capsule enhanced on MRI are important for discriminating an ancient schwannoma from other tumours, even though there are few reports of the radiological features of retroperitoneal ancient schwannomas [12
]. Our cases showed hypointense fibrous capsules and enhancements.
A rim of fat (split fat sign) is often present around deep-seated neurogenic neoplasms and has been previously described on CT scans [4
]. Because each neurovascular bundle is surrounded by fat, masses arising in such bundles maintain a rim of fat as they slowly enlarge. Although not a specific sign for peripheral nerve sheath tumours, such a finding suggests that the tumour originated in the intermuscular space around the neurovascular bundle; neurogenic neoplasms are the most common such tumours. The split fat sign is more common in benign nerve sheath tumours and lesions of large nerves, whereas malignant nerve sheath tumours less frequently demonstrate complete fat rims, reflecting the more infiltrative growth patterns of such tumours [4
]. Both our cases showed a split fat sign, which was considered as a characteristic finding reflecting benign neurogenic tumours.
A pre-operative differential diagnosis of a comparatively large soft-tissue tumour in a thigh, appearing well enhanced and with inhomogeneous signal intensity on MRI, tends to identify the tumour as malignant. Differentiation of ancient schwannoma from a malignant tumour can be achieved by considering the characteristic clinical findings and radiological features of ancient schwannoma, as described by Isobe and colleagues [2
]. First, there should be a mass with a long-term clinical course presenting with symptoms of a neurogenic tumour. Second, the mass should be palpable with local pain and characteristics similar to Tinel's sign, and should have distinct radiological features including a circumscribed degenerative area and an enhanced fibrous tumour capsule. However, one of our patients had a tumour history of only 1 year, and neither case showed tenderness. If the mass is deep in an extremity, the patient tends to ignore it until it finally reaches an excessive size, but a long-term clinical history is not essential for ancient schwannoma diagnosis. In our patients, the most characteristic MRI findings were a well-defined mass with a fibrous capsule, showing the split fat sign.
In conclusion, two cases of ancient schwannoma in the thigh are described. Two MRI features are important for differentiating ancient schwannoma from other malignant tumours: the mass should have a fibrous capsule and the split fat sign should be evident.