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1 All authors: Department of Orthopedic Surgery, Shinshu University, School of Medicine, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan.
Received November 3, 2003;
accepted after revision December 9, 2003.
Address correspondence to K. Isobe
(bechikun{at}hsp.md.shinshu-u.ac.jp).
Abstract
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MATERIALS AND METHODS. We present the clinical, radiologic, and pathologic features in seven patients with ancient schwannoma (mean age, 62 years; range, 45–80 years) treated at our department between 1998 and 2003.
RESULTS. The most characteristic clinical features were a sign like Tinel's sign and a long interval between the onset of symptoms and surgery (mean interval, 8.3 years). Ancient schwannomas can grow large; the biggest tumor seen in our study was 14 cm long. The highly accurate radiologic assessment made possible with contrast-enhanced MRI and CT scanning showed enhancement at a peridegenerative area and sometimes at a capsule. These findings differ from those of the typical schwannoma and neurofibroma patterns reported to date. Furthermore, bone scintigraphy showed uptake in the tumor, but no accumulation was seen on gallium-67 citrate scintigraphy.
CONCLUSION. The characteristic clinical and radiologic findings of ancient schwannoma should make it possible to differentiate it from malignant tumors.
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Sonography, radiography, CT, MRI, bone scintigraphy, and gallium citrate scintigraphy were all used for the examination. A 1.5-T superconductive MRI unit (Signa, GE Healthcare) was used to produce spin-echo images. T1-weighted MRI with TR/TE (ranges, 500–716/12–17) and T2-weighted imaging (ranges, 3,000–4,000/81.3–100) using a body coil and field of view of 30–40 cm depending on body size were performed for all patients. Gadolinium-enhanced scans at a dosage rate of 0.2 mL/kg were obtained for all patients, and all underwent sonography, radiography, CT, and MRI. Six patients underwent bone scintigraphy, five underwent 67Ga citrate scintigraphy, and four underwent contrast-enhanced CT. Images from all patients with ancient schwannoma who underwent radiologic examinations were interpreted independently by three authors who reached agreement by consensus. After the radiologic examinations, the tumors were surgically treated and evaluated for pathology. The radiologic features of the tumors were then assessed and compared with the MRI findings for the pathologic specimens.
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Radiologic Features of Ancient Schwannoma
Radiologic features of the ancient schwannomas are summarized in
Table 2.
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Sonography.—Sonograms showed a well-defined mass containing internal highly reflective areas or hypoechoic areas. An adjacent nerve was visualized in four cases.
CT.—CT scans showed a well-defined mass, and enhancement was observed in capsules or pericystic areas after IV contrast medium infusion (Figs. 1A and 1B). Enhancement of areas surrounding degeneration was seen in all patients who underwent enhanced CT.
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MRI findings compared with pathologic specimen results.—The gross appearance of the tumor was well circumscribed with a thin fibrous capsule, and the internal matrix was yellowish with some scattered myxoid areas and hemorrhagic, cystic, and necrotic areas ranging from red to brown (Figs. 2D and 3D).
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Microscopic examination showed that the tumors consisted mostly of Antoni type B areas that show low signal intensity on T1-weighted MR images and high signal intensity on T2-weighted images. Perivascular hyalinization and gapped vessels with organizing thrombus and hematoma or cystlike spaces mimicking vascular structures were clearly visible. These degenerative areas of hematoma or cystic formations varied in size. Contrast-enhanced MRI depicted these lesions as of various signal intensities without any enhancement. Both the Antoni type A and Antoni type B areas were enhanced by IV contrast medium. The nonenhancing areas of the tumor appeared in all cases to be areas of hemorrhage or degeneration. The circumference of these lesions was clearly enhanced, and the fibrous capsule of the tumor was sometimes enhanced. In all patients, enhanced areas surrounded the degenerative lesions, and enhancement in the capsule was seen on MRI in five patients. The Antoni type A area was reduced and seen mostly around the degenerative lesions (Figs. 2A, 2B, 2C, 2D, 2E, 2F, 2G and 3A, 3B, 3C, 3D). Calcification was histologically confirmed in two tumors, and ossification in one patient.
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Bone scintigraphy and 67Ga citrate scintigraphy.—Moderate to high uptake in the tumor was seen in five cases on the bone scintigram, but 67Ga citrate scintigraphy showed no such accumulation in six cases.
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Ancient schwannoma is usually located deep in the head and neck [6, 7], thorax [8], retroperitoneum [9, 10], pelvis [11], and extremities [12, 13] of elderly patients. It is characterized by degenerative changes typified by perivascular hyalinization, calcification, cystic necrosis, relative loss of Antoni type A tissue, and degenerative nuclei that may be misinterpreted as sarcomatous pleomorphisms [14]. Calcification is the usual degenerative change, but ossification, which was observed in our patient 7, is a rare degenerative variant [14, 15]. In our study, however, calcification and ossification could not be detected on radiologic examinations.
These degenerative changes are thought to be the result of the long-term progression of this tumor. Retroperitoneal ancient schwannoma is difficult to recognize if it has no clinical symptoms, or if it is located in the extremities with no other symptoms than local swelling, and patients tend to ignore the mass until it has finally reached an excessive size. For example, patient 1 had a tumor of the calf measuring 13.8 x 8.0 x 6.5 cm, which, to the best of our knowledge, is the largest reported ancient schwannoma in this location. However, the only complaint by the patient was of partial numbness on the side of the dorsum pedis, which he did not have medically examined until 10 years after first noticing the tumor.
Because it contains cystic areas, ancient schwannoma has been radiologically misdiagnosed as other tumor types, such as malignant fibrous histiocytoma, malignant peripheral nerve sheath tumor, liposarcoma, synovial sarcoma, or hemangiopericytoma. However, only a few reports have dealt with the radiologic features of ancient schwannoma because this tumor is so rarely encountered. Although Shultz et al. suggested that a diagnosis of ancient schwannoma should be considered when a patient presents with a hypervascular soft-tissue mass containing amorphous calcification on radiographs and cystic areas on MRI [13], calcification is not always visible on films [9, 12].
Our study suggests that MRI is the most useful technique for further evaluation of ancient schwannoma. Ordinary schwannomas show their peripheral region as a low-signalintensity area on T1-weighted images and a high-signal-intensity area on T2-weighted images—which correspond to the Antoni B area—and as low-to-intermediate-signal-intensity areas on T1- and T2-weighted images. Ordinary schwannomas are strongly enhanced by gadolinium contrast medium, which is also characteristic of the Antoni A area. On T1-weighted images, the tumors are isointense or slightly hyperintense relative to muscle, and a target pattern with a peripherally hyperintense rim and central low intensity on T2-weighted images has been reported for 52% of benign nerve sheath tumors (neurofibromas and schwannomas) [17]. This target pattern corresponds histologically to peripheral myxomatous tissue and central fibrocollagenous tissue; is absent in lesions with cystic, hemorrhagic, or necrotic degeneration; and is not visible in malignant peripheral nerve sheath tumors.
The ancient schwannoma in our series, however, showed a well-circumscribed complex cystic mass and different enhancement patterns. The circumference of the degenerative hematoma and cyst and the fibrous capsule of the tumor were enhanced. The Antoni type B area occupied most of the tumors, and the Antoni type A area had become smaller and was seen in the areas around the degenerative lesions. We therefore suspect that the Antoni type A areas had degenerated to hematomas or cysts.
Scintigraphy is also a helpful radiologic technique. A previous study found
that schwannomas with the widest diameter (
3 cm) were positive for
technetium-99m dimercaptosuccinic acid and negative for 67Ga
citrate [18]. Our patients with ancient schwannoma showed the same expression
pattern, which indicates that a scintigram should be used to examine a large
neurogenic tumor.
The preoperative differential diagnosis of a comparatively large soft-tissue tumor in an extremity that appears as a well-enhanced inhomogeneous tumor on MRI tends to identify it as malignant. Differentiation from a malignant tumor can be achieved by taking into account the characteristics of the clinical findings and radiologic features of ancient schwannoma. In this study, we made a tentative preoperative diagnosis of the tumors as schwannomas, except for patient 7. Although all the tumors had been diagnosed as ancient schwannoma, patient 7 was problematic before this study was performed because the tumor was large and had an indistinct margin, and so it was thought to be malignant.
In conclusion, the following points are important for differentiating ancient schwannoma and other tumors: The first is a mass with a long-term clinical course presenting with the symptoms of a neurogenic tumor. The second is a palpable mass with local pain and characteristics similar to Tinel's sign and distinct radiologic features that include an adjacent nerve visualized on sonography, a circumference of a degenerative area and fibrous tumor capsule enhanced on MRI, and positive accumulation seen on a bone scan but not on a gallium scan.
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