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Original Report |
1 Department of Radiology, University of Michigan Medical Center, 1500 E Medical
Center Dr., TC 2910G, Ann Arbor, MI 48109-0326.
2 Present address: Department of Radiology, Medical College of Virginia,
Virginia Commonwealth University, PO Box 980615, Richmond, VA
23298-0615.
Received July 28, 2003;
accepted after revision September 9, 2003.
Address correspondence to J. A. Jacobson.
Abstract
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CONCLUSION. Peripheral nerve sheath tumors are often hypoechoic with posterior acoustic enhancement and so may simulate a ganglion cyst. The presence of intrinsic blood flow on color Doppler sonography and peripheral nerve continuity suggests the diagnosis of peripheral nerve sheath tumor. Sonography cannot reliably distinguish neurofibromas from schwannomas.
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Sonography is often used to evaluate a palpable soft-tissue mass; the initial differential diagnosis depends on whether the mass is cystic or solid. The sonographic descriptions of peripheral nerve sheath tumors are somewhat variable, although most of these tumors are homogeneous and hypoechoic [4, 5]. Other descriptions include posterior acoustic enhancement, a target appearance (hyperechoic center and hypoechoic periphery) [6], and a pseudocystic appearance [7, 8]. Increased flow on color Doppler sonography has also been reported in schwannomas [9, 10].
In our clinical practice, we have found significant variability in the sonographic appearance of peripheral nerve sheath tumors, with several simulating ganglion cysts. The purpose of this study was to sonographically characterize pathologically proven peripheral nerve sheath tumors and to identify common sonographic features. Schwannomas and neurofibromas were also compared to determine if this differentiation could be made with sonography.
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Two fellowship-trained musculoskeletal radiologists retrospectively reviewed the sonographic images, with opinion rendered by consensus. Their length of experience with musculoskeletal sonography was 6 and 9 years. The histologic diagnosis of each peripheral nerve sheath tumor was not revealed to the reviewers. Tumors were characterized with regard to echogenicity relative to skeletal muscle (predominately anechoic, hypoechoic, isoechoic, hyperechoic, or mixed), internal homogeneity (homogeneous or heterogeneous), posterior acoustic shadowing or enhancement, nerve continuity, location relative to the involved peripheral nerve (eccentric or central), tumor extent (focal or diffuse), and vascularity on color and power Doppler sonography (when available). Peripheral nerves were identified on sonography by their characteristic appearance of alternating hypoechoic nerve fascicles and hyperechoic connective tissue [11], often surrounded by hyperechoic fat.
Sonography was performed using 5-, 7-, and 12-MHz linear and curved array transducers (HDI 5000, Philips–ATL, Bothell, WA). One patient was imaged with a Sonoline Allegra sonographic unit (Siemens Medical Solutions, Iselin, NJ) and a 7.5-MHz linear array transducer.
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Regarding echogenicity relative to skeletal muscle, five (83%) of six schwannomas were hypoechoic (Figs. 1 and 2) and one (17%) of six had a target appearance (Fig. 3). Of the four neurofibromas, two were predominately hypoechoic and two had a target appearance (Fig. 4). Of the two malignant peripheral nerve sheath tumors, one was hypoechoic and one had mixed echogenicity (Fig. 5). No peripheral nerve sheath tumors were classified as completely anechoic or hyperechoic.
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With regard to internal homogeneity, four (67%) of six schwannomas were homogeneous (Figs. 1 and 2), and two (33%) had a heterogeneous appearance. Of the four neurofibromas, two were homogeneous and two were heterogeneous (Fig. 4). Of the two malignant peripheral nerve sheath tumors, one was homogeneous and one was heterogeneous (Fig. 5).
With regard to posterior acoustic enhancement and shadowing, four (67%) of six schwannomas showed posterior acoustic enhancement (Figs. 1, 2, 3), one (17%) of six showed posterior acoustic shadowing, and one showed neither. Of the four neurofibromas, three were characterized by posterior acoustic enhancement, and one had both posterior acoustic shadowing and enhancement (Fig. 4). Both malignant peripheral nerve sheath tumors showed posterior acoustic enhancement (Fig. 5).
Continuity with the involved peripheral nerve was shown in 11 (92%) of 12 peripheral nerve sheath tumors examined (Figs. 2, 3, 4). One (8%) of 12 peripheral nerve sheath tumors, a schwannoma, did not appear to be continuous with the adjacent peripheral nerve (Fig. 1). All four neurofibromas (Fig. 4) and four (67%) of six schwannomas (Fig. 3) were centrally related to the involved peripheral nerve. One (17%) of six schwannomas was eccentrically related to the involved peripheral nerve (Fig. 2), and a relationship with the adjacent peripheral nerve was indeterminate in the single schwannoma in which nerve continuity was not shown. One of two malignant peripheral nerve sheath tumors was centrally related and the other was eccentrically related to the involved peripheral nerve. All six schwannomas, one of four neurofibromas, and both malignant peripheral nerve sheath tumors presented as focal masses. Three of four neurofibromas diffusely involved the peripheral nerve (Fig. 4).
Seven (58%) of 12 peripheral nerve sheath tumors underwent color or power Doppler sonography. Of these, two of four schwannomas (Fig. 6), zero of one neurofibroma, and two of two malignant peripheral nerve sheath tumors showed increased vascularity with prominent arterial flow.
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With regard to echogenicity, most peripheral nerve sheath tumors (8/12 or 67%) were hypoechoic and none were anechoic. The target appearance on sonography (hyperechoic center with a hypoechoic periphery) described by Lin et al. [6] was present in three of 12 peripheral nerve sheath tumors. The target appearance has been attributed to a central fibrocollagenous region and a peripheral myxomatous region [12]. No malignant peripheral nerve sheath tumors showed a target appearance, which is similar to findings described with MRI [12]. In addition, none of our cases displayed an internal echogenic ring, which has been described as rare but virtually pathognomonic of nerve tumors [9].
With regard to posterior effects, most peripheral nerve sheath tumors (9/12 or 75%) displayed posterior acoustic enhancement, including both malignant peripheral nerve sheath tumors. This effect has also been described with other solid tumors of uniform cellularity, such as lymphoma, affecting the lymph nodes [7]. Because of the common feature of posterior acoustic enhancement, a hypoechoic and homogeneous peripheral nerve sheath tumor may simulate a cystic structure such as a ganglion cyst [13, 14]. However, the presence of flow on color and power Doppler sonography excludes an uncomplicated ganglion cyst. Furthermore, the finding of peripheral nerve continuity indicates peripheral nerve sheath tumor as the cause. Posterior acoustic shadowing was present in one schwannoma and one neurofibroma, which could represent refraction shadows at the edge of a peripheral nerve sheath tumor because a hyperechoic focus suggesting calcification was not identified. Radiographs were not available to confirm this hypothesis. A schwannoma that has undergone degenerative change (an "ancient" schwannoma) may calcify [15].
Although four schwannomas had central continuity with the involved peripheral nerve, only one schwannoma was eccentric to the nerve. This finding was unexpected because schwannomas have been classically described as eccentric at MRI and at histology [9]. It is important to recognize peripheral nerve continuity because doing so significantly limits the differential diagnosis. When showing nerve continuity, the characteristic alternating hypoechoic and hyperechoic echogenicity of a peripheral nerve, described by Silvestri et al. [11], may not be as obvious with very small peripheral nerve branches. These small branches may consist of a single hypoechoic nerve fascicle and adjacent hyperechoic fat and connective tissue. The presence of peripheral nerve continuity is helpful in differentiating a peripheral nerve sheath tumor from other soft-tissue masses.
Sonography could not distinguish among schwannoma, neurofibroma, and malignant peripheral nerve sheath tumor. As stated, most schwannomas, similar to neurofibromas, were centrally positioned relative to the involved peripheral nerve. Both schwannomas and malignant peripheral nerve sheath tumors presented as a focal mass. In contrast, neurofibromas in the setting of neurofibromatosis had diffuse peripheral nerve involvement. However, most neurofibromas (90%) are described as localized and solitary [16]. Both malignant peripheral nerve sheath tumors in our study were hyperemic on color and power Doppler sonography, although hyperemia was present in only two (40%) of five benign peripheral nerve sheath tumors.
Limitations of this study include its retrospective nature, which means that we relied on the ability of the sonographer to image all pertinent sonographic findings. This may explain why peripheral nerve continuity could not be confirmed in one schwannoma at retrospective review. In addition, only seven (58%) of 12 peripheral nerve sheath tumors examined had undergone Doppler examination. Finally, our small sample of 12 peripheral nerve sheath tumors is a relative limitation.
In summary, most peripheral nerve sheath tumors are homogeneous and hypoechoic, with posterior acoustic enhancement and peripheral nerve continuity. Sonography cannot distinguish among schwannomas, neurofibromas, and malignant peripheral nerve sheath tumors. Because many peripheral nerve sheath tumors are hypoechoic and have posterior acoustic enhancement, their appearance may simulate that of a ganglion cyst. If so, internal blood flow on color and power Doppler sonography can distinguish a peripheral nerve sheath tumor from a ganglion cyst. When a solid mass is identified on sonography, peripheral nerve continuity should be evaluated, because such continuity would suggest the diagnosis of peripheral nerve sheath tumor rather than another solid mass.
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