DOI: 10.1148/radiol.2343030946
(Radiology 2005;234:899-900.)
© RSNA, 2005
The Target Sign: Extremity1
Kevin P. Banks, MD
1 From the Department of Radiology, MCHE-DR, Brooke Army Medical Center, 3851 Roger Brooke Dr, Fort Sam Houston, TX 78234. Received June 20, 2003; revision requested August 29; revision received October 8; accepted November 12. Address correspondence to the author (e-mail: kevin.banks@cen.amedd.army.mil).
 |
APPEARANCE
|
|---|
The target sign is seen at transverse T2-weighted magnetic resonance (MR) imaging of an extremity. The target sign appears as a central area of low signal intensity surrounded by an area of high signal intensity (Figure) (1,2). This appearance remains consistent on spin-echo and fast spin-echo images with and spin-echo and fast spin-echo images without the application of fat-suppression techniques.
View larger version (115K):
[in this window]
[in a new window]
[Download PPT slide]
|
Transverse T2-weighted MR image (2500/60 [repetition time msec/echo time msec]) with fat-suppression technique shows a benign peripheral nerve sheath tumor (PNST) arising from the tibial nerve. The lesion, which demonstrates the classic target sign, shows the characteristic central area of low signal intensity surrounded by a rim of high signal intensity (arrow), a finding that is typical of these neoplasms.
|
|
|
EXPLANATION
|
|---|
The target sign is seen with PNSTs and derives from the histologic composition of the tumor. The central area of low signal intensity histologically corresponds to the fibrocollagenous tissue in the lesion’s core, and the surrounding area of high signal intensity corresponds to the presence of more myxomatous tissue (3,4). This characteristic distribution of cellular versus less cellular components of PNSTs is most commonly encountered in neurofibromas. In schwannomas, the cellular components usually are distributed more randomly. As a result, the majority of PNSTs that demonstrate the target sign are benign neurofibromas (3,5). When the target sign is seen in schwannomas, it is generally attributed to a central distribution of the more cellular Antoni type A neurilemoma, with a surrounding rim of the hypocellular Antoni type B neurilemoma (3). It is postulated that, in malignant PNSTs, the areas of the lesion that demonstrate the target sign contain PNST tissue that is benign, and the areas of the lesion that are devoid of the target sign represent the portions of the original lesion that have undergone malignant transformation (5).
|
DISCUSSION
|
|---|
The incidence of PNSTs is estimated at approximately one per 100 000 individuals per year (6). The neoplasms are divided into two major categories: benign and malignant. Neurofibromas and schwannomas represent the benign entities of this group. Alternatively, malignant PNST is the generalized term that is now used in place of previous pathologic terms, such as neurofibrosarcoma and malignant schwannoma, to describe the malignant entities of this group. The term malignant PNST is preferred because the cell of origin in these lesions is often unknown (7).
Both neurofibromas and schwannomas are composed of a dominant cell line that resembles normal Schwann cells, which are the likely cells of origin for these two neoplasms (8). Beyond this commonality, neurofibromas and schwannomas have some general differences that can aid in the prospective diagnosis. Schwannomas, which represent approximately 5% of all benign soft-tissue neoplasms, are generally solitary, slow-growing lesions (9). Schwannomas are fusiform masses that arise eccentrically along the involved nerve and are encapsulated by an epineurium (10). Schwannomas commonly occur along the spinal and sympathetic nerve roots of the head and neck, as well as along the nerves in the flexor aspects of the upper and lower extremities (particularly the ulnar and peroneal nerves). The posterior mediastinum and retroperitoneum can also be affected (3). Symptoms are uncommon unless the mass becomes large enough to compress the associated nerve, and, if necessary, surgical excision can usually be performed, with sparing of the parent nerve (10). These lesions can uncommonly be associated with neurofibromatosis, and malignant degeneration is seen only in exceptional cases (1,8).
Neurofibromas, which also represent 5% of all benign soft-tissue neoplasms, are found in the setting of neurofibromatosis in up to 10% of cases; this comorbidity results in a markedly increased risk of malignant transformation of the neurofibroma (9). Neurofibromas classically have three subtypes: localized, diffuse, and plexiform (3). The localized subtype of neurofibroma is most likely to arise from cutaneous nerves, with the occasional involvement of deeper-seated nerves. The diffuse subtype more commonly originates from the nerves in the subcutaneous tissues of the head and neck (3). Both the localized and diffuse forms of neurofibromas manifest characteristics similar to those of schwannomas, which are typically painless fusiform masses. In cases of neurofibromas that originate from a nerve of the deep tissues, neurologic symptoms are also likely to be noted. The plexiform subtype of neurofibroma manifests as a diffuse tumor with tortuous expansion along the branches of the parent nerve. Unlike schwannomas, neurofibromas are intimately associated with their nerve of origin and are not encapsulated. Because of these features, the tumor and parent nerve cannot be separated, and, thus, surgical resection cannot be performed without sacrificing the parent nerve (11).
Malignant PNSTs represent approximately 6% of soft-tissue sarcomas (12). Up to 50% of these lesions are found in the presence of neurofibromatosis type 1. Malignant PNSTs typically arise along the major nerve trunks, and patients routinely present with pain and/or parathesias in addition to a soft-tissue mass (10). Like their benign counterparts, malignant PNSTs manifest as fusiform masses but frequently demonstrate areas of necrosis.
Despite these differences, the clinical distinction between benign PNSTs and malignant PNSTs is problematic because there is much overlap in the manifestation of the various entities. All of the lesions typically arise in individuals 20–50 years of age, with benign lesions arising slightly more often in the younger portion of this group (10). No sex or racial predilection has been noted (3). A palpable soft-tissue mass is the most common complaint for all subtypes, and the size of the lesion at the time of the initial complaint has been shown to be of little diagnostic importance (13). In a review of 115 PNSTs, Ogose et al (13) found that all three subtypes (schwannomas, neurofibromas, and malignant PNSTs) ranged in diameter from 0.1 to 2.5 cm at the time of surgery. While malignant PNSTs tend to be associated with pain and parethesias, benign PNSTs have also been associated with these symptoms (13).
|
ACKNOWLEDGMENTS
|
|---|
The author thanks Liem Bui-Mansfield, MD, for the use of his figures, his mentorship, and his assistance in preparing the manuscript.
|
FOOTNOTES
|
|---|
Author stated no financial relationship to disclose.
A trainee (resident or fellow) wishing to submit a manuscript for Signs in Imaging should first write to the Editor for approval of the sign to be prepared, to avoid duplicate preparation of the same sign.
|
REFERENCES
|
|---|
- Varma DG, Moulopoulos A, Sara AS, et al. MR imaging of extracranial nerve sheath tumors. J Comput Assist Tomogr 1992; 16:448-453.[Medline]
- Burk DL, Brunberg JA, Kanal E, Latchaw RE, Wolf GL. Spinal and paraspinal neurofibromatosis: surface coil MR imaging at 1.5 T1. Radiology 1987; 162:797-801.[Abstract/Free Full Text]
- Murphey MD, Smith WS, Smith SE, Kransdorf MJ, Temple HT. From the archives of the AFIP: imaging of musculoskeletal neurogenic tumors—radiologic-pathologic correlation. RadioGraphics 1999; 19:1253-1280.[Abstract/Free Full Text]
- Nishimura H, Zhang Y, Ohkuma K, Uchida M, Hayabuchi N, Sun S. MR imaging of soft-tissue masses of the extraperitoneal spaces. RadioGraphics 2001; 21:1141-1154.[Abstract/Free Full Text]
- Bhargava R, Parham DM, Lasater OE, Chari RS, Chen G, Fletcher BD. MR imaging differentiation of benign and malignant peripheral nerve sheath tumors: use of the target sign. Pediatr Radiol 1997; 27:124-129.[CrossRef][Medline]
- Hems TE, Burge PD, Wilson DJ. The role of magnetic resonance imaging in the management of peripheral nerve tumours. J Hand Surg [Br] 1997; 22:57-60.[Medline]
- Ducatman BS, Scheithauer BW, Piepgras DG, Reiman HM, Ilstrup DM. Malignant peripheral nerve sheath tumors: a clinicopathologic study of 120 cases. Cancer 1986; 57:2006-2021.[CrossRef][Medline]
- Stull MA, Moser RP, Jr, Kransdorf MJ, Bogumill GP, Nelson MC. Magnetic resonance appearance of peripheral nerve sheath tumors. Skeletal Radiol 1991; 20:9-14.[Medline]
- Kransdorf MJ. Benign soft-tissue tumors in a large referral population: distribution of specific diagnoses by age, sex, and location. AJR Am J Roentgenol 1995; 164:395-402.[Abstract/Free Full Text]
- Lin J, Martel W. Cross-sectional imaging of peripheral nerve sheath tumors: characteristic signs on CT, MR imaging, and sonography. AJR Am J Roentgenol 2001; 176:75-82.[Free Full Text]
- Suh J, Abenoza P, Galloway HR, Everson LI, Griffiths HJ. Peripheral (extracranial) nerve tumors: correlation of MR imaging and histologic findings. Radiology 1992; 183:341-346.[Abstract/Free Full Text]
- Kransdorf MJ. Malignant soft-tissue tumors in a large referral population: distribution of diagnoses by age, sex, and location. AJR Am J Roentgenol 1995; 164:129-134.[Abstract/Free Full Text]
- Ogose A, Hotta T, Morita T, et al. Tumors of peripheral nerves: correlation of symptoms, clinical signs, imaging features, and histologic diagnosis. Skeletal Radiol 1999; 28:183-188.[CrossRef][Medline]
TOP
APPEARANCE
EXPLANATION
