(Radiographics. 2001;21:601-612.)
© RSNA, 2001
Neurofibromatosis Type 1: A Diagnostic Mimicker at CT1
Brian J. Fortman, MD,
Brian S. Kuszyk, MD,
Bruce A. Urban, MD and
Elliot K. Fishman, MD
1 From the Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins Hospital, 600 N Wolfe St, Baltimore, MD 21287. Recipient of a Certificate of Merit award for a scientific exhibit at the 1999 RSNA scientific assembly. Received July 7, 2000; revision requested August 3 and received September 8; accepted September 11. Address correspondence to E.K.F. (e-mail: efishman@jhmi.edu).
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Abstract
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Neurofibromatosis type 1 (NF1) is the most common of the phakomatoses and has a variety of localized or, more frequently, systemic manifestations throughout the thorax, abdomen, pelvis, and extremities. Classic computed tomographic (CT) findings in NF1 with thoracic involvement include small, well-defined subcutaneous neurofibromas, focal thoracic scoliosis, posterior vertebral scalloping, enlarged neural foramina, and characteristic rib abnormalities due to bone dysplasia or erosion from adjacent neurofibromas. However, more atypical manifestations are occasionally seen, and magnetic resonance (MR) imaging can be useful in equivocal cases. NF1 with abdominopelvic involvement tends to arise in the retroperitoneal, mesenteric, and paraspinal regions; it may be quite extensive and therefore difficult to distinguish from adenopathy at CT. The multiplanar capabilities of MR imaging, particularly with T2 weighting, make this modality helpful in evaluating affected patients and making the diagnosis. The classic peripheral manifestations of NF1 include limb hemihypertrophy, pseudarthrosis, peripheral nerve neurofibromas, and subcutaneous common and plexiform neurofibromas. In some cases of NF1, imaging findings are inconclusive, and biopsy and subsequent pathologic analysis are required. Familiarity with the various manifestations of NF1 in different anatomic locations is important in making the diagnosis and optimizing postdiagnostic treatment.
Index Terms: Extremities, CT, 40.1211 • Extremities, neoplasms, 40.31 • Neurofibromatosis, 30.1831, 40.1831, 60.1472, 80.18 • Neuroma, 60.3161, 70.3132 • Pelvic organs, CT, 80.1211 • Pelvic organs, neoplasms, 80.31 • Thorax, CT, 60.1211 • Thorax, neoplasms, 60.31
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Describe the typical clinical features of NF1.
- Describe the characteristic imaging features of typical and plexiform neurofibromas.
- Recognize and discuss the various disease entities of the chest, abdomen, pelvis, and extremities that may appear similar to NF1 at CT.
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Introduction
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Neurofibromatosis type 1 (NF1), or von Recklinghausen disease, is a comparatively common hereditary disease in which the skin, nervous system, bones, endocrine glands, and sometimes other organs are the sites of a variety of congenital abnormalities, which often take the form of benign tumors (1). NF1 is the most common of the phakomatoses (neurocutaneous syndromes) and occurs in one of every 2,000 live births. NF1 is inherited as an autosomal dominant disorder; however, up to 50% of cases occur sporadically due to spontaneous mutation. Although many affected patients present in childhood with classic clinical findings, up to 10% of patients present later in life with forme fruste or atypical manifestations. The classic neurogenic tumors occurring outside the central nervous system include neurofibromas (Fig 1) and, more specifically, plexiform neurofibromas (Figs 2, 3). Approximately 30% of patients with a solitary neurofibroma will have NF1; virtually all patients with multiple neurofibromas or plexiform histologic findings have the disease (2). Neurofibromas arise from Schwann cells and fibroblasts and may arise in any peripheral nerve. Plexiform neurofibromas are pathognomonic for NF1, and, although they most commonly involve the fifth cranial nerve, they may occur anywhere. Plexiform tumors represent diffuse neural enlargement or multiple neurofibromas along the course of peripheral nerves.
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Figure 3. Photomicrograph (original magnification, x20; hematoxylin-eosin stain) of a plexiform neurofibroma demonstrates proliferation of neurites and fibroblasts, resulting in tortuous enlargement of nerve fascicles (solid arrow) dispersed in a loose, disordered myxoid stroma (open arrow).
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The computed tomographic (CT) findings in patients with peripheral nerve sheath tumors have been well described and depend largely on the histologic characteristics of the tumors (3,4). Although these tumors may demonstrate soft-tissue attenuation at contrast material–enhanced CT, low attenuation is characteristic and is seen in up to 73% of cases (5). Factors responsible for this low attenuation include cystic degeneration, xanthomatous features, confluent areas of hypocellularity, and lipid-rich Schwann cells. Low attenuation may be seen in plexiform tumors due to entrapped perineuronal adipose tissue. The densely cellular components or collagen-rich regions are believed to account for the areas of higher attenuation. All tumors may occasionally demonstrate confluent or, more typically, peripheral regions of contrast material enhancement due to more peripheral cellular and fibrous elements, whereas central myxomatous and cystic regions are comparatively hypovascular.
The typical clinical picture of NF1, which is usually identifiable at a glance, consists of multiple circumscribed areas of increased skin pigmentation accompanied by dermal and neural tumors of various types. In the majority of patients, spots of hyperpigmentation (café au lait spots) and cutaneous and subcutaneous tumors form the basis for the clinical diagnosis. Additional diagnostic criteria include axillary freckling, iris hamartomas, bone dysplasias, affected first-degree relatives, and multiple central nervous system tumors such as optic nerve gliomas (6). Occasionally, neurofibromas are found during surgical intervention or incidentally at radiologic imaging; affected patients usually display very mild cutaneous symptoms.
The tremendous heterogeneity of CT findings in NF1 is a direct result of primary germ layer abnormalities and the ubiquity of peripheral nerve fibers. Varying degrees of involvement by both neurofibromas and plexiform neurofibromas, in addition to primary mesodermal abnormalities, result in a wide spectrum of phenotypic manifestations. In this article, we discuss and illustrate the spectrum of CT findings in NF1 with emphasis on those that mimic other disease entities within the thorax, abdomen, pelvis, and extremities.
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Thoracic Involvement
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Given the rich distribution of peripheral nerves throughout the thorax, manifestations of NF1 may involve the ribs, chest wall, lungs, and mediastinum. Classic imaging findings include small, well-defined subcutaneous neurofibromas, focal thoracic scoliosis, posterior vertebral scalloping, enlarged neural foramina, and characteristic rib abnormalities due to bone dysplasia or erosion from adjacent neurofibromas. When these findings are seen in adolescents with typical clinical findings, the diagnosis is often straightforward. However, because of the vast distribution of nerve tissue and the heterogeneity of this disease, more atypical manifestations are occasionally seen.
Neurogenic tumors account for about 9% of primary mediastinal masses in adults and 30% of mediastinal tumors in children (7). Both common and plexiform neurofibromas may involve the mediastinum extensively and can become quite large. They typically appear as well-marginated, smooth, round or elliptic masses in the paravertebral regions or along the course of the vagus, phrenic, recurrent laryngeal, or intercostal nerves. Plexiform neurofibromas are extensive fusiform or infiltrating masses that tend to surround mediastinal vessels with loss of normally visible fat planes. They demonstrate variable contrast material enhancement and may calcify. Both common and plexiform neurofibromas may closely mimic localized or extensive adenopathy and can result in diffuse mediastinal widening as seen in numerous pathologic conditions such as lymphoma, sarcoid, lymphangiomatosis, and metastatic disease (Fig 4a, 4b). They may also insinuate themselves into adjacent mediastinal structures such as the esophagus and simulate primary disease (Fig 4c). In addition, there have been case reports of neurofibromas arising directly from the trachea and esophagus (8,9). When this occurs, imaging findings may be inconclusive, and biopsy or surgical exploration may be required for a definitive diagnosis.
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Figure 4a. (a) CT scan obtained in a 29-year-old man demonstrates multiple low-attenuation neurofibromas in the superior mediastinum and axilla. The differential diagnosis includes lymphoma, tuberculosis, Mycobacterium avium-intracellulare complex infection, sarcoid, and metastatic testicular cancer. (b) CT scan obtained in a different patient shows multiple large prevascular and paratracheal neurofibromas. (c) CT scan obtained in a 22-year-old man who presented with dysphagia demonstrates paraesophageal and pleura-based neurofibromas (arrows). Metastatic esophageal cancer and invasive thymoma may have a similar appearance. (d) CT scan obtained in a 29-year-old woman shows paraspinal neurofibromas (arrow) simulating disease processes including fibrous pleural tumors, metastatic pleural implants, and extramedullary hematopoiesis.
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Figure 4b. (a) CT scan obtained in a 29-year-old man demonstrates multiple low-attenuation neurofibromas in the superior mediastinum and axilla. The differential diagnosis includes lymphoma, tuberculosis, Mycobacterium avium-intracellulare complex infection, sarcoid, and metastatic testicular cancer. (b) CT scan obtained in a different patient shows multiple large prevascular and paratracheal neurofibromas. (c) CT scan obtained in a 22-year-old man who presented with dysphagia demonstrates paraesophageal and pleura-based neurofibromas (arrows). Metastatic esophageal cancer and invasive thymoma may have a similar appearance. (d) CT scan obtained in a 29-year-old woman shows paraspinal neurofibromas (arrow) simulating disease processes including fibrous pleural tumors, metastatic pleural implants, and extramedullary hematopoiesis.
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Figure 4c. (a) CT scan obtained in a 29-year-old man demonstrates multiple low-attenuation neurofibromas in the superior mediastinum and axilla. The differential diagnosis includes lymphoma, tuberculosis, Mycobacterium avium-intracellulare complex infection, sarcoid, and metastatic testicular cancer. (b) CT scan obtained in a different patient shows multiple large prevascular and paratracheal neurofibromas. (c) CT scan obtained in a 22-year-old man who presented with dysphagia demonstrates paraesophageal and pleura-based neurofibromas (arrows). Metastatic esophageal cancer and invasive thymoma may have a similar appearance. (d) CT scan obtained in a 29-year-old woman shows paraspinal neurofibromas (arrow) simulating disease processes including fibrous pleural tumors, metastatic pleural implants, and extramedullary hematopoiesis.
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Figure 4d. (a) CT scan obtained in a 29-year-old man demonstrates multiple low-attenuation neurofibromas in the superior mediastinum and axilla. The differential diagnosis includes lymphoma, tuberculosis, Mycobacterium avium-intracellulare complex infection, sarcoid, and metastatic testicular cancer. (b) CT scan obtained in a different patient shows multiple large prevascular and paratracheal neurofibromas. (c) CT scan obtained in a 22-year-old man who presented with dysphagia demonstrates paraesophageal and pleura-based neurofibromas (arrows). Metastatic esophageal cancer and invasive thymoma may have a similar appearance. (d) CT scan obtained in a 29-year-old woman shows paraspinal neurofibromas (arrow) simulating disease processes including fibrous pleural tumors, metastatic pleural implants, and extramedullary hematopoiesis.
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Paravertebral neurofibromas may or may not extend into the spinal canal and thus may mimic entities such as extramedullary hematopoiesis, other neurogenic tumors (eg, ganglioneuroma), or neurenteric cysts (Fig 4d). When they have very low attenuation (10–20 HU) at CT, both focal and plexiform neurofibromas may simulate congenital mediastinal cystic lesions, leading to confusion in cases in which a diagnosis of NF1 has yet to be made. Peripheral nerve tumors typically have low to intermediate signal intensity on T1-weighted magnetic resonance (MR) images. At T2-weighted imaging, the tumors are often heterogeneous. High-signal-intensity regions correspond to areas of myxoid tissue or cystic degeneration. Nodular areas of low signal intensity correspond to collagen and fibrous tissue, which may enhance after administration of gadolinium-based contrast material. MR imaging may be helpful in distinguishing neurofibromas in confusing cases, given the somewhat characteristic contrast enhancement patterns and imaging findings seen with T2-weighted sequences due to the presence of central collagen fibers (10).
Apical neurofibromas, although relatively characteristic of NF1, may occasionally be unilateral and appear more aggressive, causing confusion in cases involving older patients by simulating a primary lung malignancy such as a superior sulcus (Pancoast) tumor (Fig 5a). Both common and plexiform neurofibromas may erode, invade, or destroy adjacent bone, thereby mimicking other entities in children such as Ewing sarcoma, lymphoma, rhabdomyosarcoma, or metastatic disease (Fig 5b, 5c). In adults, lesions may simulate metastasis, soft-tissue sarcoma, or plasmacytoma. In patients with NF1 who have aggressive-appearing rib lesions and associated interval growth or pain, the possibility of malignant degeneration should be considered. Sarcomatous degeneration of peripheral soft-tissue neurofibromas is estimated to occur in 5%–15% of all patients with NF1 (11). Intercostal neurofibromas may become pedunculated and appear as pulmonary nodules on cross-sectional studies, causing confusion in cases involving known primary malignancies (Fig 5d). The multiplanar capabilities of MR imaging may prove helpful in such cases by helping identify tumor origin.
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Figure 5a. (a) CT scan demonstrates an apical neurofibroma (arrow) simulating a Pancoast tumor. (b) CT scan obtained in a different patient shows an intercostal neurofibroma (arrow) causing rib destruction and simulating a pleura-based mass such as pulmonary or metastatic adenocarcinoma, plasmacytoma, or pulmonary infection (eg, nocardiosis, actinomycosis). (c) CT scan obtained in a 29-year-old woman demonstrates a plexiform neurofibroma invading the anterior chest wall. The lesion simulates other processes such as metastatic disease, soft-tissue sarcoma, or chondrosarcoma. (d) CT scan obtained in a fourth patient shows a pedunculated intercostal neurofibroma (arrow) mimicking a solitary pulmonary nodule.
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Figure 5b. (a) CT scan demonstrates an apical neurofibroma (arrow) simulating a Pancoast tumor. (b) CT scan obtained in a different patient shows an intercostal neurofibroma (arrow) causing rib destruction and simulating a pleura-based mass such as pulmonary or metastatic adenocarcinoma, plasmacytoma, or pulmonary infection (eg, nocardiosis, actinomycosis). (c) CT scan obtained in a 29-year-old woman demonstrates a plexiform neurofibroma invading the anterior chest wall. The lesion simulates other processes such as metastatic disease, soft-tissue sarcoma, or chondrosarcoma. (d) CT scan obtained in a fourth patient shows a pedunculated intercostal neurofibroma (arrow) mimicking a solitary pulmonary nodule.
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Figure 5c. (a) CT scan demonstrates an apical neurofibroma (arrow) simulating a Pancoast tumor. (b) CT scan obtained in a different patient shows an intercostal neurofibroma (arrow) causing rib destruction and simulating a pleura-based mass such as pulmonary or metastatic adenocarcinoma, plasmacytoma, or pulmonary infection (eg, nocardiosis, actinomycosis). (c) CT scan obtained in a 29-year-old woman demonstrates a plexiform neurofibroma invading the anterior chest wall. The lesion simulates other processes such as metastatic disease, soft-tissue sarcoma, or chondrosarcoma. (d) CT scan obtained in a fourth patient shows a pedunculated intercostal neurofibroma (arrow) mimicking a solitary pulmonary nodule.
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Figure 5d. (a) CT scan demonstrates an apical neurofibroma (arrow) simulating a Pancoast tumor. (b) CT scan obtained in a different patient shows an intercostal neurofibroma (arrow) causing rib destruction and simulating a pleura-based mass such as pulmonary or metastatic adenocarcinoma, plasmacytoma, or pulmonary infection (eg, nocardiosis, actinomycosis). (c) CT scan obtained in a 29-year-old woman demonstrates a plexiform neurofibroma invading the anterior chest wall. The lesion simulates other processes such as metastatic disease, soft-tissue sarcoma, or chondrosarcoma. (d) CT scan obtained in a fourth patient shows a pedunculated intercostal neurofibroma (arrow) mimicking a solitary pulmonary nodule.
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Lateral and anterior meningoceles commonly occur in patients with NF1 due to pressure differences between the thorax and subarachnoid space superimposed on bone vertebral defects. Meningoceles usually occur in association with thoracic spine scoliosis (12). At CT, they appear as well-circumscribed, low-attenuation paravertebral masses because they contain cerebrospinal fluid, and occasionally they exhibit peripheral rim enhancement. Given their location, high definition, and low attenuation at CT, meningoceles may occasionally simulate mediastinal or lung abscesses, especially in patients who present with fever and cough (Fig 6a). Smaller meningoceles associated with vertebral body dysplasia or destruction may appear as aggressive lesions at CT and simulate processes such as metastatic disease, myeloma, or spondylitis (Fig 6b). MR imaging findings are often diagnostic, as is contrast material filling of the meningocele at CT myelography, because filling with intrathecally injected contrast material is rarely seen in other lesions.
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Figure 6a. (a) CT scan obtained in a 44-year-old man with known NF1 and severe scoliosis demonstrates an associated large, lateral thoracic meningocele (arrows) simulating a lung abscess or neurenteric cyst. The meningocele had become infected and was causing fever. (b) CT scan obtained in a different patient shows a smaller meningocele with spinal involvement (arrow) mimicking a destructive thoracic vertebral lesion such as metastatic disease, myeloma, or tuberculous spondylitis.
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Figure 6b. (a) CT scan obtained in a 44-year-old man with known NF1 and severe scoliosis demonstrates an associated large, lateral thoracic meningocele (arrows) simulating a lung abscess or neurenteric cyst. The meningocele had become infected and was causing fever. (b) CT scan obtained in a different patient shows a smaller meningocele with spinal involvement (arrow) mimicking a destructive thoracic vertebral lesion such as metastatic disease, myeloma, or tuberculous spondylitis.
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Subcutaneous neurofibromas are common in NF1 and classically appear as soft, mobile, discrete nodules that may cause varying degrees of disfigurement. At CT, neurofibromas appear as focal subcutaneous nodules with soft-tissue attenuation and are often numerous. In an adult with a known primary malignancy, this appearance may raise concern for metastatic soft-tissue implants from primary tumors such as melanoma, breast carcinoma, or lung carcinoma (13). Occasionally, subcutaneous neurofibromas may have very low attenuation (5–15 HU) and mimic lesions such as sebaceous cysts or epidermal inclusion cysts as seen in Gardner syndrome (Fig 7) (14).
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Figure 7. CT scan shows low-attenuation subcutaneous neurofibromas within the left anterior chest wall (arrow), which may occasionally simulate sebaceous or epidermal inclusion cysts.
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Abdominopelvic Involvement
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NF1 may involve numerous ganglia within the abdomen as well as the rich arborization of somatic and autonomic nerve tissue innervating the abdominal and pelvic organs. Mesenteric involvement in NF1 has been described and should not be confused with hereditary intestinal neurofibromatosis, a rare but clinically distinctive genetic disease in which involvement is limited to the small bowel and colon (15). Intestinal involvement in NF1 rarely occurs in isolation and is more typically preceded and accompanied by peripheral manifestations of the disease. Abnormal neuronal tissue may involve the entire gastrointestinal tract but is more typically seen in the small bowel and colon. Manifestations are variable, with some plexiform and common neurofibromas associated with the bowel wall and mesentery apparent at cross-sectional imaging. In other cases, disease is characterized by neuronal hyperplasia of the bowel mucosa and submucosa, manifesting as idiopathic megacolon, functional stenosis, or ileus. It is the presence of gastrointestinal mucosal neuromas that links NF1 to the multiple endocrine neoplasia type IIb syndrome, which is characterized by pheochromocytoma, medullary thyroid cancer, and marfanoid body habitus. In addition, there is a questionable association between NF1 and duodenal carcinoid tumor.
More commonly, when abdominal involvement occurs in NF1, tumors tend to arise in the retroperitoneal, mesenteric, and paraspinal regions. Focal involvement of individual organs is rare but does occur. Plexiform tumors with mesenteric infiltration may cause vascular narrowing and compromise blood flow. Affected patients typically present with secondary hypertension or occasionally with intestinal ischemia and abdominal pain. Stenosis of the mesenteric vasculature in NF1 may also be seen as a result of primary mesodermal abnormalities of the vessel wall.
At CT, multiple well-defined neurofibromas arising from nerves traversing the mesentery or retroperitoneum may be quite extensive and difficult to distinguish from adenopathy such as that seen with lymphoma (Fig 8a–8c). Often, these masses may have very low attenuation and mimic other disease processes that cause low-attenuation adenopathy (eg, tuberculosis, Whipple disease, mycobacterium avium complex, metastatic disease from tumors such as seminoma). MR imaging can be of great value in diagnosing such extensive neurogenic tumors when multiple ringlike structures are seen within the masses on T2-weighted images, likely representing nerve tissue and areas of myxoid degeneration (16). Neurofibromas may occur focally within the abdomen and, depending on location, may simulate discrete adenopathy (Fig 8d). Neural tumors may also mimic abnormalities in adjacent organs such as the adrenal glands (Fig 9). Neurofibromas may arise directly within adrenal tissue; however, glandular involvement in NF1 arises much more commonly from pheochromocytoma.
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Figure 8a. (a) CT scan demonstrates multiple neurofibromas (arrow) simulating enlarged lymph nodes in the retrocrural space and gastrohepatic ligament. (b) CT scan obtained in a different patient demonstrates neurofibromas (arrow) simulating extensive celiac adenopathy. (c) CT scan obtained in a 25-year-old woman shows a retroperitoneal plexiform neurofibroma (arrowhead) simulating other entities such as lymphoma, metastatic disease, or soft-tissue sarcoma. (d) CT scan obtained in a fourth patient demonstrates a neurofibroma (arrow) simulating retrocrural adenopathy.
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Figure 8b. (a) CT scan demonstrates multiple neurofibromas (arrow) simulating enlarged lymph nodes in the retrocrural space and gastrohepatic ligament. (b) CT scan obtained in a different patient demonstrates neurofibromas (arrow) simulating extensive celiac adenopathy. (c) CT scan obtained in a 25-year-old woman shows a retroperitoneal plexiform neurofibroma (arrowhead) simulating other entities such as lymphoma, metastatic disease, or soft-tissue sarcoma. (d) CT scan obtained in a fourth patient demonstrates a neurofibroma (arrow) simulating retrocrural adenopathy.
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Figure 8c. (a) CT scan demonstrates multiple neurofibromas (arrow) simulating enlarged lymph nodes in the retrocrural space and gastrohepatic ligament. (b) CT scan obtained in a different patient demonstrates neurofibromas (arrow) simulating extensive celiac adenopathy. (c) CT scan obtained in a 25-year-old woman shows a retroperitoneal plexiform neurofibroma (arrowhead) simulating other entities such as lymphoma, metastatic disease, or soft-tissue sarcoma. (d) CT scan obtained in a fourth patient demonstrates a neurofibroma (arrow) simulating retrocrural adenopathy.
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Figure 8d. (a) CT scan demonstrates multiple neurofibromas (arrow) simulating enlarged lymph nodes in the retrocrural space and gastrohepatic ligament. (b) CT scan obtained in a different patient demonstrates neurofibromas (arrow) simulating extensive celiac adenopathy. (c) CT scan obtained in a 25-year-old woman shows a retroperitoneal plexiform neurofibroma (arrowhead) simulating other entities such as lymphoma, metastatic disease, or soft-tissue sarcoma. (d) CT scan obtained in a fourth patient demonstrates a neurofibroma (arrow) simulating retrocrural adenopathy.
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When paravertebral neurofibromas or lateral meningoceles occur in the lumbar region, they may displace or thin the overlying psoas muscle, creating the appearance of a primary psoas mass or neoplasm (Fig 10a). When they are well-defined and have characteristic low attenuation, they may simulate a tuberculous or staphylococcal psoas abscess (Fig 10b). Lack of significant enhancement and vertebral body destruction coupled with the patient’s clinical presentation are often helpful in arriving at the correct diagnosis.
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Figure 10a. (a) CT scan demonstrates bilateral neurofibromas within the psoas muscles (arrows) simulating processes such as psoas abscess, meningocele, or lymphangioma. Note the subtle erosion of the neural foramina, a finding that suggests a spinal canal origin. (b) CT scan obtained in a different patient shows a large neurofibroma (arrow) simulating a psoas abscess.
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Figure 10b. (a) CT scan demonstrates bilateral neurofibromas within the psoas muscles (arrows) simulating processes such as psoas abscess, meningocele, or lymphangioma. Note the subtle erosion of the neural foramina, a finding that suggests a spinal canal origin. (b) CT scan obtained in a different patient shows a large neurofibroma (arrow) simulating a psoas abscess.
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Plexiform neurofibromas within the pelvis often appear as large, extensively infiltrating masses in the presacral or gluteal regions (Fig 11). These tumors often occur extraperitoneally along the axes of major nerves that course through the pelvis and may either appear hypovascular or display intense contrast material enhancement. When they appear hypovascular and have low attenuation, these large, infiltrative masses may simulate diffuse lymphangiomas; when they display intense enhancement, they may mimic soft-tissue sarcomas. MR imaging may be useful in evaluating such lesions when typical findings are observedon T2-weighted images. Ros and Eshagi (17) describe markedly increased signal intensity with multiple hypointense septations as quite specific for plexiform neurofibroma when observed in a diffusely infiltrating pelvic mass at T2-weighted MR imaging. The multiplanar capabilities of MR imaging are also useful in assessing tumor extent.
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Figure 11a. (a) CT scan obtained in a 10-year-old boy shows an infiltrating plexiform neurofibroma with extension into the gluteal region (arrows). Expansion of the sacral foramen is a clue to the diagnosis. (b) CT scan obtained in a different patient demonstrates an extensive presacral plexiform neurofibroma (arrows) simulating sarcoma or lymphangioma.
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Figure 11b. (a) CT scan obtained in a 10-year-old boy shows an infiltrating plexiform neurofibroma with extension into the gluteal region (arrows). Expansion of the sacral foramen is a clue to the diagnosis. (b) CT scan obtained in a different patient demonstrates an extensive presacral plexiform neurofibroma (arrows) simulating sarcoma or lymphangioma.
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Neurofibromas may also arise more focally within the pelvis. These focal tumors may demonstrate varying degrees of enhancement and be mistaken for enlarged lymph nodes due to their appearance and location (Fig 12). Given that major neurovascular bundles travel along the anatomic route of lymphatic pelvic drainage, neurofibromas may mimic lymph nodes in the iliac and obturator chains. In patients with a history of surgery for a malignant neoplasm, neurofibromas may cause concern for local tumor recurrence (Fig 13).
Primary bladder neoplasms are rare in infancy and adolescence. NF1 involvement of nearly every genitourinary structure has been reported, with the bladder being the most commonly involved. Urinary bladder involvement by either common or plexiform neurofibromas is pathognomonic for NF1 (Fig 14). Lesions often exhibit marked contrast material enhancement and may be confused with other benign entities such as hemangiomas, which commonly occur in children. More aggressive lesions may mimic malignant processes that affect the bladder such as leukemia, lymphoma, or rhabdomyosarcoma.
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Figure 14. CT scan shows a plexiform neurofibroma of the urinary bladder (black arrow) simulating a mass such as a hemangioma, lymphoma, or rhabdomyosarcoma. Note also the left perirectal neurofibroma (white arrow) as well as fatty infiltration of the left gluteus muscle due to atrophy.
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Pelvic involvement in NF1 may have a wide spectrum of CT appearances because plexiform neurofibromas can involve a variety of anatomic locations and tend to exhibit a permeative and infiltrative growth pattern. Superficial lesions tend to diffusely involve the subcutaneous tissues and can mimic aggressive infection or cellulitis. Plexiform lesions may extend into the underlying muscle as well, giving the appearance of an aggressive myositis and cellulitis (Fig 15). Limb hemihypertrophy with associated degenerative arthritis, which are commonly seen in NF1, may be helpful in arriving at the correct diagnosis. Tumors invading the perirectal region may cause pain and appear as an inflammatory proctitis (Fig 16). Endoscopy and biopsy may be required to exclude adenocarcinoma and make the correct diagnosis of perirectal plexiform neurofibroma.
Sacral lesions commonly occur in NF1 due either to direct involvement by neurofibromas or to remodeling caused by dural ectasia (18). Simple enlargement of neural foramina is rather characteristic for NF1 but may also be observed in conditions such as Marfan syndrome (19). When bone destruction occurs, lesions often appear more aggressive (Fig 17) and can simulate other sacral lesions such as chordoma, lymphoma, giant cell tumor, and teratoma.
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Figure 17. CT scan obtained in a 31-year-old woman demonstrates neurofibromas causing sacral destruction. The differential diagnosis includes primary bone malignancies such as giant cell tumor, chordoma, lymphoma, and metastasis.
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Neurofibromas may occur in the lower anterior abdominal wall and involve the perineum and genitalia in children. Disease may be diffuse or focal. A neurofibroma arising from the genitofemoral nerve within the inguinal canal may simulate an undescended testis (Fig 18).
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Figure 18. CT scan obtained in an 11-year-old boy with no history of neurofibromatosis shows a neurofibroma of the left spermatic cord (arrow) simulating an undescended testis, a condition that was excluded after careful physical examination.
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Peripheral Involvement
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The classic peripheral manifestations of NF1 include limb hemihypertrophy, pseudarthrosis, peripheral nerve neurofibromas, and subcutaneous common and plexiform neurofibromas. Peripheral nerve tumors may become quite large and resemble primary soft-tissue sarcomas (Fig 19a). When these tumors are multiple, they may simulate rare hypertrophic neuropathies such as Charcot-Marie-Tooth disease or Dejerine-Sottas disease (Fig 19b) (20). When they are multiple and have low attenuation (10–15 HU), they may simulate fluid collections such as abscesses or more diffuse pyomyositis, a disease characterized by multiple, usually staphylococcal intramuscular abscesses that typically occurs in immunosuppressed patients (21). However, once such severe peripheral findings become evident, cutaneous and clinical stigmas make the diagnosis of NF1 obvious in most cases.
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Figure 19a. (a) CT scan demonstrates a large sciatic nerve neurofibroma (arrow) simulating primary or metastatic soft-tissue sarcoma. Scattered low-attenuation lesions represent additional neurofibromas. (b) CT scan obtained in a 29-year-old woman shows neurofibromas simulating multiple areas of myositis or pyomyositis.
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Figure 19b. (a) CT scan demonstrates a large sciatic nerve neurofibroma (arrow) simulating primary or metastatic soft-tissue sarcoma. Scattered low-attenuation lesions represent additional neurofibromas. (b) CT scan obtained in a 29-year-old woman shows neurofibromas simulating multiple areas of myositis or pyomyositis.
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Conclusions
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NF1 is the most common of the phakomatoses and has a variety of localized or, more frequently, systemic manifestations. Up to 10% of patients present later in life with forme fruste or atypical manifestations. Neurofibromas, including plexiform neurofibromas, may have a wide variety of appearances due to the ubiquity of peripheral nerves within the chest, abdomen, pelvis, and extremities. Because of the variable expressivity of NF1, along with its predisposition for spontaneous mutation, many patients present with no preexisting diagnosis or positive family history. Furthermore, this neurocutaneous syndrome may simulate several other disease entities. In some cases, imaging findings are inconclusive, and biopsy and subsequent pathologic analysis are needed to make the correct diagnosis.
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Footnotes
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Abbreviation: NF1 = neurofibromatosis type 1
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References
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