(Radiographics. 2001;21:1141-1154.)
© RSNA, 2001
MR Imaging of Soft-Tissue Masses of the Extraperitoneal Spaces1
Hiroshi Nishimura, MD,
Yan Zhang, MD,
Kazuaki Ohkuma, MD,
Masafumi Uchida, MD,
Naofumi Hayabuchi, MD and
Shiliang Sun, MD
1 From the Department of Radiology, Kurume University School of Medicine, 67 Asahi-Machi, Kurume 830-0011, Japan (H.N., Y.Z., K.O., M.U., N.H.); and the Department of Radiology, University of Iowa College of Medicine, Iowa City (S.S.). Presented as an education exhibit at the 2000 RSNA scientific assembly. Received January 29, 2001; revision requested March 7; final revision received May 25; accepted May 25. Address correspondence to H.N. (e-mail: hnishim@med.kurume-u.ac.jp).
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Abstract
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Magnetic resonance (MR) imaging has an increasing role in evaluating soft-tissue masses of the extraperitoneal spaces. Since the MR imaging features of most soft-tissue masses are nonspecific, prediction of a specific histologic diagnosis remains a challenge for the radiologist. However, there are certain specific MR imaging appearances that are helpful for more accurate diagnosis. Some histologic components, such as myxoid stroma, collagen fibers, calcification, and fat, have characteristic MR imaging features. Recognition of these features can assist the radiologist in limiting the differential diagnosis. Dynamic enhancement patterns can reflect the vascularity of masses and may be useful in diagnosis, especially in differentiating benign from malignant soft-tissue masses. Familiarity with specific signs and knowledge of diagnostic pitfalls are also important for shortening the list of differential diagnoses. Specific signs are the target sign, the bowl of fruit sign, a whorled appearance, a flow void, speckled enhancement, associated lymphadenopathy, and extension into the intervertebral foramen. Diagnostic pitfalls are as follows: a myxoid stroma simulating cystic degeneration and necrosis, collagen fibers simulating hemorrhage, a fat component simulating extraperitoneal fat, extensive intratumoral hemorrhage simulating hematoma, benign masses mimicking malignant ones, malignant masses mimicking benign ones, and peritoneal lesions mimicking extraperitoneal masses.
Index Terms: Retroperitoneal space, MR, 87.1214 • Retroperitoneal space, neoplasms, 87.30 • Soft tissues, MR, 87.1214 • Soft tissues, neoplasms, 87.30
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Introduction
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Soft-tissue masses of the extraperitoneal spaces (eg, abdominal retroperitoneal space, pelvic extraperitoneal space) are rare but include a broad spectrum of pathologic entities (Table 1) (1–3). Although magnetic resonance (MR) imaging has become the premier method for evaluation of soft tissue, a specific histologic diagnosis cannot usually be made due to the considerable overlap in the imaging appearances of various soft-tissue masses (4–6).
This review describes and illustrates the MR imaging findings that can help suggest a specific diagnosis. These findings include (a) the presence of certain histologic components, (b) dynamic enhancement patterns, and (c) specific signs. Diagnostic pitfalls are also discussed. The MR imaging findings described are based on our 10 years of experience with 130 patients.
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Histologic Components versus MR Imaging Findings
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Although the MR imaging appearance of most soft-tissue masses is nonspecific, the presence of certain histologic components (eg, myxoid stroma, collagen fibers, calcification, and fat) can be suggested by evaluating intralesion signal intensity and enhancement patterns. Determination of the dominant histologic component can help narrow the differential diagnosis of the lesion.
Myxoid Stroma
Only a few soft-tissue masses contain a myxoid stroma (Table 2) (1,7–10). The signal intensity of a myxoid stroma is low on T1-weighted images and markedly high on T2-weighted images (11). After injection of gadopentetate dimeglumine, the degree of enhancement depends on the extent of the vascular network within the myxoid stroma. We have found that enhancement is delayed and mild (or absent) in benign masses (Fig 1) and relatively early and pronounced in malignant masses (Fig 2).
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Figure 1a. Ganglioneuroma in a 7-year-old girl. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor in the retroperitoneum, which extends across the midline and surrounds the blood vessels. (b) Axial T2-weighted spin-echo MR image shows curvilinear and linear low-signal-intensity areas (arrows) within the markedly high-signal-intensity tumor, which has a somewhat whorled appearance. The signal intensity features strongly suggest myxoid elements, and a whorled pattern is frequently seen in tumors of neural origin. (c) Coronal contrast material-enhanced dynamic spin-echo MR images show gradual and weak enhancement (pattern 4 [delayed enhancement]). This pattern is usually a relatively good indicator of a benign process. Pre = precontrast. (d) Photograph of the gross specimen shows that the tumor has a translucent appearance as a result of a myxoid stroma. (e) Photomicrograph (original magnification, x40; hematoxylineosin stain) shows interlacing bundles of Schwann cells and collagen fibers (arrows) against the background of the myxoid stroma. (Fig 1c and 1d reprinted, with permission, from reference 12.)
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Figure 1b. Ganglioneuroma in a 7-year-old girl. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor in the retroperitoneum, which extends across the midline and surrounds the blood vessels. (b) Axial T2-weighted spin-echo MR image shows curvilinear and linear low-signal-intensity areas (arrows) within the markedly high-signal-intensity tumor, which has a somewhat whorled appearance. The signal intensity features strongly suggest myxoid elements, and a whorled pattern is frequently seen in tumors of neural origin. (c) Coronal contrast material-enhanced dynamic spin-echo MR images show gradual and weak enhancement (pattern 4 [delayed enhancement]). This pattern is usually a relatively good indicator of a benign process. Pre = precontrast. (d) Photograph of the gross specimen shows that the tumor has a translucent appearance as a result of a myxoid stroma. (e) Photomicrograph (original magnification, x40; hematoxylin-eosin stain) shows interlacing bundles of Schwann cells and collagen fibers (arrows) against the background of the myxoid stroma. (Fig 1c and 1d reprinted, with permission, from reference 12.)
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Figure 1c. Ganglioneuroma in a 7-year-old girl. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor in the retroperitoneum, which extends across the midline and surrounds the blood vessels. (b) Axial T2-weighted spin-echo MR image shows curvilinear and linear low-signal-intensity areas (arrows) within the markedly high-signal-intensity tumor, which has a somewhat whorled appearance. The signal intensity features strongly suggest myxoid elements, and a whorled pattern is frequently seen in tumors of neural origin. (c) Coronal contrast material-enhanced dynamic spin-echo MR images show gradual and weak enhancement (pattern 4 [delayed enhancement]). This pattern is usually a relatively good indicator of a benign process. Pre = precontrast. (d) Photograph of the gross specimen shows that the tumor has a translucent appearance as a result of a myxoid stroma. (e) Photomicrograph (original magnification, x40; hematoxylin-eosin stain) shows interlacing bundles of Schwann cells and collagen fibers (arrows) against the background of the myxoid stroma. (Fig 1c and 1d reprinted, with permission, from reference 12.)
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Figure 1d. Ganglioneuroma in a 7-year-old girl. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor in the retroperitoneum, which extends across the midline and surrounds the blood vessels. (b) Axial T2-weighted spin-echo MR image shows curvilinear and linear low-signal-intensity areas (arrows) within the markedly high-signal-intensity tumor, which has a somewhat whorled appearance. The signal intensity features strongly suggest myxoid elements, and a whorled pattern is frequently seen in tumors of neural origin. (c) Coronal contrast material-enhanced dynamic spin-echo MR images show gradual and weak enhancement (pattern 4 [delayed enhancement]). This pattern is usually a relatively good indicator of a benign process. Pre = precontrast. (d) Photograph of the gross specimen shows that the tumor has a translucent appearance as a result of a myxoid stroma. (e) Photomicrograph (original magnification, x40; hematoxylin-eosin stain) shows interlacing bundles of Schwann cells and collagen fibers (arrows) against the background of the myxoid stroma. (Fig 1c and 1d reprinted, with permission, from reference 12.)
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Figure 1e. Ganglioneuroma in a 7-year-old girl. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor in the retroperitoneum, which extends across the midline and surrounds the blood vessels. (b) Axial T2-weighted spin-echo MR image shows curvilinear and linear low-signal-intensity areas (arrows) within the markedly high-signal-intensity tumor, which has a somewhat whorled appearance. The signal intensity features strongly suggest myxoid elements, and a whorled pattern is frequently seen in tumors of neural origin. (c) Coronal contrast material-enhanced dynamic spin-echo MR images show gradual and weak enhancement (pattern 4 [delayed enhancement]). This pattern is usually a relatively good indicator of a benign process. Pre = precontrast. (d) Photograph of the gross specimen shows that the tumor has a translucent appearance as a result of a myxoid stroma. (e) Photomicrograph (original magnification, x40; hematoxylin-eosin stain) shows interlacing bundles of Schwann cells and collagen fibers (arrows) against the background of the myxoid stroma. (Fig 1c and 1d reprinted, with permission, from reference 12.)
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Figure 2a. Myxoid malignant fibrous histiocytoma in an 89-year-old man. (a) Contrast-enhanced computed tomographic (CT) scan shows a large retroperitoneal tumor with heterogeneous enhancement. (b) Axial T1-weighted spin-echo MR image shows that the tumor has low signal intensity with slightly high-signal-intensity areas from intratumoral hemorrhage. (c) Axial T2-weighted spin-echo MR image shows that the tumor has mixed low, intermediate, and markedly high signal intensity, the so-called bowl of fruit sign, as a result of admixture of solid components, cystic degeneration, hemorrhage, myxoid stroma, and fibrous tissue. This sign is usually an indicator of malignant fibrous histiocytoma. Note the associated bone metastases (arrows). (d) Axial contrast-enhanced dynamic spin-echo MR images show relatively early, strong, and predominantly peripheral enhancement (pattern 3 [early enhancement with slow washout or without obvious washout]). This pattern is usually a relatively good indicator of a malignant process. Pre = precontrast. (Fig 2b and 2c reprinted, with permission, from reference 12.)
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Figure 2b. Myxoid malignant fibrous histiocytoma in an 89-year-old man. (a) Contrast-enhanced computed tomographic (CT) scan shows a large retroperitoneal tumor with heterogeneous enhancement. (b) Axial T1-weighted spin-echo MR image shows that the tumor has low signal intensity with slightly high-signal-intensity areas from intratumoral hemorrhage. (c) Axial T2-weighted spin-echo MR image shows that the tumor has mixed low, intermediate, and markedly high signal intensity, the so-called bowl of fruit sign, as a result of admixture of solid components, cystic degeneration, hemorrhage, myxoid stroma, and fibrous tissue. This sign is usually an indicator of malignant fibrous histiocytoma. Note the associated bone metastases (arrows). (d) Axial contrast-enhanced dynamic spin-echo MR images show relatively early, strong, and predominantly peripheral enhancement (pattern 3 [early enhancement with slow washout or without obvious washout]). This pattern is usually a relatively good indicator of a malignant process. Pre = precontrast. (Fig 2b and 2c reprinted, with permission, from reference 12.)
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Figure 2c. Myxoid malignant fibrous histiocytoma in an 89-year-old man. (a) Contrast-enhanced computed tomographic (CT) scan shows a large retroperitoneal tumor with heterogeneous enhancement. (b) Axial T1-weighted spin-echo MR image shows that the tumor has low signal intensity with slightly high-signal-intensity areas from intratumoral hemorrhage. (c) Axial T2-weighted spin-echo MR image shows that the tumor has mixed low, intermediate, and markedly high signal intensity, the so-called bowl of fruit sign, as a result of admixture of solid components, cystic degeneration, hemorrhage, myxoid stroma, and fibrous tissue. This sign is usually an indicator of malignant fibrous histiocytoma. Note the associated bone metastases (arrows). (d) Axial contrast-enhanced dynamic spin-echo MR images show relatively early, strong, and predominantly peripheral enhancement (pattern 3 [early enhancement with slow washout or without obvious washout]). This pattern is usually a relatively good indicator of a malignant process. Pre = precontrast. (Fig 2b and 2c reprinted, with permission, from reference 12.)
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Figure 2d. Myxoid malignant fibrous histiocytoma in an 89-year-old man. (a) Contrast-enhanced computed tomographic (CT) scan shows a large retroperitoneal tumor with heterogeneous enhancement. (b) Axial T1-weighted spin-echo MR image shows that the tumor has low signal intensity with slightly high-signal-intensity areas from intratumoral hemorrhage. (c) Axial T2-weighted spin-echo MR image shows that the tumor has mixed low, intermediate, and markedly high signal intensity, the so-called bowl of fruit sign, as a result of admixture of solid components, cystic degeneration, hemorrhage, myxoid stroma, and fibrous tissue. This sign is usually an indicator of malignant fibrous histiocytoma. Note the associated bone metastases (arrows). (d) Axial contrast-enhanced dynamic spin-echo MR images show relatively early, strong, and predominantly peripheral enhancement (pattern 3 [early enhancement with slow washout or without obvious washout]). This pattern is usually a relatively good indicator of a malignant process. Pre = precontrast. (Fig 2b and 2c reprinted, with permission, from reference 12.)
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Collagen Fibers
Collagen fibers typically have low signal intensity on T1- and T2-weighted images (Fig 3) (11), which is related in part to the density of collagen fibers. On contrast-enhanced images, the dense parts of collagen fibers demonstrate delayed enhancement. Extraperitoneal soft-tissue masses containing collagen fibers are listed in Table 3 (1,7–10,13,14).
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Figure 3a. Desmoid in a 32-year-old man. Axial T1-weighted (a) and T2-weighted (b) spin-echo MR images show a retroperitoneal tumor that contains hypointense areas. The signal intensity features strongly suggest the dense parts of collagen fibers. The patient had a history of colonic polyposis. This combination of MR imaging findings and medical history is usually an indicator of desmoid associated with Gardner syndrome.
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Figure 3b. Desmoid in a 32-year-old man. Axial T1-weighted (a) and T2-weighted (b) spin-echo MR images show a retroperitoneal tumor that contains hypointense areas. The signal intensity features strongly suggest the dense parts of collagen fibers. The patient had a history of colonic polyposis. This combination of MR imaging findings and medical history is usually an indicator of desmoid associated with Gardner syndrome.
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Calcification
Calcification can be seen in many extraperitoneal soft-tissue masses (Table 4) (1,7–9,11,15,16). CT is superior to MR imaging in detection and characterization of calcification. On MR images, calcifications exhibit markedly decreased signal intensity (Fig 4).
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Figure 4a. Dedifferentiated liposarcoma in a 60-year-old man. (a) CT scan shows a large retroperitoneal tumor with calcification. (b, c) Axial T1-weighted (b) and T2-weighted (c) spin-echo MR images show the calcification as areas of decreased signal intensity. Note the region of very high signal intensity (arrows) in the tumor margin, which simulates the retroperitoneal fat and may be missed. The signal intensity features strongly suggest a fatty tumor. (d) Coronal contrast-enhanced T1-weighted MR image shows a third distinct region at the bottom of the tumor. This region has very high signal intensity on the T2-weighted image (c), which strongly suggests myxoid elements. The structure of the tumor, which has three different components including mature fat, strongly suggests dedifferentiated liposarcoma. MFH = malignant fibrous histiocytoma. (e) Photograph of the gross specimen shows the myxoid liposarcoma juxtaposed with the malignant fibrous histiocytoma (MFH). (The region of well-differentiated liposarcoma is not included in this slice.) Scale is in centimeters.
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Figure 4b. Dedifferentiated liposarcoma in a 60-year-old man. (a) CT scan shows a large retroperitoneal tumor with calcification. (b, c) Axial T1-weighted (b) and T2-weighted (c) spin-echo MR images show the calcification as areas of decreased signal intensity. Note the region of very high signal intensity (arrows) in the tumor margin, which simulates the retroperitoneal fat and may be missed. The signal intensity features strongly suggest a fatty tumor. (d) Coronal contrast-enhanced T1-weighted MR image shows a third distinct region at the bottom of the tumor. This region has very high signal intensity on the T2-weighted image (c), which strongly suggests myxoid elements. The structure of the tumor, which has three different components including mature fat, strongly suggests dedifferentiated liposarcoma. MFH = malignant fibrous histiocytoma. (e) Photograph of the gross specimen shows the myxoid liposarcoma juxtaposed with the malignant fibrous histiocytoma (MFH). (The region of well-differentiated liposarcoma is not included in this slice.) Scale is in centimeters.
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Figure 4c. Dedifferentiated liposarcoma in a 60-year-old man. (a) CT scan shows a large retroperitoneal tumor with calcification. (b, c) Axial T1-weighted (b) and T2-weighted (c) spin-echo MR images show the calcification as areas of decreased signal intensity. Note the region of very high signal intensity (arrows) in the tumor margin, which simulates the retroperitoneal fat and may be missed. The signal intensity features strongly suggest a fatty tumor. (d) Coronal contrast-enhanced T1-weighted MR image shows a third distinct region at the bottom of the tumor. This region has very high signal intensity on the T2-weighted image (c), which strongly suggests myxoid elements. The structure of the tumor, which has three different components including mature fat, strongly suggests dedifferentiated liposarcoma. MFH = malignant fibrous histiocytoma. (e) Photograph of the gross specimen shows the myxoid liposarcoma juxtaposed with the malignant fibrous histiocytoma (MFH). (The region of well-differentiated liposarcoma is not included in this slice.) Scale is in centimeters.
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Figure 4d. Dedifferentiated liposarcoma in a 60-year-old man. (a) CT scan shows a large retroperitoneal tumor with calcification. (b, c) Axial T1-weighted (b) and T2-weighted (c) spin-echo MR images show the calcification as areas of decreased signal intensity. Note the region of very high signal intensity (arrows) in the tumor margin, which simulates the retroperitoneal fat and may be missed. The signal intensity features strongly suggest a fatty tumor. (d) Coronal contrast-enhanced T1-weighted MR image shows a third distinct region at the bottom of the tumor. This region has very high signal intensity on the T2-weighted image (c), which strongly suggests myxoid elements. The structure of the tumor, which has three different components including mature fat, strongly suggests dedifferentiated liposarcoma. MFH = malignant fibrous histiocytoma. (e) Photograph of the gross specimen shows the myxoid liposarcoma juxtaposed with the malignant fibrous histiocytoma (MFH). (The region of well-differentiated liposarcoma is not included in this slice.) Scale is in centimeters.
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Figure 4e. Dedifferentiated liposarcoma in a 60-year-old man. (a) CT scan shows a large retroperitoneal tumor with calcification. (b, c) Axial T1-weighted (b) and T2-weighted (c) spin-echo MR images show the calcification as areas of decreased signal intensity. Note the region of very high signal intensity (arrows) in the tumor margin, which simulates the retroperitoneal fat and may be missed. The signal intensity features strongly suggest a fatty tumor. (d) Coronal contrast-enhanced T1-weighted MR image shows a third distinct region at the bottom of the tumor. This region has very high signal intensity on the T2-weighted image (c), which strongly suggests myxoid elements. The structure of the tumor, which has three different components including mature fat, strongly suggests dedifferentiated liposarcoma. MFH = malignant fibrous histiocytoma. (e) Photograph of the gross specimen shows the myxoid liposarcoma juxtaposed with the malignant fibrous histiocytoma (MFH). (The region of well-differentiated liposarcoma is not included in this slice.) Scale is in centimeters.
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Fat
Demonstration of fat within the lesion on MR images can suggest a specific diagnosis in many cases (Table 5) (1,8,15,16). It is necessary to look for the fatty findings carefully. The signal intensity of fat is markedly high on T1-weighted images, moderately high on T2-weighted images, and low on fat-suppressed images (Figs 4, 5).
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Figure 5a. Malignant mesenchymoma in a 72-year-old woman. (a-c) Axial T1-weighted (a), T2-weighted (b), and short inversion time inversion-recovery (STIR) (c) MR images show a large retroperitoneal tumor of mixed signal intensity. Note the region of well-differentiated liposarcoma (arrow), which has signal intensity similar to that of subcutaneous fat. Two other regions appear as areas of different signal intensity: an area of very high signal intensity on the T2-weighted image (b), which strongly suggests myxoid elements, and an area of slightly high signal intensity, which indicates cystic necrosis. (d) Photograph of the gross specimen shows that the tumor has three components: leiomyosarcoma, myxoid liposarcoma, and well-differentiated liposarcoma. (Fig 5d reprinted, with permission, from reference 12.)
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Figure 5b. Malignant mesenchymoma in a 72-year-old woman. (a-c) Axial T1-weighted (a), T2-weighted (b), and short inversion time inversion-recovery (STIR) (c) MR images show a large retroperitoneal tumor of mixed signal intensity. Note the region of well-differentiated liposarcoma (arrow), which has signal intensity similar to that of subcutaneous fat. Two other regions appear as areas of different signal intensity: an area of very high signal intensity on the T2-weighted image (b), which strongly suggests myxoid elements, and an area of slightly high signal intensity, which indicates cystic necrosis. (d) Photograph of the gross specimen shows that the tumor has three components: leiomyosarcoma, myxoid liposarcoma, and well-differentiated liposarcoma. (Fig 5d reprinted, with permission, from reference 12.)
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Figure 5c. Malignant mesenchymoma in a 72-year-old woman. (a-c) Axial T1-weighted (a), T2-weighted (b), and short inversion time inversion-recovery (STIR) (c) MR images show a large retroperitoneal tumor of mixed signal intensity. Note the region of well-differentiated liposarcoma (arrow), which has signal intensity similar to that of subcutaneous fat. Two other regions appear as areas of different signal intensity: an area of very high signal intensity on the T2-weighted image (b), which strongly suggests myxoid elements, and an area of slightly high signal intensity, which indicates cystic necrosis. (d) Photograph of the gross specimen shows that the tumor has three components: leiomyosarcoma, myxoid liposarcoma, and well-differentiated liposarcoma. (Fig 5d reprinted, with permission, from reference 12.)
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Figure 5d. Malignant mesenchymoma in a 72-year-old woman. (a-c) Axial T1-weighted (a), T2-weighted (b), and short inversion time inversion-recovery (STIR) (c) MR images show a large retroperitoneal tumor of mixed signal intensity. Note the region of well-differentiated liposarcoma (arrow), which has signal intensity similar to that of subcutaneous fat. Two other regions appear as areas of different signal intensity: an area of very high signal intensity on the T2-weighted image (b), which strongly suggests myxoid elements, and an area of slightly high signal intensity, which indicates cystic necrosis. (d) Photograph of the gross specimen shows that the tumor has three components: leiomyosarcoma, myxoid liposarcoma, and well-differentiated liposarcoma. (Fig 5d reprinted, with permission, from reference 12.)
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Dynamic Enhancement Patterns
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Dynamic MR imaging performed after rapid injection of gadopentetate dimeglumine allows assessment of the hemodynamics of masses by demonstrating their enhancement patterns and may be a useful supplement to conventional MR imaging. There are four major patterns of enhancement: pattern 1 = no enhancement, pattern 2 = early enhancement with quick washout, pattern 3 = early enhancement with slow washout (or without obvious washout), and pattern 4 = delayed enhancement. On the basis of our experience and some previous series (12,17–21), pattern 1 is seen in benign masses, such as lipomas, lymphangiomas, cysts, and hematomas. Pattern 2 is also seen in benign masses, such as paragangliomas, angiomyolipomas, and Castleman disease. Pattern 3 is frequently seen in most malignant masses (Fig 2) with the exception of a few benign masses (eg, some paragangliomas, angiomyolipomas, and schwannomas) (Fig 6). Pattern 4 is frequently seen in benign masses (eg, neurogenic tumors [Fig 1], desmoids, hemangiomas, and leiomyomas) with the exception of a few malignant masses (eg, some myxoid liposarcomas, leiomyosarcomas, and malignant lymphomas) (Fig 7).
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Figure 6a. Retroperitoneal paraganglioma in a 53-year-old woman. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor with areas of high signal intensity from intratumoral hemorrhage. (b) Axial STIR MR image shows that the tumor has high signal intensity with areas of slightly low signal intensity. (c) Axial contrast-enhanced dynamic spin-echo MR images show progressive and peripheral enhancement of the tumor from the early phase (pattern 3 [early enhancement with slow washout or without obvious washout]). These findings suggest a hypervascular tumor with intratumoral hemorrhage. This enhancement pattern is usually a relatively good indicator of a malignant process, but benign paragangliomas also have this pattern. Pre = precontrast. (d) Photograph of the gross specimen shows intratumoral hemorrhage, which is seen as areas of high signal intensity on precontrast images.
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Figure 6b. Retroperitoneal paraganglioma in a 53-year-old woman. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor with areas of high signal intensity from intratumoral hemorrhage. (b) Axial STIR MR image shows that the tumor has high signal intensity with areas of slightly low signal intensity. (c) Axial contrast-enhanced dynamic spin-echo MR images show progressive and peripheral enhancement of the tumor from the early phase (pattern 3 [early enhancement with slow washout or without obvious washout]). These findings suggest a hypervascular tumor with intratumoral hemorrhage. This enhancement pattern is usually a relatively good indicator of a malignant process, but benign paragangliomas also have this pattern. Pre = precontrast. (d) Photograph of the gross specimen shows intratumoral hemorrhage, which is seen as areas of high signal intensity on precontrast images.
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Figure 6c. Retroperitoneal paraganglioma in a 53-year-old woman. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor with areas of high signal intensity from intratumoral hemorrhage. (b) Axial STIR MR image shows that the tumor has high signal intensity with areas of slightly low signal intensity. (c) Axial contrast-enhanced dynamic spin-echo MR images show progressive and peripheral enhancement of the tumor from the early phase (pattern 3 [early enhancement with slow washout or without obvious washout]). These findings suggest a hypervascular tumor with intratumoral hemorrhage. This enhancement pattern is usually a relatively good indicator of a malignant process, but benign paragangliomas also have this pattern. Pre = precontrast. (d) Photograph of the gross specimen shows intratumoral hemorrhage, which is seen as areas of high signal intensity on precontrast images.
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Figure 6d. Retroperitoneal paraganglioma in a 53-year-old woman. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor with areas of high signal intensity from intratumoral hemorrhage. (b) Axial STIR MR image shows that the tumor has high signal intensity with areas of slightly low signal intensity. (c) Axial contrast-enhanced dynamic spin-echo MR images show progressive and peripheral enhancement of the tumor from the early phase (pattern 3 [early enhancement with slow washout or without obvious washout]). These findings suggest a hypervascular tumor with intratumoral hemorrhage. This enhancement pattern is usually a relatively good indicator of a malignant process, but benign paragangliomas also have this pattern. Pre = precontrast. (d) Photograph of the gross specimen shows intratumoral hemorrhage, which is seen as areas of high signal intensity on precontrast images.
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Figure 7. Leiomyosarcoma of the pelvic extraperitoneal space in a 60-year-old woman. Axial contrast-enhanced dynamic spin-echo MR images show a tumor without early enhancement but with gradually increasing enhancement (pattern 4 [delayed enhancement]). This pattern is usually a relatively good indicator of a benign process. Although malignant masses with this pattern are very rare, it will be possible to narrow the differential diagnosis if the tumor demonstrates other malignant findings. Pre = precontrast.
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Specific Signs
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Target Sign
The target sign consists of a central area of low to intermediate signal intensity surrounded by a ring of high signal intensity on T2-weighted images (Fig 8). At histopathologic analysis, this finding corresponds to fibrous tissue centrally and myxoid tissue peripherally and is commonly seen in neurofibroma and schwannoma (22).
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Figure 8. Neurofibroma of the paraspinal region in an 11-year-old girl. Coronal T2-weighted spin-echo MR image shows a plexiform mass and numerous instances of the target sign, with a central part of moderately high signal intensity surrounded by a rim of extremely high signal intensity (arrows). The shape of the mass and the presence of this characteristic sign are suggestive of plexiform neurofibroma. (Reprinted, with permission, from reference 12.)
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Bowl of Fruit Sign
The bowl of fruit sign is a mosaic of mixed low, intermediate, and high signal intensity on T2-weighted images as a result of admixture of solid components, cystic degeneration, hemorrhage, myxoid stroma, and fibrous tissue (Fig 2c). This finding is commonly seen in malignant fibrous histiocytoma, synovial sarcoma, and Ewing sarcoma.
Whorled Appearance
A whorled appearance consists of linear or curvilinear structures of low signal intensity on T2-weighted images (Fig 1b). At histopathologic analysis, this finding corresponds to bundles of Schwann cells and collagen fibers within the mass (Fig 1e) and is commonly seen in ganglioneuroma and neurofibroma (7).
Flow Void
A flow void is commonly seen in hemangiopericytoma (Fig 9), arteriovenous hemangioma, and alveolar soft-part sarcoma (11,23).
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Figure 9a. Hemangiopericytoma of the pelvic extraperitoneal space in a 30-year-old woman. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor with a region of high signal intensity from intratumoral hemorrhage. (b) Axial T2-weighted spin-echo MR image shows that the tumor has marked high signal intensity with flow voids in the peripheral and central regions. (c) Axial contrast-enhanced T1-weighted MR image shows marked enhancement of the tumor. These findings, which represent some typical features of a richly vascularized tumor, are characteristic of hemangiopericytoma. (d) Early-phase arteriogram shows the highly vascularized mass. (Reprinted, with permission, from reference 12.)
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Figure 9b. Hemangiopericytoma of the pelvic extraperitoneal space in a 30-year-old woman. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor with a region of high signal intensity from intratumoral hemorrhage. (b) Axial T2-weighted spin-echo MR image shows that the tumor has marked high signal intensity with flow voids in the peripheral and central regions. (c) Axial contrast-enhanced T1-weighted MR image shows marked enhancement of the tumor. These findings, which represent some typical features of a richly vascularized tumor, are characteristic of hemangiopericytoma. (d) Early-phase arteriogram shows the highly vascularized mass. (Reprinted, with permission, from reference 12.)
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Figure 9c. Hemangiopericytoma of the pelvic extraperitoneal space in a 30-year-old woman. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor with a region of high signal intensity from intratumoral hemorrhage. (b) Axial T2-weighted spin-echo MR image shows that the tumor has marked high signal intensity with flow voids in the peripheral and central regions. (c) Axial contrast-enhanced T1-weighted MR image shows marked enhancement of the tumor. These findings, which represent some typical features of a richly vascularized tumor, are characteristic of hemangiopericytoma. (d) Early-phase arteriogram shows the highly vascularized mass. (Reprinted, with permission, from reference 12.)
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Figure 9d. Hemangiopericytoma of the pelvic extraperitoneal space in a 30-year-old woman. (a) Axial T1-weighted spin-echo MR image shows a low-signal-intensity tumor with a region of high signal intensity from intratumoral hemorrhage. (b) Axial T2-weighted spin-echo MR image shows that the tumor has marked high signal intensity with flow voids in the peripheral and central regions. (c) Axial contrast-enhanced T1-weighted MR image shows marked enhancement of the tumor. These findings, which represent some typical features of a richly vascularized tumor, are characteristic of hemangiopericytoma. (d) Early-phase arteriogram shows the highly vascularized mass. (Reprinted, with permission, from reference 12.)
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Speckled Enhancement
Speckled enhancement on contrast-enhanced T1-weighted images corresponds to an intratumoral septumlike structure at histopathologic analysis. In our experience, speckled enhancement is frequently seen in malignant lymphoma, leiomyosarcoma (Fig 10), and rhabdomyosarcoma.
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Figure 10a. Leiomyosarcoma in a 39-year-old woman. (a) Axial contrast-enhanced T1-weighted MR image shows a retroperitoneal tumor with speckled enhancement, which is characteristic of leiomyosarcoma, rhabdomyosarcoma, and malignant lymphoma. We were able to make an accurate diagnosis based on a combination of other findings (patient age, tumor location, and signal intensity on T2-weighted images). (b) Photograph of the gross specimen shows septumlike structures within the tumor, which correspond to the speckled enhancement.
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Figure 10b. Leiomyosarcoma in a 39-year-old woman. (a) Axial contrast-enhanced T1-weighted MR image shows a retroperitoneal tumor with speckled enhancement, which is characteristic of leiomyosarcoma, rhabdomyosarcoma, and malignant lymphoma. We were able to make an accurate diagnosis based on a combination of other findings (patient age, tumor location, and signal intensity on T2-weighted images). (b) Photograph of the gross specimen shows septumlike structures within the tumor, which correspond to the speckled enhancement.
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Associated Lymphadenopathy
Associated lymphadenopathy is commonly observed in malignant lymphoma, neuroblastoma, malignant fibrous histiocytoma, and rhabdomyosarcoma (1,10,24).
Extension into Intervertebral Foramen
Extension into the intervertebral foramen suggests a neurogenic neoplasm (Fig 11).
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Figure 11. Schwannoma in a 42-year-old woman. Axial contrast-enhanced T1-weighted MR image shows a large tumor with extensive cystic degeneration in the pelvic extraperitoneal space. The tumor appears to arise from the left sacral foramen (arrow). Such extension is frequently seen in tumors of neural origin.
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Diagnostic Pitfalls
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There are several pitfalls in diagnosis of extraperitoneal soft-tissue masses:
- A myxoid stroma can have high signal intensity on T2-weighted images and may simulate cystic degeneration and necrosis. However, a myxoid stroma usually demonstrates enhancement, particularly in malignant masses.
- Collagen fibers can have low signal intensity on T2-weighted images and may resemble hemorrhage.
- A fatty component in the margin of a mass simulates extraperitoneal fat and may be missed (Fig 4).
- Extensive intratumoral hemorrhage, which frequently occurs in malignant fibrous histiocytoma, may simulate hematoma. Other tumor components may be pushed to the mass margin by the hemorrhage; this finding will help avoid a false diagnosis (Fig 12) (9,10,25).
- Some benign masses can mimic malignant masses. For example, some schwannomas and paragangliomas show relatively marked degeneration and demonstrate early enhancement on dynamic images. Some ganglioneuromas can extend across the abdominal midline and involve the greater blood vessels (Fig 1). These findings are usually seen in malignancies.
- Some malignant masses can mimic benign masses. For example, some malignant lymphomas demonstrate homogeneous signal intensity and gradual enhancement. Some myxoid liposarcomas do not enhance and thus simulate cysts. Well-differentiated liposarcomas usually have the same signal intensity as lipomas. However, well-differentiated liposarcomas are usually larger than 5 cm in diameter (Fig 13).
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Figure 12a. Malignant fibrous histiocytoma in a 48-year-old man. (a) Axial T1-weighted spin-echo MR image shows a retroperitoneal tumor with extensive intratumoral hemorrhage. The solid component of the tumor (arrows) is pushed ventrally and may be missed; the tumor will then simulate a hematoma. (b) Axial contrast-enhanced T1-weighted MR image shows heterogeneous enhancement of the solid component. Such extensive hemorrhage and significant enlargement of the mass are occasionally seen in malignant fibrous histiocytoma. (Reprinted, with permission, from reference 12.)
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Figure 12b. Malignant fibrous histiocytoma in a 48-year-old man. (a) Axial T1-weighted spin-echo MR image shows a retroperitoneal tumor with extensive intratumoral hemorrhage. The solid component of the tumor (arrows) is pushed ventrally and may be missed; the tumor will then simulate a hematoma. (b) Axial contrast-enhanced T1-weighted MR image shows heterogeneous enhancement of the solid component. Such extensive hemorrhage and significant enlargement of the mass are occasionally seen in malignant fibrous histiocytoma. (Reprinted, with permission, from reference 12.)
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Figure 13a. Well-differentiated liposarcoma in a 72-year-old woman. Axial T1-weighted (a), T2-weighted (b), and STIR (c) and sagittal T1-weighted (d) MR images show a large retroperitoneal tumor of predominantly fatty signal intensity, which simulates a lipoma. Note the multiple and irregular thick septa within the tumor, which have low signal intensity on the T1-weighted images (a, d), low or high signal intensity on the T2-weighted image (b), and high signal intensity on the STIR image (c). This finding is characteristic of well-differentiated liposarcoma. (Reprinted, with permission, from reference 12.)
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Figure 13b. Well-differentiated liposarcoma in a 72-year-old woman. Axial T1-weighted (a), T2-weighted (b), and STIR (c) and sagittal T1-weighted (d) MR images show a large retroperitoneal tumor of predominantly fatty signal intensity, which simulates a lipoma. Note the multiple and irregular thick septa within the tumor, which have low signal intensity on the T1-weighted images (a, d), low or high signal intensity on the T2-weighted image (b), and high signal intensity on the STIR image (c). This finding is characteristic of well-differentiated liposarcoma. (Reprinted, with permission, from reference 12.)
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Figure 13c. Well-differentiated liposarcoma in a 72-year-old woman. Axial T1-weighted (a), T2-weighted (b), and STIR (c) and sagittal T1-weighted (d) MR images show a large retroperitoneal tumor of predominantly fatty signal intensity, which simulates a lipoma. Note the multiple and irregular thick septa within the tumor, which have low signal intensity on the T1-weighted images (a, d), low or high signal intensity on the T2-weighted image (b), and high signal intensity on the STIR image (c). This finding is characteristic of well-differentiated liposarcoma. (Reprinted, with permission, from reference 12.)
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Figure 13d. Well-differentiated liposarcoma in a 72-year-old woman. Axial T1-weighted (a), T2-weighted (b), and STIR (c) and sagittal T1-weighted (d) MR images show a large retroperitoneal tumor of predominantly fatty signal intensity, which simulates a lipoma. Note the multiple and irregular thick septa within the tumor, which have low signal intensity on the T1-weighted images (a, d), low or high signal intensity on the T2-weighted image (b), and high signal intensity on the STIR image (c). This finding is characteristic of well-differentiated liposarcoma. (Reprinted, with permission, from reference 12.)
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Finally, certain peritoneal lesions simulate soft-tissue masses of the extraperitoneal spaces. They mainly include extraperitoneal tumors derived from embryonic rests (eg, seminoma, embryonal cell carcinoma, choriocarcinoma, dysgerminoma, and teratoma) (26); mesenteric tumors; and gastrointestinal tumors (eg, poorly differentiated adenocarcinoma [Fig 14], mucinous adenocarcinoma, leiomyosarcoma, and malignant lymphoma).
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Figure 14a. Poorly differentiated colon cancer in a 78-year-old woman. (a-c) Axial T1-weighted (a), T2-weighted (b), and contrast-enhanced T1-weighted (c) MR images show a large retroperitoneal tumor with irregular central necrosis. These findings suggest a malignant tumor. (d) Axial contrast-enhanced dynamic spin-echo MR images show gradually increasing enhancement of the tumor (pattern 4 [delayed enhancement]]). This pattern is usually a relatively good indicator of a benign process, but leiomyosarcomas and poorly differentiated adenocarcinomas of the colon also sometimes have this pattern. Owing to the lack of ileus in this case, it is difficult to differentiate the tumor from a leiomyosarcoma. Pre = precontrast. (Fig 14a-14c reprinted, with permission, from reference 12.)
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Figure 14b. Poorly differentiated colon cancer in a 78-year-old woman. (a-c) Axial T1-weighted (a), T2-weighted (b), and contrast-enhanced T1-weighted (c) MR images show a large retroperitoneal tumor with irregular central necrosis. These findings suggest a malignant tumor. (d) Axial contrast-enhanced dynamic spin-echo MR images show gradually increasing enhancement of the tumor (pattern 4 [delayed enhancement]]). This pattern is usually a relatively good indicator of a benign process, but leiomyosarcomas and poorly differentiated adenocarcinomas of the colon also sometimes have this pattern. Owing to the lack of ileus in this case, it is difficult to differentiate the tumor from a leiomyosarcoma. Pre = precontrast. (Fig 14a-14c reprinted, with permission, from reference 12.)
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Figure 14c. Poorly differentiated colon cancer in a 78-year-old woman. (a-c) Axial T1-weighted (a), T2-weighted (b), and contrast-enhanced T1-weighted (c) MR images show a large retroperitoneal tumor with irregular central necrosis. These findings suggest a malignant tumor. (d) Axial contrast-enhanced dynamic spin-echo MR images show gradually increasing enhancement of the tumor (pattern 4 [delayed enhancement]]). This pattern is usually a relatively good indicator of a benign process, but leiomyosarcomas and poorly differentiated adenocarcinomas of the colon also sometimes have this pattern. Owing to the lack of ileus in this case, it is difficult to differentiate the tumor from a leiomyosarcoma. Pre = precontrast. (Fig 14a-14c reprinted, with permission, from reference 12.)
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Figure 14d. Poorly differentiated colon cancer in a 78-year-old woman. (a-c) Axial T1-weighted (a), T2-weighted (b), and contrast-enhanced T1-weighted (c) MR images show a large retroperitoneal tumor with irregular central necrosis. These findings suggest a malignant tumor. (d) Axial contrast-enhanced dynamic spin-echo MR images show gradually increasing enhancement of the tumor (pattern 4 [delayed enhancement]]). This pattern is usually a relatively good indicator of a benign process, but leiomyosarcomas and poorly differentiated adenocarcinomas of the colon also sometimes have this pattern. Owing to the lack of ileus in this case, it is difficult to differentiate the tumor from a leiomyosarcoma. Pre = precontrast. (Fig 14a-14c reprinted, with permission, from reference 12.)
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Conclusions
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There is a broad spectrum of soft-tissue masses that arise in the extraperitoneal spaces. Although most of these masses have a nonspecific appearance, familiarity with the MR imaging features of their histologic components and the dynamic enhancement patterns of masses, coupled with knowledge of specific signs and diagnostic pitfalls, will permit more accurate diagnosis.
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Footnotes
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Abbreviation: STIR = short inversion time inversion recovery
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Abstract
Introduction
