DOI: 10.1148/rg.231025037
(Radiographics. 2003;23:45-57.)
© RSNA, 2003
Primary Retroperitoneal Neoplasms: CT and MR Imaging Findings with Anatomic and Pathologic Diagnostic Clues1
Mizuki Nishino, MD,
Katsumi Hayakawa, MD,
Manabu Minami, MD,
Akira Yamamoto, MD,
Hiroyuki Ueda, MD and
Kosho Takasu, MD
1 From the Departments of Radiology (M.N., K.H.) and Pathology (K.T.), Kyoto City Hospital, 1-2 Higashi-takada-cho, Mibu, Nakagyo-ku, Kyoto 604-8845, Japan; the Department of Radiology, University of Tokyo, Japan (M.M.); and the Departments of Nuclear Medicine and Diagnostic Imaging, Graduate School of Medicine, Kyoto University, Kyoto, Japan (A.Y., H.U.). Presented as an education exhibit at the 2001 RSNA scientific assembly. Received February 26, 2002; revision requested April 26 and received May 21; accepted May 30. Address correspondence to M.N. (e-mail: mizuki@mbox.kyoto-inet.or.jp).
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Abstract
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Primary retroperitoneal neoplasms are a rare but diverse group of benign and malignant tumors that arise within the retroperitoneal space but outside the major organs in this space. Although computed tomography and magnetic resonance imaging can demonstrate important characteristics of these tumors, diagnosis is often challenging for radiologists. Diagnostic challenges include precise localization of the lesion, determination of the extent of invasion, and characterization of the specific pathologic type. The first step is to determine whether the tumor is located within the retroperitoneal space. Displacement of normal anatomic structures of the retroperitoneum is helpful in this regard. For tumors that are located within the retroperitoneum, the next step is to identify the organ of origin. Specific signs, including the "beak sign," the "embedded organ sign," and the "phantom (invisible) organ sign," are useful for this purpose. When there is no definite sign that suggests the organ of origin, the diagnosis of a primary retroperitoneal tumor becomes likely. Awareness of specific patterns of spread, specific tumor components, and tumor vascularity help in further narrowing the differential diagnosis. Attention to these diagnostic clues is essential in making an accurate radiologic diagnosis of primary retroperitoneal tumors and in obtaining clinically significant information.
© RSNA, 2003
Index Terms: Retroperitoneal space, CT, 80.1211 • Retroperitoneal space, MR, 80.1214 • Retroperitoneal space, neoplasms, 80.31, 80.32
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Determine whether a tumor is located in the retroperitoneal space.
- Identify the organ of origin of a retroperitoneal tumor.
- Describe specific imaging findings in a variety of retroperitoneal tumors.
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Introduction
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Primary retroperitoneal neoplasms are a diverse group of benign and malignant tumors that arise within the retroperitoneum but outside the major organs. Diagnosis of these tumors is often challenging for radiologists and consists of several steps, including determining tumor location (characterizing the retroperitoneal space and identifying the organ of origin) and recognizing specific features of various retroperitoneal tumors (evaluating patterns of spread, tumor components, and vascularity). In this article, we systematically describe and illustrate these important clues in the diagnosis of primary retroperitoneal tumors.
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Tumor Location
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Characterization of the Retroperitoneal Space
The first step is to decide whether the tumor is located within the retroperitoneal space. It is useful to observe the displacement of normal anatomic structures (1). Anterior displacement of retroperitoneal organs (eg, kidneys, adrenal glands, ureters, ascending and descending colon, pancreas, portions of the duodenum) strongly suggests that the tumor arises in the retroperitoneum (Fig 1). Major vessels and some of their branches are also found in the retroperitoneal cavity, so that displacement of these vessels can be helpful as well (Figs 2, 3).
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Figure 1. Anterior displacement of the ascending colon. Computed tomographic (CT) scan shows a bulky mass that is difficult to localize at first glance. However, anterior displacement of the ascending colon (arrow) confirms that the mass is in the retroperitoneal space. The mass proved to be liposarcoma.
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Figure 2. Anterior displacement of the aorta. Axial T2-weighted magnetic resonance (MR) image of the abdomen shows a homogeneous, hypointense mass that surrounds the aorta and displaces it anteriorly. The mass proved to be lymphoma.
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Figure 3a. Anterior displacement of the inferior mesenteric vein. (a) Transverse contrast material-enhanced CT scan shows a well-defined enhancing mass that is posterior to the pancreas but lacks the mass effect to compress adjacent major organs. (b) CT scan shows anterior displacement of the inferior mesenteric vein (arrow), a finding that confirms that the mass is located in the retroperitoneum. The mass proved to be capillary hemangioma.
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Figure 3b. Anterior displacement of the inferior mesenteric vein. (a) Transverse contrast material-enhanced CT scan shows a well-defined enhancing mass that is posterior to the pancreas but lacks the mass effect to compress adjacent major organs. (b) CT scan shows anterior displacement of the inferior mesenteric vein (arrow), a finding that confirms that the mass is located in the retroperitoneum. The mass proved to be capillary hemangioma.
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Identification of the Organ of Origin
Before a tumor can be described as primarily retroperitoneal, the possibility that the tumor originates from a retroperitoneal organ must be excluded. Some radiologic signs that are helpful in determining tumor origin include the "beak sign," the "phantom (invisible) organ sign," the "embedded organ sign," and the "prominent feeding artery sign" (2). When there is no definite sign that suggests an organ of origin, the diagnosis of primary retroperitoneal tumor becomes likely.
Beak Sign.—
When a mass deforms the edge of an adjacent organ into a "beak" shape, it is likely that the mass arises from that organ (beak sign) (Fig 4a, 4c). On the other hand, an adjacent organ with dull edges suggests that the tumor compresses the organ but does not arise from it (Fig 4b) (2).
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Figure 4a. Beak sign. (a, b) Drawings illustrate the positive beak sign (a), in which Tumor A arises from Organ B, and the negative beak sign (b), in which Tumor A does not arise from Organ B. (c) Transverse contrast-enhanced CT scan shows a huge cystic tumor with the beak sign (arrow) in its contact surface with the pancreas. This finding represents mucinous cystadenocarcinoma of the pancreas.
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Figure 4b. Beak sign. (a, b) Drawings illustrate the positive beak sign (a), in which Tumor A arises from Organ B, and the negative beak sign (b), in which Tumor A does not arise from Organ B. (c) Transverse contrast-enhanced CT scan shows a huge cystic tumor with the beak sign (arrow) in its contact surface with the pancreas. This finding represents mucinous cystadenocarcinoma of the pancreas.
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Figure 4c. Beak sign. (a, b) Drawings illustrate the positive beak sign (a), in which Tumor A arises from Organ B, and the negative beak sign (b), in which Tumor A does not arise from Organ B. (c) Transverse contrast-enhanced CT scan shows a huge cystic tumor with the beak sign (arrow) in its contact surface with the pancreas. This finding represents mucinous cystadenocarcinoma of the pancreas.
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Phantom (Invisible) Organ Sign.—
When a large mass arises from a small organ, the organ sometimes becomes undetectable. This is known as the phantom organ sign. However, false-positive findings do exist, as in cases of huge retroperitoneal sarcomas that involve other small organs such as the adrenal gland (2).
Embedded Organ Sign.—
When a tumor compresses an adjacent plastic organ (eg, gastrointestinal tract, inferior vena cava) that is not the organ of origin, the organ is deformed into a crescent shape (Fig 5a). In contrast, when part of an organ appears to be embedded in the tumor (negative embedded organ sign) (Fig 5b, 5c), the tumor is in close contact with the organ and the contact surface is typically sclerotic with desmoplastic reaction. Occasionally, the contact surface becomes ulcerative. When the embedded organ sign is present, it is likely that the tumor originates from the involved organ (2).
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Figure 5a. Embedded organ sign. (a, b) Drawings illustrate the negative embedded organ sign (a), in which Tumor A simply compresses Organ B, and the positive embedded organ sign (b), in which Tumor A arises from Organ B so that the organ appears to be embedded in the tumor. (c) CT scan shows a huge heterogeneous mass. The lumen of the duodenum is stretched toward the mass, and the wall of the duodenum appears embedded in the mass at the contact surface (arrow). These findings represent gastrointestinal stromal tumor of the duodenum with a positive embedded organ sign. (Fig 5c courtesy of K. Togashi, MD, Department of Diagnostic and Interventional Imageology, Kyoto University, Kyoto, Japan.)
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Figure 5b. Embedded organ sign. (a, b) Drawings illustrate the negative embedded organ sign (a), in which Tumor A simply compresses Organ B, and the positive embedded organ sign (b), in which Tumor A arises from Organ B so that the organ appears to be embedded in the tumor. (c) CT scan shows a huge heterogeneous mass. The lumen of the duodenum is stretched toward the mass, and the wall of the duodenum appears embedded in the mass at the contact surface (arrow). These findings represent gastrointestinal stromal tumor of the duodenum with a positive embedded organ sign. (Fig 5c courtesy of K. Togashi, MD, Department of Diagnostic and Interventional Imageology, Kyoto University, Kyoto, Japan.)
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Figure 5c. Embedded organ sign. (a, b) Drawings illustrate the negative embedded organ sign (a), in which Tumor A simply compresses Organ B, and the positive embedded organ sign (b), in which Tumor A arises from Organ B so that the organ appears to be embedded in the tumor. (c) CT scan shows a huge heterogeneous mass. The lumen of the duodenum is stretched toward the mass, and the wall of the duodenum appears embedded in the mass at the contact surface (arrow). These findings represent gastrointestinal stromal tumor of the duodenum with a positive embedded organ sign. (Fig 5c courtesy of K. Togashi, MD, Department of Diagnostic and Interventional Imageology, Kyoto University, Kyoto, Japan.)
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Prominent Feeding Artery Sign.—
Hypervascular masses are often supplied by feeding arteries that are prominent enough to be visualized at CT or MR imaging, a finding that provides an important key to understanding the origin of the mass (2).
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Imaging Features That May Complicate Differential Diagnosis
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Familiarity with the specific features of various retroperitoneal tumors often allows accurate histologic diagnosis and helps suggest proper management.
Specific Patterns of Spread
Some retroperitoneal tumors have specific patterns of growth and extension that aid in narrowing the differential diagnosis.
Lesions That Extend Between Normal Structures.—
Some tumors grow and extend into spaces between preexisting structures and surround vessels without compressing their lumina. Lymphangiomas and ganglioneuromas are examples of such tumors.
Lymphangiomas represent about 1% of all retroperitoneal neoplasms (3). Most cases are detected in the first 2 years of life on the basis of symptoms like abdominal distention or pain; however, they can manifest in older patients as a huge, asymptomatic mass (Fig 6) (4). At imaging, they appear as fluid-filled, unilocular or multilocular cystic masses with minimal contrast enhancement. Complete excision is the treatment of choice but is often difficult and has high rates of complication and recurrence (4).
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Figure 6a. Lymphangioma in a 47-year-old woman. (a, b) Transverse contrast-enhanced abdominal (a) and pelvic (b) CT scans show a multiloculated, low-attenuation cystic mass that extends between normal anatomic structures in the peritoneal cavity and retroperitoneum. (c) Postoperative photograph shows the multiloculated cystic mass. Surgery revealed that the mass extended around the transverse mesocolon, the pancreas, and the posterior wall of the cecum and contained chylous fluid. (d) Photomicrograph (original magnification, x100; hematoxylin-eosin [H-E] stain) of the mass demonstrates endothelium-lined cavities.
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Figure 6b. Lymphangioma in a 47-year-old woman. (a, b) Transverse contrast-enhanced abdominal (a) and pelvic (b) CT scans show a multiloculated, low-attenuation cystic mass that extends between normal anatomic structures in the peritoneal cavity and retroperitoneum. (c) Postoperative photograph shows the multiloculated cystic mass. Surgery revealed that the mass extended around the transverse mesocolon, the pancreas, and the posterior wall of the cecum and contained chylous fluid. (d) Photomicrograph (original magnification, x100; hematoxylin-eosin [H-E] stain) of the mass demonstrates endothelium-lined cavities.
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Figure 6c. Lymphangioma in a 47-year-old woman. (a, b) Transverse contrast-enhanced abdominal (a) and pelvic (b) CT scans show a multiloculated, low-attenuation cystic mass that extends between normal anatomic structures in the peritoneal cavity and retroperitoneum. (c) Postoperative photograph shows the multiloculated cystic mass. Surgery revealed that the mass extended around the transverse mesocolon, the pancreas, and the posterior wall of the cecum and contained chylous fluid. (d) Photomicrograph (original magnification, x100; hematoxylin-eosin [H-E] stain) of the mass demonstrates endothelium-lined cavities.
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Figure 6d. Lymphangioma in a 47-year-old woman. (a, b) Transverse contrast-enhanced abdominal (a) and pelvic (b) CT scans show a multiloculated, low-attenuation cystic mass that extends between normal anatomic structures in the peritoneal cavity and retroperitoneum. (c) Postoperative photograph shows the multiloculated cystic mass. Surgery revealed that the mass extended around the transverse mesocolon, the pancreas, and the posterior wall of the cecum and contained chylous fluid. (d) Photomicrograph (original magnification, x100; hematoxylin-eosin [H-E] stain) of the mass demonstrates endothelium-lined cavities.
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Another entity with this growth pattern is lymphoma. This neoplasm tends to surround adjacent vessels, manifesting with the "CT angiogram sign" or "floating aorta sign" (Fig 7) (5).
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Figure 7. Lymphoma in a 72-year-old woman. On a transverse contrast-enhanced CT scan, a homogeneous mass with minimal enhancement is noted around the aorta. Contrast-enhanced vessels are seen to penetrate the mass. Histologic analysis revealed B- cell lymphoma.
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Lesions That Extend along Normal Structures.—
Tumors of the sympathetic ganglia (ie, paragangliomas, ganglioneuromas) tend to extend along the sympathetic chain and have an elongated shape (6).
Characteristic Tumor Components
Some tumor contents can be clearly demonstrated at CT and MR imaging and provide strong clues that help narrow the differential diagnosis.
Fat.—
The presence of fat is easily recognized owing to its high attenuation at CT or its high signal intensity at T1-weighted MR imaging with loss of signal intensity on fat-suppressed images. The presence (or absence) of fat limits the differential diagnosis (7).
A mass that is homogeneous and well defined and consists almost entirely of fat represents lipoma (Fig 8) (6).
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Figure 8a. Lipoma. (a) Abdominal radiograph shows a huge radiolucent mass. (b) Transverse CT scan of the abdomen shows that the mass is composed primarily of fat. (c) On a T1-weighted MR image, the mass has homogeneous high signal intensity and compresses the kidney upward.
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Figure 8b. Lipoma. (a) Abdominal radiograph shows a huge radiolucent mass. (b) Transverse CT scan of the abdomen shows that the mass is composed primarily of fat. (c) On a T1-weighted MR image, the mass has homogeneous high signal intensity and compresses the kidney upward.
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Figure 8c. Lipoma. (a) Abdominal radiograph shows a huge radiolucent mass. (b) Transverse CT scan of the abdomen shows that the mass is composed primarily of fat. (c) On a T1-weighted MR image, the mass has homogeneous high signal intensity and compresses the kidney upward.
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When the mass is somewhat irregular and ill-defined but contains fat, the diagnosis of liposarcoma should be considered. Liposarcomas are the most common sarcomas of the retroperitoneum (8,9). They typically occur in the 5th and 6th decades of life and have a slight female predilection. They are classified at pathologic analysis into well-differentiated, pleomorphic, myxoid, and dedifferentiated types. Well-differentiated liposarcomas usually contain an appreciable amount of fat (Fig 9), whereas high-grade liposarcomas may not demonstrate appreciable fat, thereby appearing similar to other sarcomas (9).
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Figure 9a. Well-differentiated liposarcoma in a 49-year-old woman. (a) Transverse contrast-enhanced CT scan demonstrates a huge heterogeneous mass with predominantly fat attenuation. (b) On a T1-weighted MR image, the mass has heterogeneous hyperintensity and a partly nodular appearance.
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Figure 9b. Well-differentiated liposarcoma in a 49-year-old woman. (a) Transverse contrast-enhanced CT scan demonstrates a huge heterogeneous mass with predominantly fat attenuation. (b) On a T1-weighted MR image, the mass has heterogeneous hyperintensity and a partly nodular appearance.
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Teratomas are also characterized by the presence of fat (Fig 10), and mature teratomas can be characterized by the presence of fluid attenuation or signal intensity, fat-fluid levels, and calcifications (1).
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Figure 10a. Mature cystic teratoma in a female infant. (a) Transverse CT scan of the abdomen shows a fat-containing cystic mass with calcifications. A diagnosis of mature teratoma was suggested preoperatively. (b) Coronal T1-weighted MR image shows that the left kidney is displaced downward into the pelvis. The kidney is clearly separate from the tumor.
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Figure 10b. Mature cystic teratoma in a female infant. (a) Transverse CT scan of the abdomen shows a fat-containing cystic mass with calcifications. A diagnosis of mature teratoma was suggested preoperatively. (b) Coronal T1-weighted MR image shows that the left kidney is displaced downward into the pelvis. The kidney is clearly separate from the tumor.
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Myxoid Stroma.—
A limited number of tumors commonly contain myxoid stroma; consequently, the presence of myxoid stroma helps narrow the differential diagnosis. Myxoid stroma is characterized pathologically by a mucoid matrix that is rich in acid mucopolysaccharides (8). Myxoid stroma appears hyperintense on T2-weighted MR images and shows delayed enhancement after injection of contrast medium (10). Tumors that commonly contain myxoid stroma include neurogenic tumors (schwannomas, neurofibromas, ganglioneuromas, ganglioneuroblastomas, malignant peripheral nerve sheath tumors), myxoid liposarcomas, and myxoid malignant fibrous histiocytoma (10).
Among neurogenic tumors, schwannomas are the most common tumor of peripheral nerves (6). Schwannomas are well encapsulated and contain cells that are identical to Schwann cells. Their MR imaging appearance depends on the types of tissue they contain. Myxoid tissue is hyperintense on T2-weighted images (Fig 11b, 11c), cellular tissue is hypointense on both T1- and T2-weighted images (6), and solid fibrous tissue enhances on contrast-enhanced images. Neurofibromas tend to have high signal intensity on T2-weighted MR images and are often multiple and associated with neurofibromatosis (Fig 12) (9). Ganglioneuromas are typically located along the sympathetic chain and tend to be larger, more rounded, and contain calcification more frequently than nerve sheath tumors (Fig 13) (11). The relatively younger age of affected patients may aid in differentiating ganglioneuromas from other neurogenic tumors (6).
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Figure 11a. Schwannoma in a 70-year-old man. (a) Transverse contrast-enhanced CT scan shows a well-marginated low-attenuation mass interposed between the portal vein, superior mesenteric artery, aorta, and inferior vena cava. (b) On a T2-weighted MR image, the mass appears mostly hyperintense. (c) Photograph of the bisected gross specimen shows that the tumor is smoothly encapsulated and contains myxoid stroma. (d) Low-power photomicrograph (original magnification, x40; H-E stain) shows proliferation of fascicular interlacing spindle cells that resemble peripheral nerve tissue, along with loose fibrous stroma.
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Figure 11b. Schwannoma in a 70-year-old man. (a) Transverse contrast-enhanced CT scan shows a well-marginated low-attenuation mass interposed between the portal vein, superior mesenteric artery, aorta, and inferior vena cava. (b) On a T2-weighted MR image, the mass appears mostly hyperintense. (c) Photograph of the bisected gross specimen shows that the tumor is smoothly encapsulated and contains myxoid stroma. (d) Low-power photomicrograph (original magnification, x40; H-E stain) shows proliferation of fascicular interlacing spindle cells that resemble peripheral nerve tissue, along with loose fibrous stroma.
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Figure 11c. Schwannoma in a 70-year-old man. (a) Transverse contrast-enhanced CT scan shows a well-marginated low-attenuation mass interposed between the portal vein, superior mesenteric artery, aorta, and inferior vena cava. (b) On a T2-weighted MR image, the mass appears mostly hyperintense. (c) Photograph of the bisected gross specimen shows that the tumor is smoothly encapsulated and contains myxoid stroma. (d) Low-power photomicrograph (original magnification, x40; H-E stain) shows proliferation of fascicular interlacing spindle cells that resemble peripheral nerve tissue, along with loose fibrous stroma.
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Figure 11d. Schwannoma in a 70-year-old man. (a) Transverse contrast-enhanced CT scan shows a well-marginated low-attenuation mass interposed between the portal vein, superior mesenteric artery, aorta, and inferior vena cava. (b) On a T2-weighted MR image, the mass appears mostly hyperintense. (c) Photograph of the bisected gross specimen shows that the tumor is smoothly encapsulated and contains myxoid stroma. (d) Low-power photomicrograph (original magnification, x40; H-E stain) shows proliferation of fascicular interlacing spindle cells that resemble peripheral nerve tissue, along with loose fibrous stroma.
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Figure 13a. Ganglioneuroma in a 21-year-old man. (a) Transverse contrast-enhanced CT scan of the abdomen shows a well-marginated, minimally enhancing mass in the right paraaortic region. (b) On a T2-weighted MR image, the mass appears hyperintense. (c) Photograph of the resected specimen shows an encapsulated, elastic hard mass with a myxoid component. Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) demonstrates an arrayed proliferation of spindle cells with dispersed mature ganglion cells.
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Figure 13b. Ganglioneuroma in a 21-year-old man. (a) Transverse contrast-enhanced CT scan of the abdomen shows a well-marginated, minimally enhancing mass in the right paraaortic region. (b) On a T2-weighted MR image, the mass appears hyperintense. (c) Photograph of the resected specimen shows an encapsulated, elastic hard mass with a myxoid component. Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) demonstrates an arrayed proliferation of spindle cells with dispersed mature ganglion cells.
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Figure 13c. Ganglioneuroma in a 21-year-old man. (a) Transverse contrast-enhanced CT scan of the abdomen shows a well-marginated, minimally enhancing mass in the right paraaortic region. (b) On a T2-weighted MR image, the mass appears hyperintense. (c) Photograph of the resected specimen shows an encapsulated, elastic hard mass with a myxoid component. Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) demonstrates an arrayed proliferation of spindle cells with dispersed mature ganglion cells.
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Figure 13d. Ganglioneuroma in a 21-year-old man. (a) Transverse contrast-enhanced CT scan of the abdomen shows a well-marginated, minimally enhancing mass in the right paraaortic region. (b) On a T2-weighted MR image, the mass appears hyperintense. (c) Photograph of the resected specimen shows an encapsulated, elastic hard mass with a myxoid component. Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) demonstrates an arrayed proliferation of spindle cells with dispersed mature ganglion cells.
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Myxoid liposarcomas are of intermediate-grade malignancy and have a CT attenuation lower than that of muscle. They are homogeneously hypointense on T1-weighted MR images and markedly hyperintense on T2-weighted images. In addition, they may appear somewhat cystic before contrast enhancement but show gradual reticular enhancement after administration of contrast medium. Myxoid liposarcomas may coexist with other subtypes of liposarcomas (Fig 14) (12).
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Figure 14a. Myxoid liposarcoma as a part of well-differentiated liposarcoma. (a) Transverse contrast-enhanced CT scan shows a huge mass with fat attenuation. A higher-attenuation area (arrow) is noted within the mass. (b) On a T2-weighted MR image, the high-attenuation area within the mass is markedly hyperintense (arrow). (c) Photograph of the resected specimen shows a well-circumscribed, yellowish fatty tumor. A whitish, elastic hard portion (arrow) is present within the tumor, a finding that is consistent with the high-attenuation and high-signal-intensity areas seen in a and b, respectively. Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) of the elastic hard portion shows a pattern of myxoid liposarcoma, whereas the majority of the tumor demonstrated features of well-differentiated liposarcoma.
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Figure 14b. Myxoid liposarcoma as a part of well-differentiated liposarcoma. (a) Transverse contrast-enhanced CT scan shows a huge mass with fat attenuation. A higher-attenuation area (arrow) is noted within the mass. (b) On a T2-weighted MR image, the high-attenuation area within the mass is markedly hyperintense (arrow). (c) Photograph of the resected specimen shows a well-circumscribed, yellowish fatty tumor. A whitish, elastic hard portion (arrow) is present within the tumor, a finding that is consistent with the high-attenuation and high-signal-intensity areas seen in a and b, respectively. Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) of the elastic hard portion shows a pattern of myxoid liposarcoma, whereas the majority of the tumor demonstrated features of well-differentiated liposarcoma.
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Figure 14c. Myxoid liposarcoma as a part of well-differentiated liposarcoma. (a) Transverse contrast-enhanced CT scan shows a huge mass with fat attenuation. A higher-attenuation area (arrow) is noted within the mass. (b) On a T2-weighted MR image, the high-attenuation area within the mass is markedly hyperintense (arrow). (c) Photograph of the resected specimen shows a well-circumscribed, yellowish fatty tumor. A whitish, elastic hard portion (arrow) is present within the tumor, a finding that is consistent with the high-attenuation and high-signal-intensity areas seen in a and b, respectively. Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) of the elastic hard portion shows a pattern of myxoid liposarcoma, whereas the majority of the tumor demonstrated features of well-differentiated liposarcoma.
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Figure 14d. Myxoid liposarcoma as a part of well-differentiated liposarcoma. (a) Transverse contrast-enhanced CT scan shows a huge mass with fat attenuation. A higher-attenuation area (arrow) is noted within the mass. (b) On a T2-weighted MR image, the high-attenuation area within the mass is markedly hyperintense (arrow). (c) Photograph of the resected specimen shows a well-circumscribed, yellowish fatty tumor. A whitish, elastic hard portion (arrow) is present within the tumor, a finding that is consistent with the high-attenuation and high-signal-intensity areas seen in a and b, respectively. Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) of the elastic hard portion shows a pattern of myxoid liposarcoma, whereas the majority of the tumor demonstrated features of well-differentiated liposarcoma.
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Tumors that less commonly contain myxoid stroma include desmoid tumors, hemangiopericytomas, leiomyomas, leiomyosarcomas, malignant pericytomas, rhabdomyosarcomas, and malignant mesenchymomas (10).
Necrosis.—
Necrotic portions within tumors have low attenuation without contrast enhancement at CT and are hyperintense at T2-weighted MR imaging. Necrosis is usually seen in tumors of high-grade malignancy such as leiomyosarcomas (1).
When they occur in the retroperitoneum, leiomyosarcomas tend to develop massive cystic degeneration. They have central necrosis more commonly than other sarcomas, whereas fat and calcifications are not typically present (Fig 15) (8).
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Figure 15a. Leiomyosarcoma in a 48-year-old woman. (a) Transverse contrast-enhanced CT scan reveals a huge mass adjacent to the left kidney that displaces the spleen and pancreas anteriorly. The mass has heterogeneous enhancement with central nonenhancing foci that suggest necrosis. Enhanced vessels (arrow) are seen to penetrate the mass, a finding that reflects hypervascularity. (b) On a T2-weighted MR image, the mass is heterogeneous but relatively hypointense. The central portion has high signal intensity (arrow), a finding that represents necrosis. (c) Photograph of the resected specimen reveals that the mass is composed of yellow-white fibrous tissue with central necrosis. The mass was attached to the kidney but was separated from it at surgery. The adrenal gland was undetectable at first but was later identified at the tumor periphery, having a preserved fibrous capsule (false-positive phantom organ sign). Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) shows proliferating spindle-shaped cells with nuclear atypia and mitosis. An interlacing fascicular pattern compatible with leiomyosarcoma is also noted.
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Figure 15b. Leiomyosarcoma in a 48-year-old woman. (a) Transverse contrast-enhanced CT scan reveals a huge mass adjacent to the left kidney that displaces the spleen and pancreas anteriorly. The mass has heterogeneous enhancement with central nonenhancing foci that suggest necrosis. Enhanced vessels (arrow) are seen to penetrate the mass, a finding that reflects hypervascularity. (b) On a T2-weighted MR image, the mass is heterogeneous but relatively hypointense. The central portion has high signal intensity (arrow), a finding that represents necrosis. (c) Photograph of the resected specimen reveals that the mass is composed of yellow-white fibrous tissue with central necrosis. The mass was attached to the kidney but was separated from it at surgery. The adrenal gland was undetectable at first but was later identified at the tumor periphery, having a preserved fibrous capsule (false-positive phantom organ sign). Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) shows proliferating spindle-shaped cells with nuclear atypia and mitosis. An interlacing fascicular pattern compatible with leiomyosarcoma is also noted.
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Figure 15c. Leiomyosarcoma in a 48-year-old woman. (a) Transverse contrast-enhanced CT scan reveals a huge mass adjacent to the left kidney that displaces the spleen and pancreas anteriorly. The mass has heterogeneous enhancement with central nonenhancing foci that suggest necrosis. Enhanced vessels (arrow) are seen to penetrate the mass, a finding that reflects hypervascularity. (b) On a T2-weighted MR image, the mass is heterogeneous but relatively hypointense. The central portion has high signal intensity (arrow), a finding that represents necrosis. (c) Photograph of the resected specimen reveals that the mass is composed of yellow-white fibrous tissue with central necrosis. The mass was attached to the kidney but was separated from it at surgery. The adrenal gland was undetectable at first but was later identified at the tumor periphery, having a preserved fibrous capsule (false-positive phantom organ sign). Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) shows proliferating spindle-shaped cells with nuclear atypia and mitosis. An interlacing fascicular pattern compatible with leiomyosarcoma is also noted.
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Figure 15d. Leiomyosarcoma in a 48-year-old woman. (a) Transverse contrast-enhanced CT scan reveals a huge mass adjacent to the left kidney that displaces the spleen and pancreas anteriorly. The mass has heterogeneous enhancement with central nonenhancing foci that suggest necrosis. Enhanced vessels (arrow) are seen to penetrate the mass, a finding that reflects hypervascularity. (b) On a T2-weighted MR image, the mass is heterogeneous but relatively hypointense. The central portion has high signal intensity (arrow), a finding that represents necrosis. (c) Photograph of the resected specimen reveals that the mass is composed of yellow-white fibrous tissue with central necrosis. The mass was attached to the kidney but was separated from it at surgery. The adrenal gland was undetectable at first but was later identified at the tumor periphery, having a preserved fibrous capsule (false-positive phantom organ sign). Scale is in centimeters. (d) Photomicrograph (original magnification, x100; H-E stain) shows proliferating spindle-shaped cells with nuclear atypia and mitosis. An interlacing fascicular pattern compatible with leiomyosarcoma is also noted.
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Extremely hypervascular tumors such as paragangliomas sometimes contain hemorrhagic necrosis and manifest with fluid-fluid levels (Fig 16). Extraadrenal paragangliomas are more frequently malignant (up to 40% of cases) than adrenal pheochromocytomas (1).
Cystic Portion.—
Some tumors are completely cystic in appearance. These include lymphangiomas and mucinous cystic tumors (Fig 17) (1,3, 13). Solid tumors with a partially cystic portion include neurogenic tumors (8).
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Figure 17a. Retroperitoneal mucinous cystic tumor. (a) T2-weighted MR image shows a presacral multiloculated cystic mass without solid components. The tumor has high signal intensity. (b) Photograph of the specimen resected at laparotomy shows a large cystic tumor. The ovaries and the appendix were normal. (c) Photomicrograph (original magnification, x100; H-E stain) shows the cyst wall lined with columnar epithelial cells of the mucin-producing type. The underlying stroma resembles that of the ovary.
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Figure 17b. Retroperitoneal mucinous cystic tumor. (a) T2-weighted MR image shows a presacral multiloculated cystic mass without solid components. The tumor has high signal intensity. (b) Photograph of the specimen resected at laparotomy shows a large cystic tumor. The ovaries and the appendix were normal. (c) Photomicrograph (original magnification, x100; H-E stain) shows the cyst wall lined with columnar epithelial cells of the mucin-producing type. The underlying stroma resembles that of the ovary.
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Figure 17c. Retroperitoneal mucinous cystic tumor. (a) T2-weighted MR image shows a presacral multiloculated cystic mass without solid components. The tumor has high signal intensity. (b) Photograph of the specimen resected at laparotomy shows a large cystic tumor. The ovaries and the appendix were normal. (c) Photomicrograph (original magnification, x100; H-E stain) shows the cyst wall lined with columnar epithelial cells of the mucin-producing type. The underlying stroma resembles that of the ovary.
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Small Round Cells.—
At T2-weighted MR imaging, tumors composed of small round cells often appear as homogeneous masses with relatively hypointense areas representing densely packed cellular components (1,8,9).
Lymphomas are the most commonly encountered tumors composed of small round cells. They are homogeneous, with minimal contrast enhancement at CT and relatively low signal intensity at T2-weighted MR imaging (Fig 18). An exception is primitive neuroectodermal tumor (PNET), which often appears heterogeneous at MR imaging (Fig 19) (14).
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Figure 18a. Lymphoma in a 64-year-old woman. (a) T2-weighted MR image shows a presacral mass with homogeneous low signal intensity that represents densely packed small round cells. (b) Photomicrograph (original magnification, x100; H-E stain) reveals proliferation of atypical lymphoid cells. The diagnosis was non-Hodgkin B-cell lymphoma.
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Figure 18b. Lymphoma in a 64-year-old woman. (a) T2-weighted MR image shows a presacral mass with homogeneous low signal intensity that represents densely packed small round cells. (b) Photomicrograph (original magnification, x100; H-E stain) reveals proliferation of atypical lymphoid cells. The diagnosis was non-Hodgkin B-cell lymphoma.
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Figure 19a. PNET in a 16-year-old boy. (a) Transverse contrast-enhanced CT scan shows a large, partially ill-defined mass with heterogeneous enhancement. The spleen and the left kidney are displaced anteriorly. (b) On a T2-weighted MR image, the mass appears heterogeneous with interpersed high-signal-intensity spots. At surgery, the tumor was seen to occupy the left side of the retroperitoneum and invade the thorax through the diaphragm. (c) High-power photomicrograph (original magnification, x200; H-E stain) shows proliferation of small round cells, a finding that is compatible with PNET or neuroblastoma. Because of the patient’s young age, the location of the tumor, and the lack of secretion of catecholamines, the final diagnosis was PNET.
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Figure 19b. PNET in a 16-year-old boy. (a) Transverse contrast-enhanced CT scan shows a large, partially ill-defined mass with heterogeneous enhancement. The spleen and the left kidney are displaced anteriorly. (b) On a T2-weighted MR image, the mass appears heterogeneous with interpersed high-signal-intensity spots. At surgery, the tumor was seen to occupy the left side of the retroperitoneum and invade the thorax through the diaphragm. (c) High-power photomicrograph (original magnification, x200; H-E stain) shows proliferation of small round cells, a finding that is compatible with PNET or neuroblastoma. Because of the patient’s young age, the location of the tumor, and the lack of secretion of catecholamines, the final diagnosis was PNET.
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Figure 19c. PNET in a 16-year-old boy. (a) Transverse contrast-enhanced CT scan shows a large, partially ill-defined mass with heterogeneous enhancement. The spleen and the left kidney are displaced anteriorly. (b) On a T2-weighted MR image, the mass appears heterogeneous with interpersed high-signal-intensity spots. At surgery, the tumor was seen to occupy the left side of the retroperitoneum and invade the thorax through the diaphragm. (c) High-power photomicrograph (original magnification, x200; H-E stain) shows proliferation of small round cells, a finding that is compatible with PNET or neuroblastoma. Because of the patient’s young age, the location of the tumor, and the lack of secretion of catecholamines, the final diagnosis was PNET.
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Vascularity
Vascularity is another important feature of retroperitoneal tumors. Extremely hypervascular tumors include paragangliomas and hemangiopericytomas (Fig 20). Moderately hypervascular tumors include myxoid malignant fibrous histiocytomas, leiomyosarcomas, and many other sarcomas. Hypovascular tumors include low-grade liposarcomas, lymphomas, and many other benign tumors (10).
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Figure 20a. Malignant paraganglioma in a 57-year-old man. (a) Abdominal CT scan shows an ill-defined, hypervascular tumor with a central low-attenuation focus, findings that were interpreted as a malignant tumor with central necrosis. (b) High-power photomicrograph (original magnification, x200; H-E stain) shows "zellballen" (cell ball) growth of chief cells with invasion into surrounding vessels and lymph nodes.
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Figure 20b. Malignant paraganglioma in a 57-year-old man. (a) Abdominal CT scan shows an ill-defined, hypervascular tumor with a central low-attenuation focus, findings that were interpreted as a malignant tumor with central necrosis. (b) High-power photomicrograph (original magnification, x200; H-E stain) shows "zellballen" (cell ball) growth of chief cells with invasion into surrounding vessels and lymph nodes.
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Conclusions
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Abstract
LEARNING OBJECTIVES
