Fat-containing Retroperitoneal Lesions: Imaging Characteristics, Localization, and Differential Diagnosis1
Akram M. Shaaban, MBBCh Maryam Rezvani, MD Marc Tubay, MD Khaled M. Elsayes, MD Paula J. Woodward, MD Christine O. Menias, MD
Abbreviations: AML=angiomyolipoma, GCT= germ cell tumor, RCC = renal cell carcinoma RadioGraphics 2016; 36:710-734
Published online 10.1148/rg.2016150149
Content Codes: CT GU MR US
“From the Department of Radiology, University of Utah, 30 North 1900 East, 1A71, Salt Lake City, UT 84132 (A.M.S., M.R., P.J.W.); Imag- ing Department, U.S. Air Force Academy Medi- cal Clinic, USAF Academy, Colorado Springs, Colo (M.T.); Department of Radiology, Uni- versity of Texas MD Anderson Cancer Center, Houston, Tex (K.M.E.); and Department of Ra- diology, Mayo Clinic, Scottsdale, Ariz (C.O.M.). Presented as an education exhibit at the 2014 RSNA Annual Meeting. Received June 17, 2015; revision requested September 30 and received October 17; final version accepted December 2. For this journal-based SA-CME activity, the author M.T. has provided disclosures; all other authors, the editor, and the reviewers have dis- closed no relevant relationships. Address cor- respondence to A.M.S. (e-mail: akram.shaaban @hsc.utah.edu).
ORSNA, 2016
SA-CME LEARNING OBJECTIVES
After completing this journal-based SA-CME activity, participants will be able to:
Define imaging signs that may help identify the organ of origin for a retro- peritoneal mass.
Identify the appearance of fat at CT, MR imaging, and US.
Describe the demographic, clinical, and imaging characteristics of specific fat- containing retroperitoneal masses.
See www.rsna.org/education/search/RG.
The complex anatomy of the retroperitoneum is reflected in the spectrum of neoplastic and nonneoplastic conditions that can oc- cur in the retroperitoneum and appear as soft-tissue masses. The presence of fat within a retroperitoneal lesion is helpful in refin- ing the differential diagnosis. Fat is easily recognized because of its characteristic imaging appearance. It typically is hyperechoic at ultrasonography and demonstrates low attenuation at computed tomography (-10 to -100 HU). Magnetic resonance imaging is a more ideal imaging modality because it has better soft-tissue image contrast and higher sensitivity for depicting (a) microscopic fat by using chemical shift imaging and (b) macroscopic fat by using fat- suppression techniques. Whether a lesion arises from a retroperito- neal organ or from the soft tissues of the retroperitoneal space (pri- mary lesion) is determined by examining the relationship between the lesion and its surrounding structures. Multiple imaging signs help to determine the organ of origin, including the “beak sign,” the “embedded organ sign,” the “phantom (invisible) organ sign,” and the “prominent feeding artery sign.” Adrenal adenoma is the most common adrenal mass that contains microscopic fat, while myeloli- poma is the most common adrenal mass that contains macroscopic fat. Other adrenal masses, such as pheochromocytoma and adre- nocortical carcinoma, rarely contain fat. Renal angiomyolipoma is the most common fat-containing renal mass. Other fat-containing renal lesions, such as lipoma and liposarcoma, are rare. Fatty re- placement of the pancreas and pancreatic lipomas are relatively common, whereas pancreatic teratomas are rare. Of the primary retroperitoneal fat-containing lesions, lipoma and liposarcoma are common, while other lesions are relatively rare.
“RSNA, 2016 · radiographics.rsna.org
Introduction
The retroperitoneum is a complex space that contains solid organs and hollow viscera, lymphatics and lymph nodes, major vascular structures, and stromal tissues of the retroperitoneum. The complex anatomy of the retroperitoneum is reflected in the numerous neo- plastic and nonneoplastic conditions that can occur in this space, which appear as soft-tissue masses. Identification of a retroperitoneal mass at imaging is a challenging task for radiologists, who depend on a combination of clinical, laboratory, and imaging features to compile a list of differential possibilities. The presence of fat within a retroperitoneal lesion is helpful in refining the differential diag- nosis (Table 1). Fat is easily recognized because of its characteristic imaging appearance. Although ultrasonography (US) is rarely used for characterization of retroperitoneal masses, a large mass may be detected initially at US. Although the echogenicity of fat can be
TEACHING POINTS
Chemical shift imaging (with in-phase and out-of-phase se- quences) is the most reliable MR imaging technique for diag- nosing adrenal adenomas. Most of these lesions demonstrate a decrease in signal intensity on out-of-phase images when compared with in-phase images. A decrease in signal intensity of more than 16.5% is considered diagnostic of an adenoma.
AMLs with minimal fat are usually hyperattenuating relative to the renal parenchyma on nonenhanced CT images and hypointense on T2-weighted MR images, inherent features that reflect the predominant smooth-muscle component and classically overlap with features of papillary RCC.
Differentiating renal lipoma or well-differentiated liposarcoma from AML may be difficult: AMLs may contain large aneu- rysmal vessels, while lipomas and liposarcomas are relatively avascular and rarely occur with multiple lesions, as may be seen with AMLs.
Retroperitoneal fat necrosis is usually caused by the lipolytic effect of pancreatic enzymes released during acute pancre- atitis. It usually is accompanied by pancreatic parenchymal necrosis but can occur without visible pancreatic involvement.
Splenectomy may have a role in accelerating development of extramedullary hematopoiesis in uncommon sites.
variable at US, it typically is hyperechoic and may demonstrate posterior acoustic shadowing. Computed tomographic (CT) images derive their contrast parameters predominantly from the physical properties of tissue. This, along with the high spatial resolution of CT, provides accurate tissue attenuation measurement. CT attenua- tion measurements are quantitative, with -10 to -100 HU corresponding to fat. However, CT is limited in the demonstration of mild intracellular fat deposition or small amounts of macroscopic fat. If the fraction of fat within a voxel is small, the mean CT attenuation measurement will not reflect its presence because of volume-averaging effect.
Magnetic resonance (MR) images have lower spatial resolution than CT images but better soft- tissue contrast and greater sensitivity in detec- tion of microscopic fat (1). There are two MR imaging techniques for identification of fat within a structure: fat saturation and chemical shift imaging. Fat saturation is best for diagnosing macroscopic fat, as it requires a greater degree of steatosis. Chemical shift imaging is an ideal tech- nique for identifying microscopic fat or intracel- lular fat in a lesion or organ. Selective echo delay times are used in T1-weighted gradient-echo imaging sequences to accentuate the chemical shift phenomenon, producing images in which fat and water signals are in phase and opposed phase. Loss of signal intensity on opposed-phase images relative to in-phase images indicates the presence of fat and water within the same voxel and is therefore diagnostic of microscopic fat.
The sensitivity and specificity of chemical shift imaging for hepatic steatosis are 81% and 100%, respectively (1).
Determination of the Organ of Origin
It can be difficult to localize large abdominal masses to an anatomic space and to identify the organ of origin. Whether a lesion arises from a retroperitoneal organ or from the soft tissues of the retroperitoneal space (primary retroperitoneal lesion) is based on the relationship between the lesion and its surrounding structures. A mass that is embedded in and completely surrounded by the parenchyma of an organ undoubtedly arises from that organ. However, it is more difficult to determine the origin of a mass at the margin of an organ. The interface of the mass with an ad- jacent organ can provide insight as to whether it displaces or arises from that organ (Table 2). The “beak sign” relates to the shape of a solid organ at the edge of its interface with an adjacent mass (2). A sharp beak shape implies that the mass arises from the adjacent organ (Fig 1), whereas rounded organ edges suggest displacement by the mass (Fig 2). The “embedded organ sign” refers to the relationship between a mass and a hollow viscus or other adjacent malleable organ (ie, infe- rior vena cava) (2). If the mass only displaces the organ but does not originate from it, the organ assumes a crescent shape (Fig 3). When the mass encases an organ such that the organ appears to be embedded in it, the mass is presumed to have arisen from the organ (Fig 4). In the latter instance, the organ-tumor interface may be scle- rotic with a desmoplastic reaction. The “phantom (invisible) organ sign” is seen when a large mass arises from a small organ such that the organ of origin is obliterated and no longer visible (2). It should be noted, however, that the phantom organ sign might be falsely positive when a small organ is markedly displaced and compressed by a large mass. Lastly, hypervascular retroperito- neal masses may have large feeding arteries that can help identify the organ of origin; this is also known as the “prominent feeding artery sign” (2).
Fat-containing Adrenal Lesions
Adrenal Cortical Adenoma
Adrenal cortical adenoma is the most common adrenal lesion, found in 2%-9% of all autop- sies. Most adrenal cortical adenomas measure 2.0-2.5 cm. Rarely, adenomas can appear as larger masses, measuring up to 4 cm (3). They usually are well circumscribed and homogeneous on nonenhanced images and demonstrate homo- geneous contrast material enhancement. Adrenal
| Table 1: Differential Diagnosis of Fat-containing Retroperitoneal Masses | ||
|---|---|---|
| Organ of Origin | Common Diagnoses | Rare Diagnoses |
| Adrenal | Adenoma, myelolipoma | Carcinoma, pheochromocytoma, neurogenic tumors, AML, lipoma and liposarcoma, metas- tasis |
| Renal | AML | RCC, lipoma and liposarcoma, replacement lipomatosis |
| Pancreatic | Lipoma, focal pancreatic steatosis | Pancreatic pseudolipohypertrophy, pancreatic teratoma |
| Primary retroperitoneal | Lipoma and liposarcoma | Myelolipoma, primary and secondary GCTs, neurogenic tumors, lipomatosis, fat necrosis, extramedullary hematopoiesis |
| Note .- AML = angiomyolipoma, GCT = germ cell tumor, RCC = renal cell carcinoma. | ||
| Table 2: Positive Signs Indicating That a Retroperitoneal Tumor Arises from an Adjacent Organ | |
|---|---|
| Sign | Definition |
| Beak sign | Sharp beak shape of organ of origin occurs at the edge of the interface with the tumor |
| Embedded organ sign | Organ of origin is encased by the tumor, with or without a sclerotic interface |
| Phantom (invisible) organ sign | Organ of origin is obliterated by the tumor |
| Prominent feeding artery sign | Large feeding arteries of a hypervascular tumor point to the organ of origin |
adenomas typically contain variable amounts of microscopic fat (intracellular lipid) and can be classified as lipid rich or lipid poor according to the lipid content. The attenuation values of adenomas depend on the amount of lipid. A lipid- rich adenoma is diagnosed when an adrenal lesion measures less than 10 HU on nonenhanced CT images. Lipid-poor adenomas constitute about 10%-40% of adenomas and demonstrate higher attenuation on nonenhanced CT images (4). Threshold values of more than 60% for absolute contrast material washout and 40% for relative contrast material washout have been found to be highly sensitive and specific for diagnosing adrenal adenomas (5). Chemical shift imaging (with in-phase and out-of-phase sequences) is the most reliable MR imaging technique for diag- nosing adrenal adenomas. Most of these lesions demonstrate a decrease in signal intensity on out- of-phase images when compared with in-phase images. A decrease in signal intensity of more than 16.5% is considered diagnostic of an ad- enoma (Fig 5) (6). Rarely, foci of macroscopic fat have been reported in adrenocortical adenomas, resulting in a misdiagnosis of myelolipoma on images. Such lesions were found to be adenomas with myelolipomatous and/or lipomatous changes at postoperative pathologic evaluation (7).
Adrenal Myelolipoma
Myelolipoma is the most common macroscopic fat-containing adrenal mass. Myelolipoma is an uncommon, benign, nonfunctioning tumor that is found incidentally at imaging and ranges from 2 to 17 cm in diameter. Pathologically, a my- elolipoma is composed of adipose and hemato- poietic tissues. It commonly originates from the adrenal gland and rarely originates from an ex- tra-adrenal location. At imaging, a myelolipoma typically appears as a well-circumscribed mass that contains variable amounts of soft tissue and macroscopic fat (Fig 6). A pseudocapsule has been reported in 75% of cases, with calcification in 24% of cases (8).
Congenital Adrenal Hyperplasia
Congenital adrenal hyperplasia results from 21-hydroxylase deficiency. Prolonged stimula- tion of the adrenal cortex typically leads to enlarged adrenal glands, occasionally associated with multiple fat-containing adrenal masses (Fig 7). This appearance has been rarely reported in the literature (9,10), although this diagnosis may be underreported because these lesions mimic the appearance of myelolipomas, which seldom exist bilaterally. Chronic adrenocorticotropic hormone and androgen hyperstimulation, with
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overexpression of their receptors, is assumed to be involved in the pathogenesis of adrenal myelolipomas associated with noncompliant pa- tients with congenital adrenal hyperplasia (11).
Pheochromocytoma
The paraganglionic system is composed of neural crest cells, which are found in the adrenal medulla, parasympathetic ganglia, and chemore- ceptors. Tumors that arise from the chromaffin cells of the adrenal medulla are called pheochro- mocytomas, and those that arise in extra-adrenal locations (10%) are referred to as paraganglio- mas. Pheochromocytoma is typically symp-
tomatic, occurring with new-onset paroxysmal, refractory, or recently exacerbated hypertension (12), headache, flushing, and palpitations. Pheo- chromocytoma has been shown to follow the “rule of 10s,” with 10% being malignant, 10% being bilateral, 10% being extra-adrenal, and 10% occurring in children (13). Pheochromocy- toma can be associated with multiple endocrine neoplasia, von Hippel-Lindau disease, neurofi- bromatosis type 1, and nonsyndromic familial pheochromocytoma (14). On contrast-enhanced CT images, pheochromocytomas demonstrate heterogeneous enhancement and thus can have nonspecific imaging manifestations. One-third
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of pheochromocytomas mimic lipid-poor adeno- mas, with an absolute enhancement washout rate of more than 60% and a relative enhancement washout rate of more than 40% (15). On MR im- ages, most cases demonstrate high signal intensity on T2-weighted images, a finding formerly known as the “light bulb sign.” However, markedly in- creased signal intensity on T2-weighted images is not as common as was previously thought. Rarely, microscopic and macroscopic fat can be seen in pheochromocytomas (Fig 8) (16).
Adrenocortical Carcinoma
Although it is the most common adrenal pri- mary malignancy, adrenocortical carcinoma is rare, affecting approximately two patients per million and occurring in patients 30-70 years of age. Adrenocortical carcinoma is an aggres- sive malignancy with a poor prognosis, although less aggressive forms have been described. Most of these tumors are functional, with approxi- mately 60% of patients experiencing endocrine symptoms typically characterized by Cushing
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syndrome (40%) or a mixed hormonal picture of Cushing syndrome and virilization (17). Typi- cally, adrenocortical carcinoma appears as a large mass, usually larger than 6 cm (ranging from 4 to 25 cm in diameter). These tumors typically exhibit heterogeneous contrast enhancement and have absolute and relative contrast mate- rial washout rates of less than 60% and less than 40%, respectively. The degree of heterogeneity and large size are more reliable for diagnosis than contrast material washout measurements. Adre- nocortical carcinoma has been rarely reported to contain foci of microscopic or macroscopic fat (Fig 9) (18). Fat-containing adrenocortical carci- nomas have only a small amount of macroscopic fat relative to the overall lesion size; this may be a helpful clue when determining the diagnosis. Although small myelolipomas may contain small
amounts of fat, larger myelolipomas usually contain a substantial amount of fat. Hypertension or evidence of adrenal hormonal excess favors a diagnosis of adrenocortical carcinoma rather than myelolipoma. Myelolipomas can demonstrate en- hancement; however, a large heterogeneous mass with substantial peripheral enhancement more likely represents adrenocortical carcinoma than myelolipoma (19).
Adrenal AML
AML is a rare benign mesenchymal tumor that consists of mature adipose tissue, smooth muscle, and blood vessels. AML usually arises from the kidney. The liver is the most common extrarenal location. About 50% of AMLs are associated with tuberous sclerosis. Few cases of AML have been reported to involve the adrenal glands (Fig 10) (14), and the appearance is indistinguishable from that of adrenal myelolipoma.
Adrenal Lipoma
Lipomas are small benign tumors of mesenchymal origin that contain mature fatty tissue and are sur- rounded by a fibrous capsule; only a few cases have been reported to involve the adrenal glands (20). Adrenal lipoma appears as a well-circumscribed, homogeneous, fatty mass (Fig 11). In contrast to
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myelolipomas, no or minimal soft-tissue attenuation should be visualized within the lesion.
Management of Fat-containing Adrenal Lesions
Clinical and imaging features that help differentiate fat-containing adrenal lesions are listed in Table 3. Most benign adrenal tumors are adrenal cortical adenomas that contain varying amounts of intracy- toplasmic lipid and can be diagnosed on the basis of a combination of imaging features. Myelolipomas and AMLs are usually predominantly fatty masses. Most small, nonfunctioning, benign adrenal lesions (adenomas, myelolipomas, lipomas, and AMLs) are managed conservatively. Adrenalectomy is indi- cated for any biochemically functioning tumor and for large or symptomatic tumors. Predominantly soft-tissue masses that contain a small amount of
macroscopic fat are usually adrenocortical carci- nomas or pheochromocytomas, and both require surgical resection.
Fat-containing Renal Lesions
Renal AML
Renal AML is the most common benign mesen- chymal tumor of the kidney. Pathologically, it is composed of fat, smooth muscle, and abnormal blood vessels. AML may be sporadic or occur in association with tuberous sclerosis. The sporadic form accounts for 80%-90% of cases and is most common in middle-aged women (21). Most cases are readily diagnosed because of the predominant macroscopic fat content.
Thin-section CT is preferred for diagnosing AML when there are small amounts of fat (22).
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Loss of signal intensity on gradient-echo or spin- echo MR images acquired with fat suppression is diagnostic of AML; at chemical shift MR imag- ing, detection of “India ink” artifact at fat-water interfaces is also highly suggestive of AML (23).
AMLs with minimal fat are tumors in which intratumoral fat cannot be visualized on CT im- ages and are characterized by a predominance of blood vessels, muscle, or immature fat or a scattering of a small amount of fat within other components (24). There is substantial overlap be- tween CT and MR imaging findings of (a) AML with minimal fat and (b) RCC, with conflicting data on the ability of imaging to allow differentia-
tion between the two conditions (25,26). AMLs with minimal fat are usually hyperattenuating relative to the renal parenchyma on nonenhanced CT images and hypointense on T2-weighted MR images, inherent features that reflect the predominant smooth muscle component and classically overlap with features of papillary RCC (27). Findings suggestive of the diagnosis of AML without visible fat include an angular interface between the mass and the underlying normal renal parenchyma, a low-attenuation rim at the periphery of the mass on nonenhanced CT images, homogeneous attenuation, absence of calcification, and presence of other lesions with
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| Table 3: Differential Diagnosis of Fat-containing Adrenal Lesions | ||||||
|---|---|---|---|---|---|---|
| Parameter | Adrenal Adenoma | Adrenal Myelolipoma | Pheochro- mocytoma | Adrenocortical Carcinoma | Adrenal AML | Adrenal Lipoma |
| Incidence | Common | Common | Not uncommon | Rare | Rare | Rare |
| Likelihood of presence of fat | Common | Common | Rare | Rare | Common | Common |
| Fat quantity | Microscopic fat | Predomi- nantly fatty lesion | Microscopic or small amount of macro- scopic fat in a predomi- nantly soft- tissue mass | Microscopic or small amount of macro- scopic fat in a predomi- nantly soft- tissue mass | Predomi- nantly fatty lesion | Predomi- nantly fatty lesion |
| Clinical syn- drome | Primary aldoste- ronism or Cushing syndrome | None | Symptoms of catechol- amine excess | Cushing syn- drome with or without virilization; primary aldo- steronism | None | None |
| Calcifications | Possible | Possible | Possible | Possible | Absent | Absent |
| Texture | Homo- geneous | Heteroge- neous | Heterogeneous | Heterogeneous | Homoge- neous or heteroge- neous | Homoge- neous |
| Associated conditions | Familial adenoma- tous pol- yposis | Congenital adrenal hyper- plasia | MEN, VHL, NF1, and nonsyn- dromic familial pheochromo- cytoma | None | Tuberous sclerosis | None |
Note .- MEN = multiple endocrine neoplasia, NF1 = neurofibromatosis type 1, VHL = von Hippel-Lindau disease.
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classic imaging features of AML (28). Calcifica- tion within an AML is exceedingly rare; only six cases of AMLs that contained calcification have been reported in the literature to date (29).
Invasion of the renal vein is a rare but recog- nized complication of benign AML, and it does not imply malignancy or metastasis (Fig 12). Contributing factors to development of renal vein invasion include large size and central location of the AML (21).
Differentiating an exophytic AML from a peri- nephric liposarcoma can occasionally be difficult, as both conditions appear as a fatty mass within the perinephric space. Features that strongly fa- vor an exophytic AML are renal cortical tumoral vessels (neovascularity) or a renal parenchymal defect, usually with an angular interface with the normal renal parenchyma at the site of tumor
contact, whereas tumoral calcifications are sug- gestive of a perinephric liposarcoma (30).
Renal Cell Carcinoma
The presence of fat within an RCC is a rare phe- nomenon. It denotes fatty marrow elements in the setting of osseous metaplasia, perinephric or renal sinus fat engulfed by the tumor, or cholesterol necro- sis (31). Fat has been reported at pathologic exami- nation of nephrectomy specimens in about 0.24% of cases of RCC, mostly in the clear cell type, with fat foci ranging from 0.1 to 1.8 cm in diameter (32).
All reported cases of fat-containing RCC con- tain small foci of fat scattered within a soft-tissue mass (33-35) and can be readily differentiated from classic AML, which usually contains abun- dant macroscopic fat. Most fat-containing RCCs contain calcifications in close proximity to fat foci (Fig 13). RCCs that contain fat without calcifica- tions are extremely rare (Fig 14) (36).
Because of the smooth muscle content, AMLs with minimal fat are typically hypointense on T2- weighted MR images and can be readily differen- tiated from clear cell RCCs, which are typically hyperintense on T2-weighted images. Therefore, an enhancing renal mass that is hypoattenuating on nonenhanced CT images and hyperintense
on T2-weighted MR images is more likely to be an RCC than an AML with minimal fat. On the other hand, papillary RCCs are also typically hypointense on T2-weighted MR images and hyperattenuating on nonenhanced CT images. Therefore, a homogeneously enhancing renal mass that is hyperattenuating on nonenhanced CT images and hypointense on T2-weighted MR images could represent an AML with minimal fat or a papillary RCC. Percutaneous biopsy is the only way to distinguish between these two enti- ties, short of surgical resection (37,38). It should be noted that fat-containing oncocytomas have also been reported (31).
Renal Lipoma and Liposarcoma
Primary intrarenal lipomas are extremely rare mesenchymal neoplasms that typically arise from the renal capsule. Intrarenal lipomas are more common in middle-aged women (39) but can be seen even in young children (40). Small intrare- nal lipomas are asymptomatic, but large tumors may manifest with abdominal pain.
At imaging, renal lipoma appears as a well- circumscribed homogeneous mass that is almost exclusively composed of macroscopic fat (Fig 15). A soft-tissue component and feeding vessels are rarely seen (41).
Renal liposarcoma is a rare tumor, with only a few documented cases in the literature (42). It arises from either the renal sinus or the capsule, and in large tumors it may be difficult to deter- mine the origin of the tumor (Fig 16) (42). The imaging appearance of liposarcoma varies, de- pending on the tumor grade. Well-differentiated
liposarcomas are indistinguishable from lipomas and appear as well-defined predominantly fat- containing lesions with minimal soft-tissue at- tenuation. Poorly differentiated tumors, however, appear as locally invasive predominantly soft-tissue masses with variable amounts of fat.
Renal Replacement Lipomatosis
Renal replacement lipomatosis is a rare benign pathologic entity in which marked atrophy of the renal parenchyma is accompanied by extensive fibrofatty proliferation of the renal sinus and perinephric space (Fig 17) (43). The condition is
usually unilateral and may be segmental. In most cases, it is secondary to calculus disease, resulting in hydronephrosis and chronic inflammation (43). Renal replacement lipomatosis can be differenti- ated from fat-containing neoplasms that arise from the renal parenchyma, renal sinus, renal capsule, or perinephric space by diffuse uniform distribu- tion of fat within the renal sinus and perinephric space, marked loss of renal parenchyma, loss of renal function, and renal calculi.
Management of Fat-containing Renal Lesions
A predominantly fatty renal mass is almost always an AML. Small (<4-cm) AMLs have traditionally
been managed conservatively because of the ab- sence of malignant potential and a low propensity for hemorrhage, but follow-up is recommended to assess growth. Symptomatic tumors or tumors larger than 4 cm can be selectively embolized or resected with partial or radical nephrectomy (44). Other lesions with large fatty components include lipoma and liposarcoma. Differentiating renal lipoma or well-differentiated liposarcoma from AML may be difficult: AMLs may contain large aneurysmal vessels, while lipomas and liposarco- mas are relatively avascular and rarely occur with multiple lesions, as may be seen with AMLs. Bi- opsy is usually required in a large lesion; however, surveillance may be adequate for small purely fatty lesions. Poorly differentiated liposarcoma is more invasive and contains a variable amount of soft-tissue elements.
Fat-containing RCC may contain subtle sparse areas of fat within a predominantly soft-tissue mass and can be readily differentiated from clas- sic AML, which usually contains an abundant amount of fat. The main differential diagnostic consideration for fat-containing RCC is AML with minimal fat. The latter is almost always small (≤3 cm) and lacks the calcifications com- monly seen with RCC (34).
Fat-containing Pancreatic Lesions
Pancreatic Lipoma
Pancreatic lipomas are likely underreported in the literature. Gossner (45) identified six pancre- atic lipomas in a sample of 100 consecutive ab- dominal CT examinations. Lipomas are usually incidentally found during abdominal imaging and are more common in the pancreatic head. Pan- creatic lipomas are rarely symptomatic, although they may result in abdominal pain and obstruc- tion of pancreatic and biliary ducts (45).
At imaging, pancreatic lipomas appear as well- circumscribed homogeneous masses with a sharp interface with the normal pancreatic parenchyma (Fig 18). Calcifications are characteristically absent in pancreatic lipomas.
A large (>5-cm) fatty pancreatic mass with ir- regular borders, evidence of infiltration of adjacent structures, and areas of soft-tissue attenuation should raise suspicion for pancreatic liposarcoma; further evaluation is indicated in such cases.
Pancreatic Mature Cystic Teratoma
Pancreatic teratomas represent a rare entity, with fewer than 50 cases described in the world litera- ture. The mean age of reported patients is 36.4 years (age range, 4 months to 74 years) (46). Most pancreatic teratomas are found in the head
and/or body of the gland and are symptomatic, commonly occurring with abdominal or back pain. The imaging appearance of pancreatic terato- mas is the same as that of teratomas elsewhere, with a heterogeneous appearance due to multiple tissue components. They usually appear as fatty and/or cystic masses with fat-fluid levels. Coarse calcifications are usually present, with or without soft-tissue components (Fig 19), which help dif- ferentiate mature cystic teratoma from the more common pancreatic lipoma (47). The echogenicity of pancreatic teratomas varies on abdominal or endoscopic US images, ranging from hypoechoic (46,48) to hyperechoic (47) findings relative to the normal underlying pancreatic parenchyma.
Pancreatic Lipomatosis
Pancreatic lipomatosis, or fatty replacement of the pancreas, is the most common pathologic finding in the adult pancreas. It can be focal or diffuse, and in some instances, it can mimic a pancreatic mass (49). Complete replacement of the pancreas by fat is seen most commonly in patients with cystic fibrosis or, occasionally, in rare conditions such as Shwachman-Diamond syndrome or Johanson-Blizzard syndrome. In adult patients with cystic fibrosis, complete fatty replacement is the most common pancreatic finding on images (Fig 20), with a reported prevalence of 51%-75%; it is most commonly identified in older patients (50). Pancreatic cal- cifications occur in roughly 7% of patients with cystic fibrosis and are usually found along the course of the pancreatic duct. Pancreatic cysts are relatively common in patients with cystic fibrosis and probably occur secondary to duct obstruction by inspissated secretions. Cysts are usually small, measuring 1-3 mm, and are best demonstrated on T2-weighted MR images or MR cholangiopancreatographic images (50,51). A gland that is markedly enlarged with fatty re- placement has been termed lipomatous pseudohy- pertrophy of the pancreas (Fig 20). Lesser degrees of diffuse fatty replacement are more common in patients with diabetes, metabolic syndrome, steroid use, chronic pancreatitis, or obesity and in the elderly.
Fatty replacement of the pancreas can be focal and may resemble a pancreatic mass (Fig 21); it typically involves the anterior head, with sparing of the posterior head and the uncinate process (52). It is postulated to be secondary to the dif- ferent histologic characteristics of the ventral and dorsal pancreatic anlages and may demonstrate mass effect and contrast enhancement due to entrapped normal parenchymal tissue. Nonen- hanced CT is usually adequate for depicting the negative attenuation values of a focal pancreatic
lesion and to confirm its fatty nature. However, a mild degree of focal fatty change in the pancreas can be difficult to diagnose with CT alone be- cause the attenuation values may be in the range of fluid attenuation values. Areas of focal fatty replacement may show contrast enhancement on CT images because of enhancement of normal pancreatic parenchyma entrapped between fatty replaced parenchyma. Chemical shift MR imag- ing is diagnostic for confirming focal fatty change in the pancreas by demonstrating signal intensity loss on opposed-phase images when compared with in-phase images (49).
Management of Fat-containing Pancreatic Lesions
Pancreatic lipomatosis is a diffuse process that involves the entire pancreas and is easily di- agnosed at imaging. Focal pancreatic lesions include lipoma, liposarcoma, mature cystic teratoma, and focal fatty replacement. Lipoma is well circumscribed and homogeneous, fea- tures that help to differentiate it from focal fatty replacement. The presence of fat-fluid levels and coarse calcifications, with or without soft-tissue components, can differentiate pancreatic mature cystic teratoma from pancreatic lipoma. Dif- ferentiation of fat-containing pancreatic lesions
does not have clinical significance, as none of these lesions require intervention or treatment. The most important issue in these cases is that the presence of fat helps to exclude the diag- noses of adenocarcinoma and neuroendocrine tumors, which are not known to contain fatty elements. The only exception for a fat-contain- ing pancreatic mass that requires intervention is the rare pancreatic liposarcoma. Those tumors are usually large and heterogeneous and contain soft-tissue elements together with fatty compo- nents, and biopsy or surgical excision is required in such cases.
Fat-containing Primary Retroperitoneal Lesions
Nonneoplastic Lesions
Retroperitoneal Lipomatosis .- Lipomatosis is a benign metaplastic overgrowth of mature unencapsulated white fat. The condition usually affects the extraperitoneal pelvis and less com- monly affects the abdominal retroperitoneum- specifically the perinephric space. The patient may be asymptomatic or may present with non- specific symptoms related to organ compres- sion, including abdominal pain and urinary or gastrointestinal symptoms. Lipomatosis is more common in men (with a ratio of men to women of 18:1), particularly African-American men, with a mean age of 48 years. There is no statisti- cally significant association with obesity (53). CT and MR images show expansion of the pelvic or retroperitoneal fat, with mass effect on the pelvic and abdominal structures push- ing the bowel loops anteriorly and compressing the urinary bladder, resulting in a “pear-shaped” bladder (Fig 22). A few fibrous tissue strands may be present, but there is no enhancing soft-tissue component. The ureters may be displaced medi- ally, and the femoral veins may be stretched (54).
Human Immunodeficiency Virus-related Li-
podystrophy .- Long-term use of antiretroviral therapy in patients with human immunodeficiency virus or acquired immunodeficiency syndrome is associated with disturbances of adipose tissue distribution, accompanied by metabolic and endo- crine disorders, collectively referred to as antiret- roviral therapy-associated lipodystrophy syndrome. In this syndrome, clinical conditions are grouped together that describe changes in body fat distri- bution, including lipoatrophy and/or lipoaccu- mulation. Lipoatrophy is related to subcutaneous fat loss and typically involves the limbs (pre- dominance of lower limbs), face, and buttocks. Lipohypertrophy consists of the accumulation of
adipose tissue and usually involves the retroperi- toneum, breast, anterior neck, and dorsocervical (“buffalo hump”) regions (55). At imaging, it re- sembles retroperitoneal lipomatosis, with a notable paucity of subcutaneous fat (Fig 23).
Retroperitoneal Panniculitis .- Mesenteric pan- niculitis is an inflammatory process of the mesenteric fat. It is a rare condition characterized by chronic mesenteric inflammation, likely an immunoglobulin G4-related sclerosing condition. It usually involves the mesentery of the small bowel, especially at its root, but occasionally it involves the mesocolon, peripancreatic region, omentum, retroperitoneum, or pelvis. It is characterized by an infiltration of lipid- laden macrophages and is associated with variable degrees of inflammation and fibrosis (56). The CT appearance varies from subtle increased attenuation of the mesenteric and retroperitoneal fat to a solid soft-tissue mass. Preservation of fat around the mesenteric vessels (“fat ring sign”) and a tumoral pseudocapsule have been described in patients with mesenteric panniculitis (Fig 24) (57). Retro- peritoneal involvement can be differentiated from other conditions involving the retroperitoneal fat by the typical coexisting involvement of the mesentery.
Retroperitoneal Fat Necrosis .- Retroperitoneal fat necrosis is usually caused by the lipolytic effect of pancreatic enzymes released during acute pancreatitis. It usually is accompanied by pancreatic parenchymal necrosis but can occur without visible pancreatic involvement (58). On CT images, the involved retroperitoneal fat shows increased attenuation, more than that of normal retroperitoneal fat, with scattered nodules and thick septa. The process may extend into the various retroperitoneal compartments, the small bowel mesentery, and the transverse mesocolon. The process may traverse the strong perinephric fascia to involve the perinephric compartment (59). A clinical history of a prior episode of acute pancreatitis is usually helpful in establishing the diagnosis. Other causes of retroperitoneal fat necrosis have been reported, including long-term use of steroids (Fig 25) (60).
Encapsulated Fat Necrosis .- Encapsulated fat necrosis is a degenerative process that involves mature adipose tissues and is much more com- mon in the breasts and extremities. It is thought to result from a traumatic or ischemic insult that causes fat degeneration; however, the exact pathogenesis is unknown. Its imaging appear- ance may be complex; fat-attenuation lesions contain linear or masslike fluid or show soft- tissue attenuation and are surrounded by a thick
capsule. The capsule may weakly enhance after administration of contrast material and can cal- cify in chronic lesions. There is no local invasion and only minimal mass effect on surrounding structures. The size and appearance can change over time. A fat-fluid level may be seen in areas of fat necrosis (Fig 26) (61). On MR im- ages, encapsulated fat necrosis usually appears heterogeneous on both T1- and T2-weighted images, with areas of high and low signal inten- sity; inhomogeneous enhancement is seen after
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intravenous gadolinium-based contrast material administration. A peripheral rim of low signal intensity on both T1- and T2-weighted images has been described and corresponds to the fibrotic peripheral capsule (62).
Differentiating encapsulated fat necrosis from liposarcoma can be challenging. Patients with encapsulated fat necrosis usually have a docu- mented history of surgery or trauma. Encapsu- lated fat necrosis is well circumscribed and has a surrounding capsule, whereas liposarcoma is usually less defined, lacks a capsule, and may invade surrounding structures. Encapsulated fat necrosis tends to decrease in size over time, whereas liposarcoma tends to progressively increase in size (63).
Neoplastic Lesions
Tumors of Adipose Origin
Lipoma is a benign mesenchymal tumor com- posed of mature adipose tissue, whereas liposar- coma is its malignant counterpart. These tumors can be encountered virtually anywhere in the body at any age. Retroperitoneal liposarcomas can be asymptomatic for a long period and com- monly occur in older patients (in the 5th-7th decades of life). At CT, lipomas appear as well- defined homogeneous masses with fat attenua- tion. Areas of soft-tissue attenuation may be seen within the tumor and may represent fat necrosis, septa, or normal adjacent structures. However, the presence of a predominantly solid soft-tissue
component or adjacent organ invasion should raise suspicion for liposarcoma. Retroperito- neal liposarcomas are classified into five groups: well-differentiated liposarcoma, myxoid lipo- sarcoma, round cell liposarcoma, pleomorphic liposarcoma, and dedifferentiated liposarcoma. The imaging appearance of liposarcoma varies, depending on the tumor group. Well-differenti- ated liposarcomas appear as well-defined pre- dominantly fat-containing lesions with minimal soft-tissue attenuation (Fig 27) and commonly contain thin septa. The appearance may be indis- tinguishable from lipoma, and any retroperitoneal purely fatty lesion should be considered a well- differentiated liposarcoma rather than a lipoma until proven otherwise with histologic examina- tion (64,65). Because dedifferentiation always occurs within a well-differentiated liposarcoma, a dedifferentiated liposarcoma appears identical to a well-differentiated liposarcoma, and dedif- ferentiation is suggested by focal nodular nonli- pomatous regions larger than 1 cm (Fig 28) (66). At CT, myxoid liposarcomas typically have lower attenuation than adjacent muscle but higher at- tenuation than simple fluid. They demonstrate low signal intensity on T1-weighted images and high signal intensity on T2-weighted images and resemble cysts because of the large amounts of extracellular myxoid material. However, areas of high signal intensity on T2-weighted images do enhance after contrast material administration (Fig 29). Most myxoid liposarcomas have enough fat to suggest the diagnosis of liposarcoma. Thick septa and patchy or nodular soft-tissue com- ponents are commonly seen. Round cell lipo- sarcoma and pleomorphic liposarcoma exhibit soft-tissue tumor attenuation and signal intensity with a minimal amount of fat (Fig 30) (66).
In an infant or child, the most likely diagnosis for a well-circumscribed fat-containing lesion is lipoblastoma. Poorly circumscribed lesions with an infiltrative growth pattern are described as lipoblastomatosis. Most patients develop lipo- blastoma before the age of 3 years; however, the diagnosis has been assigned in newborns and in patients as old as 16 years of age. The main feature of retroperitoneal lipoblastoma is the presence of fat. However, the amount of fat var- ies, depending on the cellular composition of the lesion and the presence of myxoid material (Fig 31). Calcifications are rare. Lipoblastomas tend to be encapsulated with internal septa, whereas lipo- blastomatosis is unencapsulated and shows signs of infiltration of surrounding structures (64,67).
A hibernoma is a rare benign tumor that con- tains immature fat cells (brown fat), which are commonly found in large quantities in hibernat- ing animals. On US images, hibernomas are
echogenic and may have well-defined or ill-de- fined borders. Increased blood flow in large ves- sels on the surface of a hibernoma may be visual- ized at Doppler US. On CT images, hibernomas appear as fat-attenuating lesions, with attenu- ation slightly higher than that of subcutaneous fat. They usually contain enhancing soft-tissue septa, and calcifications are notably absent. Their appearance can be heterogeneous because they contain variable amounts of brown and white fat, myxoid material, and spindle cell elements. The presence of a large branching vessel within a fatty-appearing lesion is strongly suggestive of hi- bernoma (68). On MR images, most hibernomas are slightly hypointense relative to subcutaneous fat on T1-weighted images and demonstrate
a.
b.
c.
variable signal intensity on T2-weighted images, owing to their variable tissue contents. Less commonly, they demonstrate signal intensity that parallels that of subcutaneous fat. Contrast enhancement varies from none to intense and from focal to diffuse. Positron emission to- mography with fluorine 18 fluorodeoxyglucose (FDG) has the potential to allow differentiation of hibernomas from other fat-containing lesions because of the presence of brown fat, which has an extremely high metabolic rate, with a maxi- mum standardized uptake value greater than 10 owing to the considerable mitochondrial activity of the tumor. Liposarcomas show low to intermediate FDG uptake, with the value depending on histologic subtype. Pleomor- phic liposarcomas demonstrate the most FDG uptake (median standardized uptake value, 4.6; range, 2.6-9.1), whereas well-differentiated liposarcomas display the least uptake (median standardized uptake value, 2.0; range, 1.2-3.2)
(64). Thus, it has been proposed that high FDG uptake values in fat-containing lesions are sug- gestive of hibernomas. Hibernomas may also demonstrate fluctuation in standardized uptake values over time, another finding that may be helpful for differentiating hibernomas and ma- lignant fatty tumors (69).
Retroperitoneal Myelolipomas
Extra-adrenal myelolipomas are benign lesions that are histologically identical to adrenal myelolipomas. They contain mature adipose cells and “trilineage” hematopoietic cells (red blood cells, white blood cells, and platelets) in distributions that are identi- cal to those of normal bone marrow. They usually occur in the presacral region but have also been re- ported in the perinephric space (70). Presacral my- elolipomas occur predominantly in elderly women (mean age, 65 years; ratio of women to men, 2:1) and are usually asymptomatic but may manifest with symptoms related to mass effect on the pelvic organs. The tumors may be large at presentation, ranging from 5 to 19 cm in diameter.
On US images, these tumors vary in their echogenic pattern and may be hyperechoic or hypoechoic, depending on the predominance of fat or hematopoietic elements, respectively. On CT images, they are well-circumscribed encapsulated fatty masses interspersed with avidly enhancing hematopoietic soft-tissue elements. The attenuation
value of fat in myelolipomas is usually higher than that of retroperitoneal fat because of the admixture with hematopoietic tissue (54). Areas of intratu- moral hemorrhage or calcification may also be seen. On MR images, areas of high signal intensity on T1-weighted images and intermediate to high signal intensity on T2-weighted images with loss of signal intensity on fat-suppressed images confirm intratumoral fat (Fig 32). Myelolipomas show vari- able enhancement, depending on the proportion of hematopoietic tissue. Technetium 99m-sulfur colloid scintigraphy has been suggested to confirm the presence of erythroid elements in myelolipomas (71); however, it has not proven useful in the cases we have encountered.
The differential diagnosis for fat-containing presacral masses includes lipoma, liposarcoma, teratoma, extramedullary hematopoiesis, and lipomatosis. Unlike liposarcomas, myelolipomas are typically well defined and do not exhibit an infiltrative growth pattern. Teratomas are usually seen in younger individuals and contain fluid-filled cystic components, whereas myelolipomas are typically seen in elderly patients and do not show cystic changes. Differentiating presacral myeloli- pomas from other fat-containing lesions may not always be possible with imaging alone. Presacral myelolipomas occur predominantly in elderly women, which is the single most useful feature for differentiating this tumor from other fat-contain- ing presacral lesions (72).
Retroperitoneal Fat-containing GCTs
Teratomas are GCTs that are commonly com- posed of multiple cell types, derived from one or more of the three germ cell layers. Teratomas that contain only one or two germ cell layer compo- nents are considered monodermal or bidermal, respectively. Primary retroperitoneal teratomas account for 1%-11% of retroperitoneal neoplasms and are most commonly found in neonates and young adults. About 1%-2.5% of GCTs originate in an extragonadal location from aberrant primor- dial germ cell rests; the retroperitoneum is the second most common extragonadal site for GCTs, after the mediastinum.
Mature Cystic Teratoma .- Primary retroperito- neal mature cystic teratomas are extremely rare. They are more common in female patients, with a female-to-male ratio of 3.4:1, and have a bimodal age distribution, with a peak in the first 6 months of life and a second peak in early adulthood (73). Typically, mature cystic teratoma appears as a well- circumscribed complex cystic mass that contains a variable amount of fluid, fat, or sebum in the form of a fat-fluid level and calcification in a congealed or linear strand pattern (Figs 33, 34) (73).
While most mature cystic teratomas are benign, a variety of malignant components may be present or develop from clonal transformation. Primary ret- roperitoneal mature teratomas can rarely undergo malignant transformation. Adenocarcinoma is the most common somatic malignancy that arises in retroperitoneal mature cystic teratomas, as opposed to malignant transformation in ovarian mature cystic teratomas (in which squamous cell carcinoma occurs in most cases) (74).
Sacrococcygeal Teratoma .- Sacrococcygeal tera- tomas are commonly seen in infants and chil- dren, with a prevalence of approximately one in every 40 000 live births and a female-to-male ratio of about 4:1 (75). Sacrococcygeal teratoma rarely occurs in adults and its true incidence is
Figure 32. Presacral myelolipoma in a 74-year- old woman. Sagittal T2-weighted MR image (a), axial T1-weighted MR image (b), and axial fat-sup- pressed T1-weighted MR image (c) show a lobu- lated presacral mass (arrows). The mass demon- strates heterogeneous predominantly high signal intensity relative to that of muscle in a and b and loss of signal intensity in c.
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C.
2
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not known, but a review of the current literature demonstrates that fewer than 120 cases have been reported worldwide (76). In contrast to the neonatal presentation of sacrococcygeal terato- mas, in which 90% are visible externally, most sacrococcygeal teratomas in adults manifest as intrapelvic masses. Clinically, presacral teratomas may be asymptomatic or may manifest as a pelvic mass, with recurrent pilonidal infections or pres- sure symptoms, including constipation. There is a 20%-30% risk for infection and a 1%-12% risk for malignant transformation in adult patients- usually adenocarcinoma (76,77). Because benign and malignant tissues can coexist within a sacro- coccygeal teratoma, surgical excision and careful histopathologic analysis are necessary to exclude malignancy in adults. At imaging, sacrococcygeal teratomas may have varied appearances, ranging from cystic to solid masses; areas of intratumoral fat may also be seen.
Malignant Teratoma .- Most malignant retro- peritoneal GCTs are metastases from primary gonadal GCTs, and they are seen in 30% of pa- tients with primary gonadal GCTs (54). Careful US examination of the testes is crucial in every patient with a retroperitoneal GCT to evaluate
the patient for primary testicular GCTs. Oc- casionally, the primary testicular tumor is not visible, or small intratesticular scars are found in patients with retroperitoneal GCTs; these scars represent regressed “burned out” GCTs (78). Gonadal GCTs that contain immature tera- tomatous elements can manifest as retroperito- neal masses that contain fat.
Retroperitoneal GCTs are usually large at pre- sentation. The presenting symptoms and signs in- clude a palpable mass with or without pain, weight loss, constipation, hip and back pain, dyspnea, leg edema, fever, varicocele, and urinary retention. Encasement, displacement, and compression of the abdominal vessels are common. On images, both primary and metastatic malignant teratomas appear as enhancing soft-tissue masses with foci of fat and calcifications (Fig 35).
Retroperitoneal Paraganglioma
Retroperitoneal paragangliomas arise predomi- nantly from the paraganglia, which are symmetri- cally distributed along the abdominal aorta and are closely related to the sympathetic nervous system (79). The retroperitoneum accounts for 85% of all extra-adrenal paragangliomas. In the retroperito- neum, the most common site for paragangliomas is the organ of Zuckerkandl, which is located anterior to the aorta at the level of the origin of the inferior mesenteric artery (80).
Paragangliomas are commonly seen in the 3rd- 4th decades of life, with no sex predilection. They can be associated with type 1 neurofibromatosis, multiple endocrine neoplasia syndrome, and von Hippel-Lindau disease. There is an increased inci- dence of extra-adrenal paragangliomas in patients with genetic mutations within the succinate dehy- drogenase B subunit and the succinate dehydroge- nase D subunit. Extra-adrenal paragangliomas are more likely to be malignant (40%) than are adrenal pheochromocytomas (2%-11%). Patients with functioning paragangliomas (40%) present with a variety of symptoms that result from catecholamine excess, including headaches, palpitation, excessive sweating, tremor, vomiting, chest pain, constipation, and visual disturbances. Patients with nonfunction- ing paragangliomas usually present late after disease onset, with symptoms resulting from compression of surrounding structures. Rarely, paraganglioma can manifest with acute abdomen caused by retro- peritoneal hemorrhage.
On CT images, paraganglioma is usually seen as a large, well-defined, avidly enhancing tumor with areas of hemorrhage and necrosis. The tumor is highly vascular, and large vessels can be seen around it. Punctate calcification is seen in 15% of cases, and a fluid-fluid level can be seen due to intratumoral hemorrhage. On MR images, paragan- gliomas are usually hypointense or isointense com- pared with the liver parenchyma, with signal voids on T1-weighted images. Small tumors are markedly hyperintense, while larger tumors usually show vari- able signal intensity on T2-weighted MR images. Radionuclide imaging performed after administra- tion of m-iodobenzylguanidine shows high uptake in paragangliomas and is a sensitive technique for localizing these lesions in patients who present with suggestive symptoms and who show labora- tory evidence of catecholamine excess (79,81). As with adrenal pheochromocytoma, microscopic and macroscopic fat is rarely found in retroperitoneal paraganglioma (16).
Extramedullary Hematopoiesis
Extramedullary hematopoiesis is characterized by abnormal deposits of hematopoietic tissue outside the bone marrow as a compensatory
mechanism for inefficient hematopoiesis by the bone marrow. Although extramedullary hemato- poiesis is not a true neoplasm, it can manifest as a mass in the retroperitoneum. The condition is seen in hemoglobinopathy, myelofibrosis, leuke- mia, lymphoma, and carcinoma. The retroperi- toneum is an uncommon site for extramedullary hematopoiesis, which is typically found in the liver, spleen, and lymph nodes. Common sites of involvement in the retroperitoneum include the paravertebral, perinephric, and presacral regions. Perinephric involvement of extramedullary hema- topoiesis is rare and appears as soft-tissue masses intermixed with fat or as a diffuse uniform infiltrative process that surrounds the kidneys (82,83). Splenectomy may have a role in accel- erating development of extramedullary hemato- poiesis in uncommon sites (82). The presence of fat in extramedullary hematopoiesis masses is indicative of old inactive lesions. The typical CT appearance is round or lobulated masses with soft-tissue attenuation, with or without macro- scopic fat. The MR imaging appearance varies and depends on the relative proportion of active and inactive marrow. Active extramedullary red marrow usually appears as a well-demarcated isointense or hypointense mass on T1- and T2- weighted images. After contrast material ad- ministration, there is always some enhancement in active extramedullary hematopoiesis lesions. Areas of fatty infiltration within old inactive dis- ease demonstrate high signal intensity on T1- and T2-weighted images. Areas of low signal inten- sity can be seen on T1- and T2-weighted images because of the red marrow or hemosiderin content (84). The diagnosis is evident when the patient has a known history of hemoglobinopathy, hepato- splenomegaly, and characteristic skeletal changes of a marrow infiltrative process (54).
Retroperitoneal Ganglioneuroma
Ganglioneuromas are rare tumors that arise from the autonomic ganglion cells of the pe- ripheral nervous system-usually the sympa- thetic ganglion. They represent the benign end of the spectrum for the ganglion cell lineage, whereas ganglioneuroblastomas and neuroblas- tomas comprise the malignant counterparts. Ganglioneuromas may arise anywhere along the peripheral autonomic ganglion sites but are most common within the posterior mediasti- num (39%-43% of cases) and retroperitoneum (32%-52% of cases). Less commonly, they may arise in the adrenal medulla, parapharyngeal region, visceral ganglia, or cranial nerve ganglia. They are usually asymptomatic but can manifest with pain or a mass. Ganglioneuromas occasion- ally secrete hormones such as catecholamines,
vasoactive intestinal peptides, or androgenic hormones. This tumor is commonly seen in the 20-40-year-old age group, with no sex predilection. Histopathologically, ganglioneuromas are composed of Schwann cells, ganglion cells, and nerve fibers. In the retroperitoneum, the tumor is commonly seen along the paraverte- bral sympathetic ganglia (59% of cases) or, less commonly, in the adrenal medulla. Lipomatous ganglioneuroma, previously called “ganglioneu- roma with fatty replacement,” is an extremely rare variant of ganglioneuroma that was first reported in 1999 by Hara et al (85). It is char- acterized by a mature adipocytic component admixed with conventional ganglioneuromas (86-89). The presence of fat can be explained by adipocytic differentiation or lipometaplasia (86).
On CT images, ganglioneuroma is depicted as a well-circumscribed, lobulated, hypoattenuating mass that may surround a blood vessel without narrowing the lumen (Fig 36). Ganglioneuromas may grow through the adjacent intervertebral neuroforamen and extend into the spinal canal with a “dumbbell” or “hourglass” configuration. Discrete punctate calcifications are seen in 20%- 30% of ganglioneuromas. Necrosis and hemor- rhage are uncommon. The presence of fat is unusual; when present, it appears as locules of fat attenuation intermingled with soft-tissue compo- nents or peripheral fat replacement surrounding soft-tissue elements (90). After contrast material administration, ganglioneuromas usually demon- strate characteristic mild enhancement during the arterial phase and progressive delayed retention of contrast material on delayed phase images. On MR images, a ganglioneuroma is homogeneously hypointense on T1-weighted images, with varying signal intensity on T2-weighted images depend- ing on the myxoid, cellular, and collagen compo- nents. A whorled appearance on MR images has been reported and is due to interlacing bundles of longitudinal and transverse Schwann cells or collagen fibers. Generally, ganglioneuromas are larger and have more calcification than do nerve sheath tumors. The prognosis is good after surgi- cal resection.
Conclusion
The relationship between a retroperitoneal mass and the retroperitoneal organs determines whether the mass arises from or displaces neighboring organs. Fat is easily recognized because of its char- acteristic imaging appearance, and its presence within a retroperitoneal lesion helps in refining the differential diagnosis. Most fat-containing retro- peritoneal lesions can be characterized on the basis of a patient’s demographics, clinical presentation, and imaging features. When there is any doubt,
imaging can guide percutaneous biopsy to obtain tissue for pathologic examination.
Acknowledgments .- Special thanks to Kent Sanders, MD, for the medical illustrations.
Disclosures of Conflicts of Interest .- M.T. Activities related to the present article: disclosed no relevant relationships. Activities not related to the present article: royalties and consulting fees from Amirsys. Other activities: disclosed no relevant relationships.
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