|ECR 2018 / C-3029|
|Fat-containing tumors in abdomen and pelvis: the radiologic-pathology correlation|
Findings and procedure details
Appearances of fat on different imaging modalities
On ultrasound (US) images, fat tissues usually appear hyperechoic. However, there are exceptions and this technique is nonspecific and with limited sensitivity in the detection of fat.
On computed tomography (CT), fat appears to have low attenuation with a range of -10 to -100 Hounsfield units (HUs). Identification of macroscopic fat is usually simple, however if the proportion of fat within a voxel is small, then the mean CT number will increase and fat may be difficult to identify. For this reason, CT is not as sensitive for detecting microscopic fat as magnetic resonance imaging (MRI).
On magnetic resonance imaging (MRI) macroscopic fat-containing lesions are hyperintense on T1- and intermediately to hyperintense on T2- weighted images (WIs) with loss of signal on fat-saturated MR images. However, these features alone are not sufficient to characterize the presence of microscopic fat. For this reason, there are a number of fat suppression techniques that allow to differentiate the composition of fat within a lesion, such as in-phase/out-of-phase acquisitions, which demonstrate a drop in signal intensity within the fat containing areas.
Lipomas are benign mesenchymal tumors that can be found almost anywhere in the body where there is adipose tissue. When found intra-abdominally, they frequently arise from the gastrointestinal tract, although they can be found in other rare locations, namely liver and pancreas.
A presumptive diagnosis of lipoma can be made if the lesion is purely fat containing and is small in size (< 3 cm).
On US lipomas appear as soft variably echogenic masses, with no acoustic shadowing and with no or minimal colour doppler flow. A heterogeneous echotexture, a more than minimal colour Doppler flow, or large sizes are suspicious for liposarcoma.
Lipomas typically demonstrate homogenous fatty attenuation at CT (Fig. 1) with homogeneous signal intensity identical to that of fat with all MR imaging pulse sequences and loss of fat signal intensity in fat-saturated sequences (FS).
A negligible enhancement is noted on the dynamic studies after endovenous administration of gadolinium. Thin fibrous septa of low signal intensity on T1- and T2- weighted images may be present. (Fig. 2)
Also some lipomas have prominent fibrous septa and nodularity which can mimic well-differentiated liposarcomas at imaging.
Macroscopically, lipomas are an encapsulated, lobular, yellow and greasy mass. (Fig. 3) Histologically, they lack the widened septa and atypical lipocytes of well-differentiated liposarcoma, however a mild variation in lipocyte cell size and shape is possible. (Fig. 4)
Adrenal adenomas are the most common adrenal mass lesion and are age-related. They are often found incidentally during abdominal imaging ("adrenal incidentalomas").
The majority of lesions are not functioning, so patients usually are asymptomatic.
Usually they are sharply defined, homogeneous, round masses of < 3 cm in size. A small adrenal mass with these characteristics, that remain stable in size and apperance on follow-up examination (within 6 months or more) is very likely benign.
On US they usually are well-defined circumscribed lesions, but they have no specific ultrasound features. (Fig. 5)
CT is the primary modality for both detection and characterization of adrenal masses. On CT scans, adrenal adenomas appear as small, well-defined homogeneous masses that are typically hypo-attenuating relative to the liver. (Fig. 6)
The vast majority of adenomas are lipid-rich, which means that they have precontrast attenuation of -2 to 10 HU.
Lipid–poor adenomas have precontrast attenuation of 20-25 HU, approaching soft tissue density and, for that reason, they are more difficult to diagnose.
It is estimated that up to 30% of adenomas do not contain enough lipids to have low attenuation at CT. Thus, although nonenhanced CT can be used to identify 70% of adenomas, it does not allow reliable differentiation of the 30% that are lipid-poor.
Regardless of lipid content, adenomas typically have rapid contrast washout, whereas non-adenomas lesions tend to wash-out more slowly. Benign adenomas have an absolute percentage washout (APW) of more than 60% and a relative percentage washout (RPW) of more than 40% on delayed images (15 minutes after contrast).
Chemical-shift MR techniques is the most reliable technique for diagnosing adrenal adenomas by demonstrating loss of signal intensity on out-of-phase images. (Fig. 7) Enhancement patterns on MRI have also been investigated as a mean of differentiation benign from malignant lesions, and similar to their appearance at CT, adenomas vigorously enhance and show early wash-out.
Lipid-poor adenomas are generally diagnosed in follow-up or by dynamic postcontrast studies, where they show fast wash-in and early wash-out.
Importantly, none of these techniques (US, CT and MRI) distinguish functioning from nonfunctioning adenomas.
At pathologic analysis, both hyperfunctioning and nonfunctional adenomas have variable amount of intracytoplasmatic fat. The tumor may consist of one or more of the normal cell types (cells of zona glomerulosa, zona fasciculata and zona reticularis), and usually shows an intact thin capsule around the tumor. Generally, in contrast to carcinomas, no necrosis or hemorrhage is seen in adenomas. (Fig. 8)
AML is a benign mesenchymal tumor composed of fat, smooth muscle and thick-walled blood vessels. Individual cells have a mature, benign appearance, but the tissue architecture is abnormal. The fat component typically predominates.
The kidney is by far, the most frequente affected organ and the vast majority arise from renal cortex.
There are two distinct epidemiologic forms: sporadic (80%) or in association with tuberous sclerosis (20%).
The two clinical forms are histologically indistinguishable but vary in the imaging features.
On US they typical appear hyperechoic, located in the cortex with posterior acoustic shadowing. (Fig. 9)
On CT they typically appear as well defined masses with predominant fat-attenuation (Fig. 10). The size is usually < 5 cm. Heterogeneous soft tissue attenuation, due to hemorrhage, fibrosis or vascular components may be evident.
Contrast enhancement is variable and depends on the amount of soft tissue and vascularity.
The demonstration of mature fat inside a smooth kidney nodule (high signal intensity in T1-WI and loss of signal on FS sequences) is enough for an accurately diagnosis. (Fig. 11)
Macroscopically, AML are yellow and gray, can vary in size, and are typically centered within the renal parenchyma, perirenal fat or hilum of the kidney. (Fig. 12)
As the name indicates, they have 3 types of components in varying proportions: epithelioid, immature and sometimes atypical, smooth muscle component, prominent vasculature and mature fat. The fat component typically predominates. (Fig. 13)
Myelolipoma is a benign tumor composed of mature fat and interspersed hematopoietic elements that resemble bone marrow.
They occur almost exclusively in an otherwise normal adrenal gland. Extraadrenal myelolipomas are significantly less common, but other sites include the mediastinum, abdomen, and muscle fascia.
Usually asymptomatic and discovered incidentally at cross-sectional imaging, myelolipoma occasionally causes discomfort due to compression or hemorrhage.
At US, the typical appearance of myelolipoma is that of a hyperechoic mass with more hypoechoic regions in the predominantly myeloid components. The interspersed areas of fatty and myeloid tissue appear to be the most echogenic whereas regions of pure fat may be hypoechoic. The margins of the lesion are often difficult to define due to a lack of contrast with the surrounding retroperitoneal fat.
On CT, lesions usually have a negative Hounsfield unit value owing to macroscopic fat. Because of intermixed hematopoietic tissue, the attenuation is usually higher than that of retroperitoneal fat. High attenuation regions may be seen due to hemorrhage or calcifications. (Fig. 14).
In MR imaging, the fatty component is usually hyperintense on T1-WI and heterogeneously hyperintense on T2-WI due to nonuniform admixture of fat and marrow components.
Macroscopic fat can be demonstrated by loss of signal intensity with a fat-suppressed pulse sequence when compared with an identical sequence without fat suppression. Fat-suppressed techniques may be the best method for demonstrating the fat in a myelolipoma and the presence of marrowlike elements or hemorrhage results in persistent areas of increased intensity on fat-suppressed MR images. Opposed-phase imaging should demonstrate low signal intensity in voxels containing both fat and water tissue.
A thin peripheral rim of mild enhancement is often seen on contrast-enhanced T1-WI. Associated retroperitoneal hemorrhage may occasionally be seen as well.
The diagnosis of myelolipoma is practically certain after detecting macroscopic fat within an adrenal mass.
Myelolipomas appear grayish-red, typically contain a pseudocapsule, and can grow to large sizes. (Fig. 15)
Microscopically, it is composed of mature benign white fat and marrow elements. The myeloid component is best characterized by the large megakaryocytes admixed with other smaller marrow cell types. (Fig. 16)
By definition, a teratoma must contain tissue derivatives of germ cell layers—namely, ectoderm, mesoderm, and endoderm—and evidence of tissue differentiation.
They usually occur in ovaries, testes, anterior mediastinum, presacral and coccygeal areas, and retroperitoneum.
Ovarian teratomas are the most frequent germ cell neoplasms and comprise 3 histologic types (mature cystic teratoma, immature teratoma, and monodermal teratoma).
The most frequent teratomas are mature cystic teratoma (dermoid cyst) and contain well-differentiated tissue from at least two of the three germ cell layers.
The neoplasms that demonstrate malignant behavior are immature teratomas.
The morphologic features of teratoma extend from predominantly cystic to completely solid lesions.
Most contain a large amount of fat in the form of liquid sebum, which is hypoechoic on sonography, of low density on conventional radiographs, and of high signal on MRI.
On US they are primarily cystic tumors, and typical findings include a complex echogenic mass with solid and cystic components that may or not have an acoustic shadowing (depending on the presence of calcification).
Dermoid plug (or Rokitansky nodule) are the most common sonographic feature, appear as an echogenic mass within the cyst, and represent a conglomeration of tissues that typically contain fat, calcium or hair. On the other hand, dermoid mesh, as the name implies, is characterized by multiple small hyperechoic lines and dots (small hair) within a cyst, resembling a "mesh-like" picture.
Pure sebum within the cyst may be hypoechoic or anechoic. A fat-fluid is very characteristic and it is believed to be the result of layering of serous fluid and sebum.
On CT, fat attenuation within a cyst, with or without wall calcification, is indicative of mature cystic teratoma. If fat appears with the configuration of sebum (that is fat attenuation or signal that horizontally interfaces with a nonfatty material) this feature is almost patognomonic for teratoma.
MR imaging demonstrates the same spectrum of findings, which usually includes a complex mass with both solid and fluid components. The dermoid cysts have very high signal intensity on T1-WI and variable signal intensity on T2-WI, which can also be seen in an intracystic hemorrhage. Both fat suppression techniques and chemical shift artifact can be used to confirm the presence of fat. (Fig. 17)
Occasionally, a low signal intensity fibrous pseudocapsule may be visualized on T1-WI and T2-WI, and this feature helps to exclude invasion of adjacent structures. A dermoid plug may also be seen along the inner surface of a cystic component with variable signal intensity depending on what components present within it.
Histologically they often have cysts lined by various types of epithelium of ectodermal and endodermal origins and mesodermal components that frequently include bone and cartilage.
Adipose tissue may be among mesodermal elements that occupy sufficient volume to be detectable at radiologic evaluation. (Fig. 18)
During the early phases of hepatocarcinogenesis, hepatocytes may accumulate fat.
Low-grade dysplastic nodules, high-grade dysplastic nodules, and early HCCs may become more steatotic. On the cirrhotic liver, fatty change in a dysplastic nodule has been recognized as one of the hepatocellular expressions of malignant transformation.
The fat in hepatocellular carcinoma tends to be diffuse in smaller tumors (< 3.5 cm) and focal in larger lesions.
In CT, HCC arterial hypervascularity varies with the grade of malignancy. HCC larger than 2 cm typically shows avid contrast enhancement in the arterial phase and wash-out in portal venous or equilibrium phase.
Small (< 2cm), high grade tumors typically demonstrate bright homogeneous enhancement on arterial-phase with characteristic wash-out in portal venous phase. This finding is the hallmark for detection of small HCC.
HCC with fatty change appears hyperintense on T1-weighted images and demonstrates signal intensity drop on chemical shift images. (Fig. 21)
Nevertheless, the hyperintensity of HCC on T1-weighted images might also be attributed to a number of factors, such as content of glycogen, subacute hemorrhage, clear cell formation, and excessive copper accumulation.
Therefore the dual-echo sequences aid in the diagnosis of intralesional fat content.
Larger lesions are heterogeneous due to fibrosis, fatty change, necrosis and calcification. Well-differentiated hepatocellular carcinoma, can synthesize and store various components of hepatocytes such as lipids (shown in the photomicrograph). (Fig. 22 and Fig. 23)
Most often there are large fat vacuoles (macrosteatosis) as opposed to very small fat droplets (microsteatosis).
This feature helps the diagnosis in difficult cases, once the presence of fat in well differentiated tumors is very common.
Liposarcoma is a malignant tumor of mesenchymal origin that may arise in any fat-containing region of the body and are the most common primary retroperitoneal malignant neoplasm.
Liposarcomas are histologically subdivided ranging from nearly entirely composed of mature adipose cells with low grade malignancy (well-differentiated) to tumors with sparse adipose elements, considered high grade malignancies (pleomorphic) with extensive metastatic potential.
The appearances vary according to the histologic subtypes.
A well-differentiated liposarcoma appears as a heterogeneous, multilobulated, typically well-defined mass. In CT and MR imaging, they usually have a relatively charateristic appearance as a predominantly adipose mass (generally greater than 75%) containg nonlipomatous components, which most often are seen as proeminet thick septa (> 2 mm) that may show nodularity. Focal nodular or globular nonadipose areas may also be apparent (Fig. 24).
Higher grade lesions are often devoid of macroscopic fat making them difficult to distinguish from other non-lipomatous tumors or sarcomas. Frequently they are a large, relatively well-defined, non-specific soft-tissue mass, with areas of necrosis and/or hemorrhage and small focal areas of fat.
Well differentiated liposarcomas are yellow, soft, greasy, may contain lobules and white septa. They usually resemble lipomas although liposarcomas tend to be larger and have dense collagen bands.
Their appearance can be difficult to distinguish from a lipoma, however lipomas are rarely found within the retroperitoneum, so fatty lesions located here should be approached with suspicion. (Fig. 25)
Dedifferentiated components are often visible as firm and well-demarcated areas, embedded within the soft yellow fat of the background tumor. Histologically, they can have spindle cells, with enlarged hyperchromatic and multinucleated nuclei or most commonly, abundant large clear vacuoles with their nucleus displaced at the periphery.