
 lipoma is a rare tumor but can be found at various sites in the abdominal cavity ( fig .
2 ) . if it is of a large size , it can compress adjacent tissues or organs and become heterogeneous . in such cases ,
imaging is incapable of distinguishing it from liposarcoma.lipoblastoma is a type of a benign tumor with embryonic fat .
it is usually found in the extremities and torso whereas abdominal cavity belongs to its rather rare sites . in us , such tumors are echogenic .
lipoblastomatosis is a form of this tumor characterized by aggressive localized growth.hibernoma is a benign tumor , mainly composed of brown fat , in whose cells the numbers of mitochondria responsible for enhanced metabolic activity are impressive .
these tumors are rarely encountered in the retroperitoneal space but are also echogenic with well - or ill - defined margins .
their characteristic feature is evident flow in color doppler.other benign tumors with a fat component , such as myolipoma , angiomyolipoma and myelolipoma , apart from teratoma ( which is usually benign ) , tend to be homogeneous with echogenicity increased to various degrees ( fig . 3 , 4 ) .
only gonadal germ cell tumors , such as teratoma and dermoid cyst that arise from several germ layers , frequently have irregular echotexture due to the presence of fluid sebum , hair , soft tissues and bony elements .
that is why they are encountered in solid , cystic or solid - cystic forms ( fig .
mature fat tissue can also be found in choristoma which , however , is not a neoplasm .
lipoma is a rare tumor but can be found at various sites in the abdominal cavity ( fig .
2 ) . if it is of a large size , it can compress adjacent tissues or organs and become heterogeneous .
it is usually found in the extremities and torso whereas abdominal cavity belongs to its rather rare sites . in us , such tumors are echogenic .
hibernoma is a benign tumor , mainly composed of brown fat , in whose cells the numbers of mitochondria responsible for enhanced metabolic activity are impressive .
these tumors are rarely encountered in the retroperitoneal space but are also echogenic with well - or ill - defined margins .
other benign tumors with a fat component , such as myolipoma , angiomyolipoma and myelolipoma , apart from teratoma ( which is usually benign ) , tend to be homogeneous with echogenicity increased to various degrees ( fig . 3 , 4 ) .
only gonadal germ cell tumors , such as teratoma and dermoid cyst that arise from several germ layers , frequently have irregular echotexture due to the presence of fluid sebum , hair , soft tissues and bony elements .
that is why they are encountered in solid , cystic or solid - cystic forms ( fig .
mature fat tissue can also be found in choristoma which , however , is not a neoplasm .
two views show lipoma ( arrows ) as a slightly echogenic lesion in the supraperitoneal fat three lipomas ( l ) in the small bowel mesentery , which in computed tomography showed density ranging from 57 to 74 hounsfield units two views show angiomyolipoma ( l ) located entirely in the adipose capsule of the right kidney .
an arrow points to the site of regrowth in the form of the beak sign myelolipoma in the right suprarenal field ( arrow ) as a visible hyperechoic mass a heterogeneous mass arising from the retroperitoneal space is a mature teratoma with slight calcifications ( arrows )
liposarcoma is the only malignancy deriving from adipose tissue that is relatively frequently encountered in the retroperitoneal space ( it constitutes nearly 1/3 of sarcomas at this site and 1015% of all neoplasms of this type ) . according to the who , these tumors can be divided into five histological subtypes depending on the grade of their differentiation ( well - differentiated , dedifferentiated , myxoid , round cell , pleomorphic ) , which is relevant in prognosis .
its presence can be indicated only by uneven margins and heterogeneous enhancement upon contrast agent administration .
poorly differentiated subtypes account for even 40% of local relapses , and metastases are observed in 17% of cases . as has already been mentioned , well - differentiated liposarcoma is difficult to distinguish from lipoma also in sonography .
one of the differential criteria can be tumor 's reaction to compression with a transducer ( fig .
more aggressive forms of this tumor , however , tend to be unevenly delineated , present heterogeneous echotexture and infiltrate adjacent tissues ( fig .
myxoid liposarcoma is characterized by a cystic structure and should be distinguished e.g. from lymphangioma , teratoma or cystic mesothelioma .
no lesion compressibility when pressure is applied with the transducer ( arrow ) heterogeneous retroperitoneal liposarcoma infiltrates into adjacent tissues ( arrows ) mesenteric liposarcoma with pathological vascularity finally , immature teratoma must be mentioned .
it is a primary germinal tumor that can contain adipose tissue and occurs at similar sites .
they are characterized by the predominance of a solid component , and the only sign suggesting a malignancy is poorly circumscribed margins . a chance for malignant transformation
cystic lesions can correspond to mesenteric cysts , lymphatic or vascular angiomatosis and even a cystic form of peritoneal mesothelioma , which is considered a benign lesion ( fig .
the differential diagnosis of these pathologies should include ovarian cystic tumors , endometrial cysts and cystic tumors growing beyond organs , e.g. of the pancreas , kidneys or liver .
solid tumors found at these sites include : leiomyoma and rhabdomyoma , angiomas as well as stromal , neurogenic and desmoid tumors ( fig .
desmoid tumors , also called fibromatosis , are an interesting group of benign tumors that exhibit local aggressiveness and have considerable tendency to recur ( in approximately 50% of cases ) .
all lesions were hypoechoic with slightly irregular echo patterns , 59% of cases were well - defined and 41% had irregular margins .
the differential diagnosis should include sarcomas , mainly the two types that are most frequently found in soft tissues : histiocytic fibrosarcoma and liposarcoma , i.e. lesions characterized by higher echogenicity .
it is of note that desmoid tumors frequently coexist with familial adenomatous polyposis , which is estimated at 918% of cases . of all imaging modalities ,
some authors recommend punch or even surgical biopsy in the event of doubts . in this case ,
mesenteric lymphangioma as a multilocular lesion without flow ( arrow ) retroperitoneal neuroblastoma in a 13-year - old ( arrows ) .
aorta , v  inferior vena cava desmoid , poorly vascularized tumor ( d ) in the small bowel mesentery
these changes are rarely encountered but they represent a rich spectrum of mainly mesenchymal neoplasms : fibrosarcoma , malignant fibrous histiocytoma , hemangiopericytoma , leiomyosarcoma , rhabdomyosarcoma and gastrointestinal stromal tumors ( fig .
an epithelioid malignant neoplasm encountered at this site is malignant peritoneal mesothelioma , a lesion that almost exclusively develops in males in the fifth and sixth decade of life .
it can assume two morphological forms : irregular peritoneal infiltration without ascites ( dry appearance ) ( fig .
13 ) and nodular thickening of the peritoneum and greater omentum with ascites ( wet appearance ) .
lesions of this type must be distinguished from papillary serous carcinoma of the peritoneum ( a rare neoplasm found in elderly women ) , abdominal carcinomatosis , in which ascites prevails over implants , and peritoneal tuberculosis , in which there are no tumorous lesions ( but ascites is present , the greater omentum is thickened with preserved smooth surface and the mesenteric lymph nodes are enlarged ) .
lymphomas , however , are characterized by considerable abdominal lymphadenopathy , rare and slight ascites and no omental involvement .
rhabdomyosarcoma with rich vascularity in the retroperitoneal space ( arrows ) two views present poorly vascularized malignant mesothelioma of the greater omentum ( m )
this is the most numerous group with prevailing carcinoma foci , mainly deriving from abdominal organs , such as pancreas , stomach , large bowel or ovaries .
these lesions spread by direct extension , via the lymphatic system or blood stream and through the peritoneal cavity . their echotexture can vary .
14 ) and thickened parietal peritoneum ( its normal thickness is up to 1.5 mm ) ( fig .
echogenic lesions that only slightly differ from the surrounding tissues are the hardest to find ( fig .
peritoneal sites at which carcinoma implants tend to develop are : the recto - uterine pouch , right iliac fossa , right paracolic gutter and the region of the upper sigmoid mesentery .
moreover , ovarian carcinoma cells frequently migrate to the supra- and subhepatic area ( fig .
well - designed prospective studies have proven ultrasound imaging to be highly useful in detecting abdominal carcinomatosis .
it occurred that lesions in the greater omentum were the easiest to detect ( 91% ) whereas those in the small bowel mesentery were the hardest to find ( 67% ) .
moreover , ultrasound guidance enables one to obtain diagnostically valuable material for cytological and histological analysis in the vast majority of such cases .
metastasis of malignant melanoma ( m ) from the interscapular space to the small bowel mesentery peritoneal carcinomatosis of ovarian carcinoma . thickened hypoechoic parietal peritoneum with signs of vascular flow ( arrows ) two views show a hyperechoic implant of ovarian carcinoma in the gastrocolonic ligament ( arrows ) ovarian carcinoma implants in the pouch of morison ( arrows ) another rare neoplasm encountered in fatty bodies of the abdominal cavity is non - hodgkin lymphoma .
the involvement of the small bowel mesentery , with prominent , large mesenteric lymph nodes , is observed to occur in a half of patients with this neoplasm .
these lymph nodes , together with the superior mesenteric vessels , create a so - called  sandwich  image ( fig .
it is usually a small neuroendocrine neoplasm located in the small bowel . sometimes , the first morphological sign of its existence is the presence of mesenteric abnormalities in the form of enlarged lymph nodes or a hypoechoic mass .
mesenteric thickening or contraction , which can lead to small bowel obstruction , is more rarely a predominant element of the image . any doubts in the assessment of these lesions require verification in computed tomography and , sometimes , magnetic resonance imaging . ultrasound imaging , however , enables precise , percutaneous sampling for cytological and histological analysis , which is worth remembering and applying . 
sandwich  sign in the small bowel mesentery ( arrows ) created by enlarged lymph nodes surrounding the superior mesenteric vessels  a manifestation of non - hodgkin lymphoma extranodal location of follicular lymphoma in the small bowel mesentery ( arrow ) lesions in the lymph nodes , which are relatively frequently affected by various neoplastic and non - neoplastic processes , have been presented only briefly since these issues are too broad to be included in this review .
authors do not report any financial or personal connections with other persons or organizations , which might negatively affect the contents of this publication and/or claim authorship rights to this publication .
 this article focuses on various cancerous lesions that are found beyond organs in the intra - abdominal fat and can be visualized with ultrasonography . 
 these lesions are divided into five groups . 
 the first group includes primary benign tumors containing adipocytes , such as lipoma , lipoblastoma , hibernoma and other lesions with an adipose tissue component , such as myolipoma , angiomyolipoma , myelolipoma and teratoma . 
 the second group comprises primary malignant adipocytecontaining tumors , including liposarcoma and immature teratoma . 
 the third group contains primary benign tumors without an adipocyte component that are located in intra - abdominal fat . 
 this is a numerous group of lesions represented by cystic and solid tumors . 
 the fourth group encompasses primary malignant tumors without an adipocyte component that are located in intra - abdominal fat . 
 these are rare lesions associated mainly with sarcomas : fibrosarcoma , malignant fibrous histiocytoma , hemangiopericytoma and leiomyosarcoma . 
 an epithelioid tumor at this site is mesothelioma . 
 the last but not least group includes secondary malignant tumors without an adipocyte component located in intra - abdominal fat . 
 this is the most numerous group with prevailing carcinoma foci . for each of these groups , 
 the authors present ultrasound features of individual lesions and discuss their differential diagnosis . in the vast majority of cases , 
 the material for cytological and histological analysis can be obtained during ultrasound - guided procedures . 
 this is the advantage of this imaging modality . 
Primary benign adipocyte-containing tumors located in intra-abdominal fat
Liposarcoma
Primary benign tumors without an adipocyte component located in intra-abdominal fat
Primary malignant tumors without an adipocyte component located in intra-abdominal fat
Secondary malignant tumors without an adipocyte component located in intra-abdominal fat
Conflict of interest