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retroperitoneal sarcomas are rare tumors accounting for only 1%2% of all solid malignancies . of all sarcomas ,
only 10%20% of sarcomas are retroperitoneal sarcomas , and the overall incidence is 0.3%0.4% per 100000 of the population .
the peak incidence is in the 5th decade of life , although they can occur in any age group .
the most common types of retroperitoneal soft tissue sarcomas in adults vary from study to study .
however , in most studies , the most frequently encountered cell types are liposarcomas , leiomyosarcomas and malignant fibrous histiocytomas ( mfh ) .
recently , the frequent diagnosis of mfh in the retroperitoneum has been - disputed . with the use of immunohistochemistry ,
many of these fibrous tumors have now been shown to represent other sarcoma types such as leiomyosarcomas or dedifferentiated liposarcomas [ 3 , 4 ] .
for this reason , it is anticipated that the number of these neoplasms that will be considered as mfh will be dramatically reduced in the future .
patients with sarcomas present late , because these tumors arise in the large potential spaces of the retroperitoneum and can grow very large without producing symptoms [ 5 , 6 ] .
moreover , when symptoms do occur , they are nonspecific , such as abdominal pain and fullness , and are easily dismissed as being caused by other less serious processes .
imaging is important in the diagnostic workup of these patients , being required not only for tumor detection , staging , and operative planning , but also for guiding percutaneous or surgical biopsy of these tumors .
as other neoplastic processes , such as lymphoma and metastatic disease , which are treated differently , may mimic retroperitoneal sarcomas , tissue diagnosis is of paramount importance .
therefore , image - guided and surgical biopsies have a relatively greater role to play in the diagnosis of retroperitoneal sarcomas than is the case for sarcomas elsewhere in the body [ 811 ] .
once the diagnosis is made , the surgical team needs to determine if the retroperitoneal sarcoma can be resected .
therefore one of the first determinations to be made is whether the tumor is localized , its local extent , and also if there is evidence of intra- or extra - abdominal metastatic spread of tumor .
the location and size of the tumor , its relationship to adjacent organs , presence or absence of local extension , relationship to and/or involvement of major vascular structures , as well as the presence of normal anatomic variants and anomalies of major abdominal arteries and veins , are all crucial pieces of information that need to be provided .
since resection of one kidney is not uncommon , any radiographic evidence of unilateral renal dysfunction involving the kidney that is not adjacent to the tumor should be relayed to the surgical team . while it may be unavoidable that the patient will be left with a single poorly functioning kidney , the surgeon must be provided with all relevant information prior to attempted tumor resection . in evaluating preoperative imaging studies
, the radiologist should be cognizant of two facts : ( a ) up to 75% of retroperitoneal sarcoma resections involve concomitant resection of at least one adjoining intra - abdominal visceral organ ( commonly large or small bowel or kidney ) ; ( b ) the most common types of vascular involvement precluding resection are involvement of the proximal superior mesenteric vessels or involvement of bilateral renal vessels .
accordingly , since these tumors tend to invade organs with which they are contiguous , such contiguity must be mentioned even in the absence of imaging evidence of gross tumor invasion of these organs .
also , the mesenteric and renal vessels must be carefully examined and their relationship to a mid or upper retroperitoneal tumor described .
accurate staging is important as it facilitates determination of appropriate surgery , establishes prognosis , and provides a guide for adjunctive therapy .
the american joint committee staging system ( tables 1 and 2 ) of extremity soft tissue sarcomas , which is based on the tnm classification , is most commonly used for most retroperitoneal soft tissue sarcomas , although it is better suited for extremity sarcomas .
this staging system takes into consideration histological grade , tumor size and depth relative to the superficial muscular fascia , presence or absence of lymph node involvement , and the presence or absence of distant metastases ( table 2 ) .
nearly all retroperitoneal sarcomas are large and > 5 cm and are deep to the superficial fascia .
therefore localized retroperitoneal sarcomas are nearly always classified as stage iib ( large , low - grade , and deep ) or stage iii ( large , high - grade and deep ) neoplasms , with the distinction between these two stages being made only on the basis of histologic grade .     
imaging can not be reliably used to predict the cell types of most sarcomas ( fig .
1(a ) , ( b ) ) , with rare exceptions being liposarcoma and intracaval leiomyosarcomas . the presence of macroscopic fat enables one to make the diagnosis of a liposarcoma ( fig .
3 ) . some of these tumors may be composed almost entirely of soft tissue and fluid components . in these instances ,
a tumor within the lumen of the inferior vena cava with expansion of its lumen and enhancing tumor thrombus is pathognomonic of an intracaval leiomyosarcoma ( fig .
fortunately , it has been shown that in the vast majority of sarcomas , cell type has no impact on treatment and long - term survival .
the major factors that affect survival are the tumor grade and resectability [ 10 , 11 ] .
patients who have had a successful complete resection and also have low - grade tumors have the best survival rates .
the exception is a liposarcoma , and , in general , if a liposarcoma contains mostly fat and very little soft tissue , it is likely to be a low - grade tumor ( fig .
5(a ) , ( b ) ) . however , the converse is not true .
a liposarcoma containing a large amount of soft tissue and with little or no macroscopic fat , may be a low- , intermediate- or high - grade tumor .
calcification or ossification within a liposarcoma has been shown to be a poor prognostic feature , often indicating dedifferentiation .  
ct is the most commonly used modality for identification , localization , and staging of retroperitoneal sarcomas [ 5 , 8 , 1820 ] .
the use of magnetic resonance imaging is generally reserved for selected problem solving ; such as to address questions regarding vascular invasion , and evaluate problematic indeterminate liver lesions .
more recently pet - fdg imaging has been used in an effort to assess the tumor grade as well as to evaluate patients for tumor recurrence ( fig .
chemotherapy and radiotherapy without surgical debulking have rarely been beneficial , when used alone or in combination .
pre- , intra- or post - operative radiotherapy has , however , been of benefit in some patients , but , in most instances , does not improve patient prognosis [ 5 , 2427 ] . as these tumors are locally invasive , extensive and aggressive local resection of the tumor and any adjacent organs should be performed at the time of presentation .
resection of the tumor en - bloc with adjacent adrenals , kidneys , or segments of small bowel , or colon is often required [ 28 , 29 ] . in a study of 28 patients with liposarcomas ,
adjacent organ resection was carried out in more than half the cases , with partial or total resection of the kidneys in 60% , colon in 50% and adrenal glands in 35% .
despite  complete resections , 5- and 10-year survival rates are poor , being 51% and 36% respectively [ 30 , 31 ] .
most tumor recurrences occur within 2 years of initial surgical resection [ 7 , 32 ] .
since subsequent prognosis in these patients is affected by the ability to completely resect the local recurrences , early detection of tumor recurrence is important .
when re - resections are performed early , they are successful in up to 90% of the patients [ 30 , 31 ] .
unfortunately , many recurrences are diagnosed late in the course of the disease , leading to incomplete resection , which then leads to re - recurrence in about 50% of patients .
as most recurrences are local , a careful scrutiny of the surgical bed for subtle changes on follow - up imaging should be made [ 2832 ] .
clinical follow is usually up not helpful as up to 50% of patients are asymptomatic , and if symptoms are present , they are usually nonspecific .
soft tissue attenuation recurrences may not be easily distinguished from post - operative scarring / fibrosis in the surgical bed .
detection of local recurrence in liposarcomas is especially difficulty as , when small , recurrent liposarcomas can be difficult to distinguish from normal retroperitoneal fat on imaging ( fig .
frequently , closer scrutiny may show that the fat in a recurrent liposarcoma is of slightly higher ct attenuation when compared to normal retroperitoneal fat .
also , at times , recurrent liposarcomas can have different imaging characteristics than that of the primary tumor .
in one ct study of fat - containing liposarcomas , four of the eight recurrent tumors did not contain any visible fat .
as recurrent tumors are best treated with repeat surgical resection , all tumor - bearing sites should be identified to enable optimal and complete re - resection [ 32 , 33 ] .  
regional metastases are also frequent and a thorough search of the draining nodes , peritoneal surfaces , and liver should be made prior to evaluation for distant metastases .
follow - up imaging is usually performed with ct or mri with the frequency of follow - up being often dictated by the completeness of the tumor resection , tumor type and grade .
one suggested follow up scheme is to obtain imaging at regular intervals ( i.e. ct or mri every 34 months for 2 years , then every 46 months for 35 years , and every 12 months thereafter . follow up for greater than 5 years is recommended as although most sarcomas ( whether high - grade or low - grade ) recur within 2 years , marked delay in appearance of recurrent disease is not unusual .
contrast - enhanced axial ct shows two large retroperitoneal heterogenous neoplasms with areas of low density due to degeneration or necrosis .
these proved to be malignant : a peripheral nerve sheath tumor ( a ) and retroperitoneal leiomyosarcoma ( b ) , respectively .  
contrast - enhanced axial ct shows large right retroperitoneal liposarcoma ( arrow ) composed predominantly of fat but also has areas of soft tissue density and calcific components .  
contrast - enhanced axial ct shows left - sided predominantly soft - tissue density abdominal tumor ( arrow ) which proved to be a high - grade pleomorphic liposarcoma .  
large inferior vena caval leiomyosarcoma ( arrow ) seen on ( a ) contrast - enhanced ct and ( b ) coronal contrast - enhanced gradient echo mr image ( arrow ) .
note intraluminal tumor extension on both images .   well - differentiated low - grade and high - grade liposarcomas .
( a ) contrast - enhanced axial ct shows well encapsulated fatty mass ( arrow ) with no septations or soft tissue component .
( b ) contrast - enhanced axial ct shows a predominantly soft tissue pelvic mass ( arrow ) which proved to be a high - grade liposarcoma .  
( a ) coronal pet - fdg image shows peritoneal metastatic nodule ( arrow ) .
( b ) axial contrast - enhanced ct shows this metastatic nodule ( arrow ) , which was not prospectively identified as it was thought to be part of the left lobe of the liver
( a ) contrast - enhanced axial ct shows right retroperitoneal mass ( arrow ) which as a liposarcoma .
( b ) on a 6-month post - resection follow - up contrast - enhanced axial ct , there is suggestion of recurrence ( arrow ) .
( c ) at 12-month follow - up contrast - enhanced axial ct , the mass ( arrow ) has shown interval growth and is more obvious .
retroperitoneal sarcomas are rare neoplasms . ct or mr imaging 
 is performed in patients with these tumors to detect local extent and distant metastases of the tumor and for preoperative surgical planning . 
 most sarcomas can not be characterized as to cell type with ct or mr , with the exceptions being liposarcomas and intracaval leiomyosarcomas . 
 similarly histological grading can not be made definitively with imaging alone , the exception being liposarcoma since well differentiated liposarcomas contain more macroscopic fat than do less differentiated liposarcomas . 
 after surgery , follow up imaging with ct or mr and careful scrutiny of the tumor bed and resection site are essential to detect early recurrences , which can often be managed with re - resection .