CT and MR Imaging of Retroperitoneal Sarcomas: A Practical Guide for the Radiologist

Simple Summary The diagnosis of retroperitoneal sarcoma may be challenging for the radiologist. Current guidelines report postsurgical margin as the strongest predictive factor for disease-specific survival and recurrence, as well as histologic subtype and grade. The role of the radiologist is indeed important in RPS diagnosis, management, and follow-up, as the ability to promptly recognize local progression, invasion of nearby structures, and complications has a direct impact on patients’ survival. A practical guide is provided to radiologists with an overview of the current knowledge regarding cross-sectional CT/MRI imaging features of patients with retroperitoneal sarcomas, presenting tips and tricks to improve imaging diagnosis of RPS. Abstract Primary retroperitoneal sarcomas (RPS) represent around 10–16% of all sarcomas, with liposarcomas and leiomyosarcomas being the most common subtypes. RPS have some peculiar characteristics, imaging appearances, worse prognosis, and complications compared to other locations of sarcoma. Commonly, RPS primarily present as large masses, progressively encasing adjacent structures, causing mass effect, and complications. RPS diagnosis is often challenging, and these tumors may be overlooked; however, failure to recognize RPS characteristics leads to a worse prognosis for the patients. Surgery is the only recognized curative treatment, but the anatomical constraints of the retroperitoneum limit the ability to achieve wide resection margins; therefore, these tumors have a high rate of recurrence, and require long-term follow-up. The radiologist has an important role in the diagnosis of RPS, the definition of their extent, and their follow-up. Specific knowledge of the main imaging findings is required to reach an early diagnosis, and, ultimately, to guarantee the best patient management. This article provides an overview of the current knowledge regarding cross-sectional imaging features of patients with retroperitoneal sarcomas, presenting tips and tricks to improve imaging diagnosis of RPS.

LPS can contain large quantities of gross fat, thus making their extent assessment very difficult [32][33][34]. The most frequent mistakes on RPS imaging are indeed to underestimate the extension of the well-differentiated part of a LPS that presents both a well-differentiated and a dedifferentiated component, and to miss extension across abdominal wall openings such as the inguinal ligament, or extension into the scrotum or adductor compartment of the thigh or posterior extension out of the sciatic notch [7][8][9][10][11]. Considering that LPS represent up to 70% of all RPS, the diagnostic process should start with searching for the presence of abnormal macroscopic fat and RP distortions [6]. Imaging is also challenging in LPS with unclear gross fat, or, in LMS, when an obvious origin from venous structures is not seen. When several organs such as adrenal glands and kidneys are invaded, assessing the origin of the primary tumor is tricky [32].
Radiologists should describe tumor location, size in the three-dimensional planes, the relationships and mass effect to other structures and vessels, parietal wall, nervous and vascular structures, and spine, as well as the presence of fat, myxoid, necrotic, and hemorrhagic content [9,33]. Regarding vascular and nervous extension, CT can provide some information on the encasement of arteries, veins, and nerves. With regards to MRI, a study by Holzapfel et al. [35] found a sensitivity of 84.6%, 84.6%, and 77.8%, and specificity of 93.8−97.5%, 94.7, and 97.3%, for T2-and fat-suppressed contrast-enhanced T1-weighted imaging in arteries, veins, and nerves encasement, respectively. It is important to effectively describe vessels involvement since it has many repercussions on both surgery and clinical management. Vascular compression can be seen in all RPS, but some subtypes, such as leiomyosarcoma, are prone to macroscopic vascular invasion or can originate primarily from RP vessels, such as the inferior vena cava (IVC). Leiomyosarcomas arising from the IVC require more specialized surgical treatment than their primary retroperitoneal counterparts [36]. Radiologists should describe each anatomical compartment and structure invaded, aponeurotic spreading, satellite tumors (i.e., multifocality), lymphadenopathy, extension to skin, bone, artery, veins, and nerves. Table 1 includes some of the main differential diagnoses of RPS based on the different internal components that might be seen on imaging.  [2][3][4][5][6][7][8][9][10][11][12][13][14]34], and typically occur in adult patients (50-70 years). LPS commonly present as round, oval, or lobulated intra-abdominal fat-attenuating masses that exert mass effect on adjacent structures. They are histologically subdivided into five different subgroups based on the WHO 2020 classification [8]. LPS with both well-differentiated and dedifferentiated components are often poorly evaluated and described as multifocal masses, because only the dedifferentiated or solid components are demarked, while the well-differentiated fatty mass is often not recognized [37,38]. An incomplete report may lead to incomplete surgery, which worsens the prognosis [11,12]. Lungs are the primary metastatic site [11]. LPS subtypes have some peculiar characteristics that may allow their differentiation, as follows: A. Well-differentiated liposarcomas ( Figure 1) are low-grade tumors. Characteristic CT features include macroscopic fat in at least 75% of the whole tumor with smooth and lobular margins, thick septa (>3 mm), tendency to be nodular, and mild or inconstant low enhancement [5,[37][38][39][40]. Calcifications are rare [34] and can indicate dedifferentiation or inflammation. These tumors can recur, but do not tend to metastasize [5]. A. Well-differentiated liposarcomas ( Figure 1) are low-grade tumors. Characteristic CT features include macroscopic fat in at least 75% of the whole tumor with smooth and lobular margins, thick septa (>3 mm), tendency to be nodular, and mild or inconstant low enhancement [5,[37][38][39][40]. Calcifications are rare [34] and can indicate dedifferentiation or inflammation. These tumors can recur, but do not tend to metastasize [5].  [41]. They are heterogeneous, lobular, with internal septations and features often described as "pseudocystic" due to myxoid components. Compared to true cystic lesions, they gradually enhance on delayed postcontrast phases, with progressive accumulation of contrast within the myxoid matrix [37,38]. In more than 50% of cases, there is no fat component [5,37]. Calcifications are rare. Unlike other sarcoma subtypes, it has a propensity for pulmonary, and extrapulmonary metastases, that do not uptake contrast on FDG-PET [41]. Of note, there is a preponderance of spinal metastases, not clearly visible on CT. Clinical practice guidelines have therefore included spine MRI as part of MLS staging [42]. C. Dedifferentiated liposarcomas ( Figure 2) are high-grade tumors with poor prognosis.
Characteristic features include heterogeneous nonlipomatous mass within, adjacent to, or surrounding a fatty mass [37,38]. There may be no evidence of fat-density tissue in up to 20% of cases, making the imaging diagnosis difficult [40]. Enhancing septa within the fatty portions are frequently seen [37]. Calcifications are rare (around 25% of cases) and are poor prognostic factors [39].
D. Pleomorphic liposarcomas contain little or no fat and myxoid components. They are considered high-grade malignancies with high rates of local recurrence and distant metastases [38]. They are heterogeneous masses, isoattenuating to muscles on CT and commonly have internal areas of low attenuation representing necrosis. Calcifications are rare [12]. E. Undifferentiated pleomorphic liposarcoma: imaging features are nonspecific. It manifests as a large, well-circumscribed soft-tissue mass with heterogeneous enhancement and myxoid components. Areas of necrosis and hemorrhage may be seen but are less extensive than leiomyosarcomas. Calcifications occur in up to 20% of cases with a ring-like pattern. Direct invasion of adjacent organs may be present [37].
The main differential diagnosis is with simple lipomas (Figure 3) that will present as purely adipocytic tumors [38]. In this case, when the lesion is ≤10 cm, the patient can be managed without a biopsy. Beyond 10 cm, patients should still undergo MRI and biopsy [9]. Another diagnosis to keep in mind is renal angiomyolipoma, which, contrary to LPS, is hypervascular and presents with a large vessel extending into the renal cortex. Moreover, the presence of a renal parenchymal defect at the site of tumor contact favors exophytic angiomyolipoma. Other rarer occurrences to consider are adrenal myelolipomas, which share similar imaging appearance with angiomyolipoma, and ovarian teratomas. Features that should favor the latter are the presence of fat-fluid levels or tooth-like calcifications [38].
propensity for pulmonary, and extrapulmonary metastases, that do not uptake contrast on FDG-PET [41]. Of note, there is a preponderance of spinal metastases, not clearly visible on CT. Clinical practice guidelines have therefore included spine MRI as part of MLS staging [42]. C. Dedifferentiated liposarcomas ( Figure 2) are high-grade tumors with poor prognosis.
Characteristic features include heterogeneous nonlipomatous mass within, adjacent to, or surrounding a fatty mass [37,38]. There may be no evidence of fat-density tissue in up to 20% of cases, making the imaging diagnosis difficult [40]. Enhancing septa within the fatty portions are frequently seen [37]. Calcifications are rare (around 25% of cases) and are poor prognostic factors [39]. D. Pleomorphic liposarcomas contain little or no fat and myxoid components. They are considered high-grade malignancies with high rates of local recurrence and distant metastases [38]. They are heterogeneous masses, isoattenuating to muscles on CT and commonly have internal areas of low attenuation representing necrosis. Calcifications are rare [12]. E. Undifferentiated pleomorphic liposarcoma: imaging features are nonspecific. It manifests as a large, well-circumscribed soft-tissue mass with heterogeneous enhancement and myxoid components. Areas of necrosis and hemorrhage may be seen but are less extensive than leiomyosarcomas. Calcifications occur in up to 20% of cases with a ring-like pattern. Direct invasion of adjacent organs may be present [37]. The main differential diagnosis is with simple lipomas (Figure 3) that will present as purely adipocytic tumors [38]. In this case, when the lesion is ≤10 cm, the patient can be managed without a biopsy. Beyond 10 cm, patients should still undergo MRI and biopsy [9]. Another diagnosis to keep in mind is renal angiomyolipoma, which, contrary to LPS, is hypervascular and presents with a large vessel extending into the renal cortex. Moreover, the presence of a renal parenchymal defect at the site of tumor contact favors exophytic angiomyolipoma. Other rarer occurrences to consider are adrenal myelolipomas, which share similar imaging appearance with angiomyolipoma, and ovarian teratomas. Features that should favor the latter are the presence of fat-fluid levels or tooth-like calcifications [38]. After surgery, retroperitoneal LPS have a high tendency to recur locally, while metastases are less common. Intraoperatively, low-grade LPS is grossly like normal fat, thus making the resection challenging, as frozen section evaluation is considered unhelpful [2]. Preoperative resection planning is guided by asymmetry shown on imaging, knowledge of functional anatomy, and experience with patterns of recurrence [2].

Leiomyosarcoma
LMS is a smooth muscle tumor [38], more common than LPS in younger age groups [6]. Its most frequent location is the retroperitoneum, near the inferior vena cava [43]. LMS are heterogeneous masses (Figure 4), with irregular peripheral enhancement and enhancing solid portions, mixed with cystic, hemorrhagic, or necrotic areas. LMS show marked T2 hypointensity on MRI and are similar in attenuation to uterine myometrial smooth muscle on CT [39]. Fatty components and calcifications are usually absent [34,38]. The evidence of a large, heterogeneously enhancing, necrotic retroperitoneal mass After surgery, retroperitoneal LPS have a high tendency to recur locally, while metastases are less common. Intraoperatively, low-grade LPS is grossly like normal fat, thus making the resection challenging, as frozen section evaluation is considered unhelpful [2]. Preoperative resection planning is guided by asymmetry shown on imaging, knowledge of functional anatomy, and experience with patterns of recurrence [2].

Leiomyosarcoma
LMS is a smooth muscle tumor [38], more common than LPS in younger age groups [6]. Its most frequent location is the retroperitoneum, near the inferior vena cava [43]. LMS are heterogeneous masses (Figure 4), with irregular peripheral enhancement and enhancing solid portions, mixed with cystic, hemorrhagic, or necrotic areas. LMS show marked T2 hypointensity on MRI and are similar in attenuation to uterine myometrial smooth muscle on CT [39]. Fatty components and calcifications are usually absent [34,38]. The evidence of a large, heterogeneously enhancing, necrotic retroperitoneal mass contiguous with a vessel, with extra-and intraluminal involvement, is highly suggestive of LMS [12,37,43]. It could stem from the inferior vena cava, extending into the intrahepatic portal veins or superior mesenteric vein, or even from small vessels such as renal or gonadic veins. Differentiation from extrinsic compression can be challenging [12,40].

Solitary Fibrous Tumors (SFT)
Although their imaging appearance is nonspecific, most solitary fibrous tumors ( Figure 5) show heterogeneity and are highly vascular, with prominent collateral vessels [38,44,45]. These tumors are well defined, with intense delayed contrast enhancement due to the fibrous stroma [40]. SFT may rarely exhibit cystic degeneration, necrosis, and calcifications [44]. MRI signal intensity is variable and dependent on cellularity and the abundance of myxoid stroma and/or fibrous tissue [40]. Solitary fibrous tumors are typically hypo-to-isointense to muscle on T1-weighted MRI and hypointense on T2weighted sequences [45]. Flow voids may be present because of prominent intralesional vessels [45,46].

Nerve Sheath Tumors (NST)
In the retroperitoneum, a mass originating proximally to the spine, in the psoas muscle, in the sciatic nerve, or sacral plexus, or involving the neural foramina (which must

Solitary Fibrous Tumors (SFT)
Although their imaging appearance is nonspecific, most solitary fibrous tumors ( Figure 5) show heterogeneity and are highly vascular, with prominent collateral vessels [38,44,45]. These tumors are well defined, with intense delayed contrast enhancement due to the fibrous stroma [40]. SFT may rarely exhibit cystic degeneration, necrosis, and calcifications [44]. MRI signal intensity is variable and dependent on cellularity and the abundance of myxoid stroma and/or fibrous tissue [40]. Solitary fibrous tumors are typically hypo-to-isointense to muscle on T1-weighted MRI and hypointense on T2weighted sequences [45]. Flow voids may be present because of prominent intralesional vessels [45,46].

Solitary Fibrous Tumors (SFT)
Although their imaging appearance is nonspecific, most solitary fibrous tumors ( Figure 5) show heterogeneity and are highly vascular, with prominent collateral vessels [38,44,45]. These tumors are well defined, with intense delayed contrast enhancement due to the fibrous stroma [40]. SFT may rarely exhibit cystic degeneration, necrosis, and calcifications [44]. MRI signal intensity is variable and dependent on cellularity and the abundance of myxoid stroma and/or fibrous tissue [40]. Solitary fibrous tumors are typically hypo-to-isointense to muscle on T1-weighted MRI and hypointense on T2weighted sequences [45]. Flow voids may be present because of prominent intralesional vessels [45,46].

Nerve Sheath Tumors (NST)
In the retroperitoneum, a mass originating proximally to the spine, in the psoas muscle, in the sciatic nerve, or sacral plexus, or involving the neural foramina (which must be checked) can suggest a neural origin [11,47]. Schwannoma is the most frequent benign tumor (Figure 6), while MPNST is the most frequent malignant one [11]. Benign sporadic

Nerve Sheath Tumors (NST)
In the retroperitoneum, a mass originating proximally to the spine, in the psoas muscle, in the sciatic nerve, or sacral plexus, or involving the neural foramina (which must be checked) can suggest a neural origin [11,47]. Schwannoma is the most frequent benign tumor (Figure 6), while MPNST is the most frequent malignant one [11]. Benign sporadic schwannomas have an unknown etiology, but if multiple lesions are seen, neurofibromatosis should always be ruled out, as it is associated with a higher risk of malignant degeneration [12,34,47]. Benign NST are usually rounded and well defined, with subtle, homogeneous enhancement, but the differential diagnosis with MPNST is oftentimes not doable on imaging [11,40]. The presence of ill-defined, infiltrative, margins, inhomogeneous peripheral enhancement, osseous involvement or encasement of nearby structures and vessels, sudden fast growth and pain are all factors pointing towards MPNST [11,12,34,35]. MPNST MRI signal is inhomogeneous, with T1 isointensity and T2 hyperintensity, cystic degeneration and lack of capsulation [40]. Table 2 summarizes the types of RPS described in this section.  Benign NST are usually rounded and well defined, with subtle, homogeneous enhancement, but the differential diagnosis with MPNST is oftentimes not doable on imaging [11,40]. The presence of ill-defined, infiltrative, margins, inhomogeneous peripheral enhancement, osseous involvement or encasement of nearby structures and vessels, sudden fast growth and pain are all factors pointing towards MPNST [11,12,34,35]. MPNST MRI signal is inhomogeneous, with T1 isointensity and T2 hyperintensity, cystic degeneration and lack of capsulation [40]. Table 2 summarizes the types of RPS described in this section.  • The beak (claw) sign: when a mass deforms the edge of an adjacent organ into a "beak" shape, it is likely that the mass arises from that organ (e.g., a notch or an infiltration of the renal hilum, suggests a kidney origin of the tumor in the differential diagnosis between LPS and angiomyolipoma ( Figure 7) [11,46]

Tips and Tricks: CT
Some CT features can help to identify the organ of origin of RPS: • The beak (claw) sign: when a mass deforms the edge of an adjacent organ into a "beak" shape, it is likely that the mass arises from that organ (e.g., a notch or an infiltration of the renal hilum, suggests a kidney origin of the tumor in the differential diagnosis between LPS and angiomyolipoma ( Figure 7) [11,46]). • The embedded organ sign: when there is intimate contact between the mass and the organ of origin, a desmoplastic reaction and sometimes ulcerations are observed (positive sign). On the contrary, a moldable, compressed organ will be deformed into a crescent shape (negative sign). The latter is useful for example in the differential • The embedded organ sign: when there is intimate contact between the mass and the organ of origin, a desmoplastic reaction and sometimes ulcerations are observed (positive sign). On the contrary, a moldable, compressed organ will be deformed into a crescent shape (negative sign). The latter is useful for example in the differential diagnosis between RP leiomyosarcoma and primitive inferior cava leiomyosarcoma [36,46].

•
The phantom or invisible organ sign: if an organ cannot be seen, it is probably the origin of the mass. This sign can lead to false positives diagnoses, as large sarcomas can invade the adrenal gland. • Prominent feeding artery sign: hypervascular masses in most cases are supplied by feeding prominent arterial vessel that can be studied on CT and can help to understand the origin of the mass (e.g., identifying a prominent vessel originating from the kidney balances the diagnosis towards angiomyolipoma).

•
The sandwich (or hamburger) sign ( Figure 8): presence of a mesenteric nodal mass that envelops mesenteric vessels on both sides, without grossly infiltrating them. This is a classic sign of mesenteric lymphoma (typically non-Hodgkin) or posttransplant lymphoproliferative disorder in transplanted patients.
Cancers 2023, 15, x FOR PEER REVIEW 12 of 18 • The phantom or invisible organ sign: if an organ cannot be seen, it is probably the origin of the mass. This sign can lead to false positives diagnoses, as large sarcomas can invade the adrenal gland. • Prominent feeding artery sign: hypervascular masses in most cases are supplied by feeding prominent arterial vessel that can be studied on CT and can help to understand the origin of the mass (e.g., identifying a prominent vessel originating from the kidney balances the diagnosis towards angiomyolipoma).

•
The sandwich (or hamburger) sign ( Figure 8): presence of a mesenteric nodal mass that envelops mesenteric vessels on both sides, without grossly infiltrating them. This is a classic sign of mesenteric lymphoma (typically non-Hodgkin) or posttransplant lymphoproliferative disorder in transplanted patients. CT is also useful to identify patterns of growth and dissemination, which is fundamental for surgical and treatment planning in multidisciplinary contexts, and useful to identify the sarcoma subtype. Lesions surrounding normal structures and vessels without invading or compressing them and affecting more than one compartment will more likely be lymphangiomas and lymphomas; conversely, lesions that are located along the sympathetic chain will most likely be paragangliomas or ganglioneuromas.
The assessment of the enhancement pattern is another fundamental aspect to search on CT to understand the nature of a lesion, as follows: • Absence of enhancement is commonly seen in cysts, hematomas, and simple lipomas.

•
Early enhancement and rapid washout is seen in paragangliomas and Castleman's disease.

•
Early enhancement and delayed or imperceptible washout is unspecific, as it is seen in most tumors (benign or malignant), such as paragangliomas.
Lastly, CT can also help to identify tumoral components such as fat, myxoid stroma, necrosis, cystic degeneration, and calcifications. Calcifications can be present and indicate dedifferentiation and poor prognosis or sclerosing or inflammatory variants of well-differentiated LPS [6]. CT is also useful to identify patterns of growth and dissemination, which is fundamental for surgical and treatment planning in multidisciplinary contexts, and useful to identify the sarcoma subtype. Lesions surrounding normal structures and vessels without invading or compressing them and affecting more than one compartment will more likely be lymphangiomas and lymphomas; conversely, lesions that are located along the sympathetic chain will most likely be paragangliomas or ganglioneuromas.
The assessment of the enhancement pattern is another fundamental aspect to search on CT to understand the nature of a lesion, as follows:

•
Absence of enhancement is commonly seen in cysts, hematomas, and simple lipomas. • Early enhancement and rapid washout is seen in paragangliomas and Castleman's disease. • Early enhancement and delayed or imperceptible washout is unspecific, as it is seen in most tumors (benign or malignant), such as paragangliomas.
Lastly, CT can also help to identify tumoral components such as fat, myxoid stroma, necrosis, cystic degeneration, and calcifications. Calcifications can be present and in-dicate dedifferentiation and poor prognosis or sclerosing or inflammatory variants of well-differentiated LPS [6].

Tips and Tricks: MRI
Contrast-enhanced MRI is the best modality to characterize a soft-tissue tumor, but also for local staging and prognosis. The diagnostic orientation can be facilitated on MRI by the identification of myxoid stroma (high fluid-like signal on T2, hypointense on T1, with contrast uptake), fatty content (Dixon [9] and Dual Echo), necrosis, hemorrhage, cystic degeneration, and fibrotic content. For instance, the presence of myxoid stroma can point towards myxoid round cells LPS, low-grade fibromyxoid chondrosarcoma, rhabdomyosarcoma (Figure 9), or small extraskeletal myxoid chondrosarcoma [9]. Undifferentiated sarcoma, instead, can present as massively hemorrhagic tumors, therefore being similar to a large hematoma, that does not have an explanation, nor a rapid appearance or resolution [6,9].

Tips and Tricks: MRI
Contrast-enhanced MRI is the best modality to characterize a soft-tissue tumor, but also for local staging and prognosis. The diagnostic orientation can be facilitated on MRI by the identification of myxoid stroma (high fluid-like signal on T2, hypointense on T1, with contrast uptake), fatty content (Dixon [9] and Dual Echo), necrosis, hemorrhage, cystic degeneration, and fibrotic content. For instance, the presence of myxoid stroma can point towards myxoid round cells LPS, low-grade fibromyxoid chondrosarcoma, rhabdomyosarcoma (Figure 9), or small extraskeletal myxoid chondrosarcoma [9]. Undifferentiated sarcoma, instead, can present as massively hemorrhagic tumors, therefore being similar to a large hematoma, that does not have an explanation, nor a rapid appearance or resolution [6,9]. On MRI, in peripheral NST, specific signs such as the target sign or the split fat sign and the entering nerve sign can be seen [9,34]. In all RPS, when infiltrative patterns are present, wider margins of resection should be obtained, as they are predictive of grade III tumors; therefore, the radiologist must acknowledge them [9]. Other elements indicative of a grade II-III tumor is high necrotic content, peritumoral edema, or peritumoral enhancement [9].
In myxofibrosarcoma and myxoid LPS, the water-like sign on T2-w sequences is related to high myxoid matrix content (≥75%) and has been associated with an increased risk of local recurrence and worse prognosis [48], as it is associated with shorter metastasis-free survival [27]. A higher amount (≥50%) of fatty content also seemed to correlate with higher grade and shorter metastasis-free survival [27] (Figure 10). On MRI, in peripheral NST, specific signs such as the target sign or the split fat sign and the entering nerve sign can be seen [9,34]. In all RPS, when infiltrative patterns are present, wider margins of resection should be obtained, as they are predictive of grade III tumors; therefore, the radiologist must acknowledge them [9]. Other elements indicative of a grade II-III tumor is high necrotic content, peritumoral edema, or peritumoral enhancement [9].
In myxofibrosarcoma and myxoid LPS, the water-like sign on T2-w sequences is related to high myxoid matrix content (≥75%) and has been associated with an increased risk of local recurrence and worse prognosis [48], as it is associated with shorter metastasis-free survival [27]. A higher amount (≥50%) of fatty content also seemed to correlate with higher grade and shorter metastasis-free survival [27] (Figure 10). In LMS, instead, a prognostic factor is the quantification of necrosis, as higher necrosis is associated with higher grade of the disease [43].
In all RPS, evidence of osseous invasion is linked to lower disease-free survival and overall survival, while vascular or nerve invasion and multifocality are associated with higher local recurrence rates. Nodal invasion instead correlates also with higher probability of metastases [9,35]. MRI helps also in the differential diagnosis with lymphoma facing a hypointense T1 homogeneous mass with a homogeneous enhancement and strong diffusion restriction, without necrotic, blood, fibrotic or fat signal, no peritumoral enhancement or abnormal intra-and peritumoral vasculature, and a possible infiltrative growth pattern [9].

Complications
Complications are usually related to tumor invasion of adjacent structures or to mass effect caused by the tumor and include hydronephrosis, pulmonary embolisms secondary to compression of inferior cava vein, nerve compression, bowel obstruction ( Figure 11) [49], and even intestinal perforation [50]. Postoperative complications are divided into early and late complications and include pulmonary embolism, ileus, fluid collections, hemorrhage [49], splenic injuries, sepsis, multisystem organ failure. For these reasons, guidelines recommend that immediate postsurgical care should be held in subspecialized facilities [2,19,[51][52][53]. In LMS, instead, a prognostic factor is the quantification of necrosis, as higher necrosis is associated with higher grade of the disease [43].
In all RPS, evidence of osseous invasion is linked to lower disease-free survival and overall survival, while vascular or nerve invasion and multifocality are associated with higher local recurrence rates. Nodal invasion instead correlates also with higher probability of metastases [9,35]. MRI helps also in the differential diagnosis with lymphoma facing a hypointense T1 homogeneous mass with a homogeneous enhancement and strong diffusion restriction, without necrotic, blood, fibrotic or fat signal, no peritumoral enhancement or abnormal intra-and peritumoral vasculature, and a possible infiltrative growth pattern [9].

Complications
Complications are usually related to tumor invasion of adjacent structures or to mass effect caused by the tumor and include hydronephrosis, pulmonary embolisms secondary to compression of inferior cava vein, nerve compression, bowel obstruction (Figure 11) [49], and even intestinal perforation [50]. Postoperative complications are divided into early and late complications and include pulmonary embolism, ileus, fluid collections, hemorrhage [49], splenic injuries, sepsis, multisystem organ failure. For these reasons, guidelines recommend that immediate postsurgical care should be held in subspecialized facilities [2,19,[51][52][53].

Treatment Options and the Role of the Radiologist
An optimal management of RPS is facilitated by a comprehensive pretreatment diagnosis and staging, for which the radiologist has a primary role. Pretreatment pathologic diagnosis is needed, but complete surgical resection with negative margins is the cornerstone for nonmetastatic RPS and is the only proved curative strategy. This is achieved by

Treatment Options and the Role of the Radiologist
An optimal management of RPS is facilitated by a comprehensive pretreatment diagnosis and staging, for which the radiologist has a primary role. Pretreatment pathologic diagnosis is needed, but complete surgical resection with negative margins is the cornerstone for nonmetastatic RPS and is the only proved curative strategy. This is achieved by en bloc resection of the tumor and all surrounding structures, even if not overtly infiltrated on imaging or surgical exploration [2,7,11,17,18,51], with the aim of obtaining R0 margins; however, this goal is difficult, especially in nonspecialized centers. Since patients with R1 margins tend to have significantly lower mid-term survival rates and higher recurrence rates [52][53][54][55], but at the same time a far too demolitive surgery can have a certain impact on a patient's life, the optimal extent of resection is still debated and considers several factors, such as histological subtype, preoperative therapies, or anatomical barriers. Novel surgical techniques are being adopted to improve radical excision [52,53]. Current guidelines recommend referring patients to multidisciplinary discussion and decision making with radiologists, surgeons, and oncologists who have consolidated experience in the management of soft-tissue sarcomas and in specialized centers [2,11,12,18,[54][55][56].
In cases of recurrence, surgery should be aimed at achieving macroscopic complete resection, including surrounding organs only when overtly infiltrated [2]. Grossly incomplete resections are questionable and potentially harmful but could be planned out as palliative surgery in selected patients [2].
Currently, there is no evidence to support the routine use of adjuvant radiation therapy (RT) or adjuvant chemotherapy in RPS [56]. Neoadjuvant RT has been prospectively evaluated in a recent randomized controlled trial (STRASS) [51], suggesting an improved abdominal recurrence-free survival only in well-differentiated LPS, and lack of effect for LMS or high-grade dedifferentiated LPS, since these two subtypes have a predominant pattern of distant rather than local progression [18]. This evidence has prompted new studies on neoadjuvant chemotherapy (EORTC1809-STRASS2), targeted for these histological subtypes [1,3,18]. Future evidence may emerge from novel systemic therapy options with targeted and immunotherapeutic therapies.

Conclusions
Radiologists play a primary role in preoperative diagnosis and staging of RPS, detection of recurrence, and assessment of postoperative complications. Many imaging patterns can overlap among the various subtypes of sarcomas. Nonetheless, some specific imaging appearances can guide the radiologist. This is of the utmost importance, as imaging has a central place in each phase of the management of patients, and both nonspecialized and specialized radiologists can have a strong impact in the management and survival of patients with RPS. Indeed, the radiologist is commonly among the first physicians to raise the suspicion of the diagnosis of sarcoma. Imaging features with patterns of involvement, associated to clinical features and a multidisciplinary approach through discussions in tumor board, is fundamental.