Introduction

A wide range of benign and malignant pathologies arise from the retroperitoneal spaces of the abdomen and pelvis. In general, a mass or process is considered to be primary to the retroperitoneum if it originates from the soft tissues, lymphatics or neural tissue of the retroperitoneum and not from its solid organs. This heterogeneous group of diseases often poses a diagnostic challenge for radiologists.

Two Main categories of primary retroperitoneal pathologies are recognized: neoplastic and non-Neoplastic. The majority of primary retroperitoneal neoplasms are malignant (Table 1) [1,2,3,4]. Although a definitive diagnosis often cannot be established on imaging, the primary role of the radiologist is to determine whether a biopsy is necessary and/or if the mass is resectable.

Table 1 Imaging, clinical features and differential considerations of retroperitoneal neoplasms
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Primary retroperitoneal non-neoplastic pathologies include Castleman disease, extramedullary erythropoiesis Erdheim–Chester disease and amyloidosis many of which demonstrate characteristic imaging findings that are important to recognize in order to avoid unnecessary biopsy or surgery (Table 2).

Table 2 Imaging, clinical features, and differential considerations of non-neoplastic primary retroperitoneal conditions
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In this article, we review neoplastic and non-neoplastic primary retroperitoneal conditions, their relevant pathophysiology, clinical presentation, and characteristic CT and MRI imaging findings, particularly key features that can aid in establishing an accurate diagnosis and guide therapy. Additionally, we will provide a comprehensive diagnostic approach for the differentiation of neoplastic from non-neoplastic pathologies that can serve as a road map for narrowing the differential diagnosis and allow for appropriate and timely management.

Anatomy of the retroperitoneum at a glance

A comprehensive knowledge of retroperitoneal anatomy is invaluable to accurately characterize and diagnose its conditions. The retroperitoneum can be grossly divided into several spaces: the anterior and posterior pararenal spaces, the perirenal space, and the fat-containing great vessel space. These can be further divided by separating the anterior pararenal space into the pancreatoduodenal space (containing the pancreas and duodenum) and the pericolonic space (surrounding the ascending and descending colon). Below the level of the kidneys, the anterior and posterior pararenal spaces merge to form an infrarenal retroperitoneal space, which in turn communicates inferiorly with the prevesical space and extraperitoneal compartments of the pelvis (Fig. 1) [5]. Several signs can be used to differentiate peritoneal from retroperitoneal masses, via determination of the organ of tumor origin (Table 3) [6].

Fig. 1
figure 1

Schematic diagram of retroperitoneal anatomy with CT correlation. The anterior pararenal space a, b is delineated by the peritoneum anteriorly and the anterior pararenal facia posteriorly and contains the pancreas and transverse colon. The posterior pararenal space (a, b), primarily composed of fat, is bounded by the posterior pararenal fascia anteriorly and the transversals fascia posteriorly. The great vessel space a contains the aorta, the inferior vena cava, fat, neural tissue, and lymphatics. Many of the retroperitoneal spaces are contiguous with extraperitoneal spaces of the pelvis (c). The perirenal space b, d, e is bounded by the anterior and posterior perirenal fasciae and contains fat as well as bridging septa that extend from the renal capsule to the perirenal fascia (e)

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Table 3 Signs used to differentiate peritoneal from retroperitoneal masses via determination of the organ of tumor origin
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Diagnostic imaging approach to differentiate neoplastic from non-neoplastic pathologies and suggested management

Imaging plays a major role in the detection and characterization of retroperitoneal pathologies, and recognition of key imaging features can help in narrowing the differential diagnosis. Both contrast-enhanced CT and MRI are useful for tissue characterization, accurate determination of mass origin and extent, and assessment of benign or malignant potential.

Morphology is an important clue as to whether a retroperitoneal soft tissue mass is malignant. On imaging, benign processes often have a non-mass-like appearance surrounding and encasing adjacent anatomical structures, such as mantle-like encasement of the ureters and vessels with tethering and luminal narrowing. Inflammatory processes may cause wall thickening and pseudoaneurysm formation of the aorta or mesenteric arteries. Malignant tumors present with more well-defined borders and mass-like appearances and are commonly associated with adjacent organ and/or structure displacement/distortion.

A mass centered in the retroperitoneum often indicates malignancy. For most primary retroperitoneal malignancies, the only curative treatment is complete resection with a tumor-free margin. Therefore, when a diagnosis of primary malignancy is likely, patients usually are referred for surgical evaluation.

In contrast, chemotherapy is usually the first line treatment for lymphoma and metastatic disease. When these entities are considered on imaging (specifically when a primary neoplasm is detected elsewhere), biopsy should be suggested for confirmation (also see Table 4). Individual conditions, grouped by tissue type, will be discussed in greater detail in the subsequent section.

Table 4 Imaging findings of retroperitoneal pathologies grouped by clinical management recommendation
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Imaging findings that suggest a specific primary retroperitoneal malignancy: resection should be considered

Fat components

The presence of gross fat in a primary retroperitoneal mass that also has myxoid, solid, infiltrative or nodular soft tissue components strongly suggests liposarcoma (Fig. 2). Tumors not primary to the retroperitoneum, such as adrenal myelolipoma or renal angiomyolipoma, also contain gross fat intermixed with soft tissue; however, these are centered or completely contained within their respective organs of origin (Fig. 3). Retroperitoneal lipomas appear as well-defined masses with homogenous fat attenuation. Although benign, lipomas cannot be distinguished from well-differentiated liposarcomas on imaging and any purely fatty mass should be treated as a liposarcoma until confirmed otherwise by histology [7]. Other primary retroperitoneal tumors to consider when gross fat is present include teratoma (usually demonstrates calcification and heterogeneous enhancement) and extra-adrenal myelolipoma (a rare entity, most commonly in the presacral space) [1, 8].

Fig. 2
figure 2

a Liposarcoma in a middle-aged man. a Contrast-enhanced CT shows a mixed fat and soft tissue attenuation mass in the pelvis (arrow). b Axial T1W FS post-contrast MRI shows enhancement of the soft tissue components of the mass (arrowhead). b Liposarcoma in a middle-aged man. a Contrast-enhanced CT shows a mixed fat and soft tissue attenuation mass in the pelvis (arrow). b/c Axial T1W FS pre- (b) and post-contrast c MRI shows enhancement of the soft tissue components of the mass (arrowhead). c Liposarcoma in a middle-aged man. a Contrast-enhanced CT shows a mixed fat and soft tissue attenuation mass in the pelvis (arrow). b/c Axial T1W FS pre- (b) and post-contrast (c) MRI shows enhancement of the soft tissue components of the mass (arrowhead)

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Fig. 3
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Myxoid liposarcoma. Contrast-enhanced CT in the portal venous phase shows a mass (arrows) with mixed fat and soft tissue attenuation, including several fluid attenuating components (asterisk) corresponding to the myxoid components within a liposarcoma

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Intravascular extension

A primary retroperitoneal lesion with intravascular growth is characteristic of leiomyosarcoma (Fig. 4). The main differential consideration is intravascular spread of an abdominal or pelvic solid organ primary malignancy, most commonly hepatocellular carcinoma, adrenal cortical carcinoma, renal cell carcinoma, or uterine metastasizing leiomyoma. Therefore, evidence of a primary mass in any of these organs should be sought.

Fig. 4
figure 4

a Leiomyosarcoma in a middle-aged woman, endovascular growth. Contrast-enhanced CT in the portal venous phase shows a heterogeneous soft tissue mass completely filling and expanding the infrahepatic (arrow in b) and intra-hepatic (arrowheads in a) segments of the IVC. b Leiomyosarcoma in a middle-aged woman, endovascular growth. Contrast-enhanced CT in the portal venous phase shows a heterogeneous soft tissue mass completely filling and expanding the infrahepatic (arrow in b) and intra-hepatic (arrowheads in a) segments of the IVC

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Myxoid stroma

Neurogenic tumors, myxoid liposarcomas, and myxofibrosarcomas are the most common retroperitoneal tumors that contain myxoid matrices, which on MRI has characteristic marked T2 hyperintensity as well as delayed contrast enhancement [2, 9]. Progressive enhancement peaking on the delayed phase distinguishes myxoid stroma from internal necrosis within a mass, which does not enhance.

Necrotic component

Necrosis is a characteristic finding of large or high-grade neoplasms. When necrosis is seen in retroperitoneal tumors leiomyosarcoma, undifferentiated pleomorphic sarcoma, PEComas, and pleomorphic liposarcomas are the main differential considerations. Necrotic tissue is heterogeneously hyperintense on T2WI and demonstrates no enhancement on post-contrast imaging [6].

Hypervascularity

Among the hypervascular lesions are paragangliomas, leiomyosarcoma, myxofibrosarcoma, other sarcomas, and arteriovenous malformations and fistulas. Hypervascularity differentiates these from entities that are usually hypovascular such as lymphoma, multiple myeloma, and low-grade sarcomas [10]. Other imaging characteristics of these lesions that further narrow the differential will be subsequently discussed.

Imaging findings for which clinical management is uncertain: Surgery versus watchful waiting

Cystic contents

Many benign neoplasms have relatively large cystic components and may pose a diagnostic challenge. Examples of cystic masses include cystic lymphangiomas cystic mesotheliomas (rare benign neoplasms of mesothelial origin), ganglioneuromas, cystic schwannomas, paragangliomas, cystadenomas, and epidermoid cysts.

Imaging findings that are nonspecific but suggest malignancy: biopsy should be considered

Lymphoma and metastatic disease typically require tissue confirmation. Imaging findings indicative of either should prompt recommendation of biopsy.

Enlarged abdominopelvic lymph nodes can indicate lymphoma or metastatic disease. Lymphoma usually produces nodes in multiple contiguous anatomic distributions. The nodes are homogenous, mildly enhancing, and surround but not occlude the vasculature. Splenomegaly with or without splenic nodules may be present. Metastatic adenopathy is typically heterogeneous. Necrosis may be present if the nodes are large and they may obstruct vasculature, bowel, or the ureters. When metastatic disease is suspected, parenchymal organs should be closely examined for evidence of a primary mass or other metastases.

Differential considerations of malignant retroperitoneal tumors based on tissue type

Primary retroperitoneal lesions with characteristics of a mass are often malignant [1,2,3,4]. These lesions are typically asymptomatic until they become large and come to clinical attention when symptoms develop from mass effect. With the exception of a few select tumor types that are discussed below, there are no laboratory or clinical abnormalities that suggest a specific tumor. For clinical and demographic characteristics of these conditions please refer to Table 1.

Primary retroperitoneal neoplasms

Mesenchymal tumors

Liposarcoma

Liposarcomas are most common primary tumors to arise in the retroperitoneum [11,12,13]. The only potentially curative treatment is resection with a tumor-free margin. Local or regional recurrence is the most common cause of death. Distant metastases are most often to the lungs [14].

Liposarcomas are divided into 5 histologic subtypes: well differentiated, dedifferentiated, myxoid, pleomorphic and not otherwise specified [15]; of these differentiated/dedifferentiated liposarcomas are the most common [16]. More than one histologic subtype can co-exist within the same tumor, in these cases, overall prognosis depends on the least differentiated subtype.

Imaging has a critical role both in diagnosis and staging. Most liposarcomas present as large well-defined masses of mixed fluid and soft tissue components that avidly enhance with intravenous contrast (Fig. 2). Gross fat is not always present, but when found in a heterogeneous retroperitoneal mass not arising from a solid organ strongly suggests liposarcoma. Differential considerations for a gross fat-containing retroperitoneal mass include lipomas, which are homogenous and entirely composed of fat with no soft tissue components, and PEComas, which have a dominant soft tissue component but may contain smaller amounts of gross fat.

The imaging appearance of liposarcomas correlates with their histologic composition [17]. A higher ratio of soft tissue compared to lipid is found in high-grade tumors [18, 19]. Well-differentiated liposarcomas typically present as well-circumscribed masses nearly completely composed of gross fat, with a small nodular, reticular, or hazy soft tissue component (Fig. 2). Dedifferentiated liposarcomas, considered intermediate grade, contain a larger proportion of internal nonlipid soft tissue [20]. Myxoid components in liposarcomas are indicative of higher grade and can be readily distinguishable on imaging, as the myxoid components are similar to fluid on non-contrast CT and show T2 hyperintense signal when compared to muscle on MRI. On post-contrast imaging, unlike fluid-containing structures, myxoid tumors show gradual reticular internal enhancement (Fig. 3) [15, 19].

Leiomyosarcoma

Leiomyosarcomas are mesenchymal tumors that show smooth muscle differentiation. The retroperitoneum is the most common site of origin of leiomyosarcomas. Treatment is surgical. Retroperitoneal leiomyosarcomas are classified into IVC and non-IVC origin tumors; although the two subtypes are resected through a different surgical approach, they are similar in their internal composition, metastatic pattern and long-term prognosis [21].

The majority of non-IVC origin leiomyosarcomas arise in the perirenal space of the retroperitoneum. Tumors that originate from the IVC wall and most commonly arise at or caudal to the level of the retrohepatic IVC, presenting at an earlier stage due to development of clinical symptoms related to IVC obstruction. Metastases are typically hematogenous, most commonly to the lungs, liver, bone, soft tissue and the peritoneum [22]. Local lymph node metastases are uncommon. This distinguishes leiomyosarcomas from angiosarcomas which also present as a soft tissue mass arising from the IVC but more commonly metastasize to local lymph node chains [23].

On imaging, leiomyosarcomas present as a large, heterogeneous, infiltrative, lipid-free soft tissue mass, similar to muscle on CT and T1WI, hyperintense to muscle on T2WI with heterogeneous enhancement. A central necrotic or avascular area is also often present [24, 25]. Although most leiomyosarcomas are extraluminal, if a mass is centered within the IVC lumen, it can be confidently diagnosed as a leiomyosarcoma of IVC origin (Fig. 4) [26]. The vessel lumen may be partially or completely occluded and is often expanded [24].

Undifferentiated pleomorphic sarcoma (UPS)

Undifferentiated pleomorphic sarcomas (UPS), previously called malignant fibrous histiocytomas, are tumors of mesenchymal origin composed of fibroblastic or myofibroblastic cells. UPS is the third most common type of primary malignancy in the retroperitoneum, following liposarcoma and leiomyosarcoma [4, 27]. Treatment is surgical.

On imaging, UPS present as large heterogeneous tumors with similar signal characteristics to muscle except for hyperintense signal on T2WI, strong arterial enhancement and delayed hypoenhancement [28]. Although less common than in leiomyosarcomas, central necrosis may occur in large tumors (Fig. 5). Approximately 20% of UPS show internal calcification, which is uncommon in leiomyosarcomas [29, 30].

Fig. 5
figure 5

A Undifferentiated pleomorphic sarcoma (formerly malignant fibrous histiocytoma) in a middle-aged man. a coronal T1W FS pre-contrast, b T1W FS post-contrast, c T2 TSE coronal images of the abdomen. T1 hyperintense signal within the mass is consistent with hemorrhage (arrow in a) and the mass also demonstrates heterogeneous enhancement (arrowhead in b). The T2W TSE image shows anatomic relationships of the mass (arrows in c) centered in the retroperitoneum. b Undifferentiated pleomorphic sarcoma (formerly malignant fibrous histiocytoma) in a middle-aged man. a coronal T1W FS pre-contrast b T1W FS post-contrast C: T2 TSE coronal images of the abdomen. T1 hyperintense signal within the mass is consistent with hemorrhage (arrow in a) and the mass also demonstrates heterogeneous enhancement (arrowhead in b). The T2W TSE image shows anatomic relationships of the mass (arrows in c) centered in the retroperitoneum. c Undifferentiated pleomorphic sarcoma (formerly malignant fibrous histiocytoma) in a middle-aged man. a coronal T1W FS pre-contrast, b T1W FS post-contrast, c T2 TSE coronal images of the abdomen. T1 hyperintense signal within the mass is consistent with hemorrhage (arrow in a) and the mass also demonstrates heterogeneous enhancement (arrowhead in b). The T2W TSE image shows anatomic relationships of the mass (arrows in c) centered in the retroperitoneum

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Similar to other soft tissue malignancies, including liposarcoma and leiomyosarcoma, UPS show diffusion restriction on MRI. Although diffusion restriction is useful to distinguish retroperitoneal malignancy from benign entities, its use in determining the specific type of malignancy is limited [31].

Less common sarcomas

Other rare histologic subtypes of retroperitoneal sarcomas include rhabdomyosarcoma, angiosarcoma, chondrosarcoma, and synovial sarcoma. These tumors appear similar on imaging: a large soft tissue mass, heterogenous but similar to muscle in CT attenuation, primarily isointense to muscle on T1WI hyperintense on T2WI, with heterogenous enhancement. A few imaging characteristics that may help suggest a specific diagnosis include high flow vessels that show prominent T2 signal void within angiosarcomas, chondroid matrix with stippled, punctate and arc-like calcifications occasionally present in chondrosarcomas, and a prominent component of hemorrhage, necrosis, and calcification in synovial sarcomas [32,33,34,35].

Perivascular epithelioid cell tumor (PEComa)

PEComas are mesenchymal neoplasms composed of epithelioid and spindle cells arranged around blood vessels. Most PEComas are benign (such as angiomyolipoma and lymphangioleiomyomatosis), a minority of PEComas are malignant sarcomas that may arise in the retroperitoneum [36]. These tumors are associated with tuberous sclerosis, and have a female predominance and a poor prognosis [37]. Treatment is surgical.

PEComas do not have pathognomonic imaging findings and therefore tissue sampling is usually necessary for diagnosis. On imaging, retroperitoneal PEComas are large (> 5 cm) infiltrative heterogeneous masses that demonstrate avid arterial enhancement, areas of necrosis, hemorrhage, and occasional punctate foci of calcification [38]. Gross fat may be present, which can make these neoplasms indistinguishable from liposarcomas. Invasion of retroperitoneal organs may also be seen [39].

Lymphoma

Most lymphomas involving the retroperitoneum originate from the urinary tract; a minority are of primary retroperitoneal origin. Treatment is chemotherapy. Characteristic imaging findings include well-circumscribed nodular or lobulated soft tissue masses of homogenous CT attenuation, T1WI hypointense and T2WI hyperintense signal and homogenous enhancement that peaks in the delayed phase. Involved retroperitoneal lymph nodes typically form a confluent lobulated soft tissue mass that extends into the pelvis or the mesentery (Fig. 6, Video 1). Lymphoma-related adenopathy tends to insinuate between rather than narrow adjacent vascular structures (“sandwich sign”) [40, 41]. Necrosis, calcification, and hemorrhage are rare (Figs. 7, 8, 9, 10, 11, 12).

Fig. 6
figure 6

Lymphoma in a 40-year-old man. Contrast-enhanced CT shows confluent mass-like lymphadenopathy in the retroperitoneum (black arrow) showing homogenous low-level enhancement with lymphadenopathy also extending to the root of the mesentery (arrowhead). The nodes displace and surround in spite of the degree of adenopathy do not obstruct the vasculature (arrow). A nodal mass surrounding the vasculature has been described as the “sandwich sign”

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Fig. 7
figure 7

A. Schwannoma in a 52-year-old male. a Coronal Fat Sat T2-weighted image of the pelvis demonstrating T2 hyperintense mass to left of the L4–L5 disk space (asterisk). b Coronal T1W FS post-contrast image showing heterogeneous enhancement (white arrow) of the mass (asterisk). An L4–L5 nerve appears to be entering the mass (black arrows in a, b). The mass is associated with the left L4–L5 neural foramina. b Schwannoma in a 52-year-old male. a Coronal Fat Sat T2-weighted image of the pelvis demonstrating T2 hyperintense mass to left of the L4–L5 disk space (asterisk). b Coronal T1W FS post-contrast image showing heterogeneous enhancement (white arrow) of the mass (asterisk). An L4–L5 nerve appears to be entering the mass (black arrows in a, b). The mass is associated with the left L4–L5 neural foramina

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Fig. 8
figure 8

A Diffuse plexiform neurofibroma in a 23-year-old male with history of Neurofibromatosis Type 1 (NF1). a coronal T2W FS and b sagittal T2 W TSE image shows an infiltrative mass (asterisk in a and arrows and asterisk in b) extending along a nerve and surrounding the urinary bladder. Note widening of the thecal sac in this patient with sacral Tarlov’s cysts (not shown). b Diffuse plexiform neurofibroma in a 23-year-old male with history of Neurofibromatosis Type 1 (NF1). a coronal T2W FS and b sagittal T2 W TSE image shows an infiltrative mass (asterisk in a and arrows and asterisk in b) extending along a nerve and surrounding the urinary bladder. Note widening of the thecal sac in this patient with sacral Tarlov’s cysts (not shown)

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Fig. 9
figure 9

A Ganglioneuroma in a young female. b Axial subtraction T1W FS image demonstrates a large (5 × 7 cm) heterogeneously enhancing, well -circumscribed pelvic, midline mass (asterisk). The mass extends/arises from the widened right S1-2 neural foramen (arrows). The mass abuts the right ovary and right iliac artery but it does not encase the vasculature and there is no evidence of right ovarian involvement (not shown, B = urinary bladder) b Sagittal T2-weighted image reveals heterogeneous hyperintensity signal within the mass. Note mass effect on the adjacent posterior wall of the urinary bladder (b). Again appreciated is the extension on the mass into the right S1-2 neural foramen (white arrows). B Ganglioneuroma in a young female. a Axial subtraction T1W FS image demonstrates a large (5 × 7 cm) heterogeneously enhancing, well-circumscribed pelvic, midline mass (asterisk). The mass extends/arises from the widened right S1-2 neural foramen (arrows). The mass abuts the right ovary and right iliac artery but it does not encase the vasculature and there is no evidence of right ovarian involvement (not shown, B = urinary bladder) b. Sagittal T2-weighted image reveals heterogeneous hyperintense signal within the mass. Note mass effect on the adjacent posterior wall of the urinary bladder (b). Again appreciated is the extension on the mass into the right S1-2 neural foramen (white arrows)

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Fig. 10
figure 10

A Sacrococcygeal Primary Germ Cell Tumor (Yolk Sac type and Mature Teratoma) in a young male patient. a Axial T1W FS post-contrast image demonstrates a large 8.5 × 6 cm heterogeneously enhancing retroperitoneal bilobed mass that is centered within the presacral and precoccygeal space. The mass is composed of two main components: one is more heterogeneously enhancing presacral solid component (asterisk, representing a yolk sac tumor on pathology); and a second more anteriorly positioned minimally to none enhancing fat-containing component (star, representing mature teratoma component of the tumor). b Axial T2W TSE image demonstrates heterogeneous T2 signal of the posterior yolk sac component (asterisk) and hyperintense fat (asterisk) and hemorrhage (short white arrows) containing anterior mature teratoma component (star). b Sacrococcygeal Primary Germ Cell Tumor (Yolk Sac type and Mature Teratoma) in a young male patient. a Axial T1W FS post-contrast image demonstrates a large 8.5 × 6 cm heterogeneously enhancing retroperitoneal bilobed mass that is centered within the presacral and precoccygeal space. The mass is composed of two main components: one is more heterogeneously enhancing presacral solid component (asterisk, representing a yolk sac tumor on pathology); and a second more anteriorly positioned minimally to none enhancing fat-containing component (star, representing mature teratoma component of the tumor). b Axial T2W TSE image demonstrates heterogeneous T2 signal of the posterior yolk sac component (asterisk) and hyperintense fat (asterisk) and hemorrhage (short white arrows) containing anterior mature teratoma component (star)

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Fig. 11
figure 11

a Retroperitoneal hemorrhage with active extravasation from a ruptured pseudoaneurysm (PSA) in a 52-year-old female status post catheter directed spasmolysis one day ago. a IV contrast-enhanced CT image obtained in the coronal plane demonstrates a large right retroperitoneal hemorrhage (H) with an active extravasation (black arrows in a). b Selective arteriogram of the common iliac artery demonstrates a small pseudoaneurysm of the right distal external iliac artery (R EIA) (arrow in b). Active extravasation was noted on dynamic imaging (not shown). The patient underwent successful thrombin injection of the PSA. b Retroperitoneal hemorrhage with active extravasation from a ruptured pseudoaneurysm (PSA) in a 52-year-old female status post catheter directed spasmolysis one day ago. a IV contrast-enhanced CT image obtained in the coronal plane demonstrates a large right retroperitoneal hemorrhage (H) with an active extravasation (black arrows in a). b Selective arteriogram of the common iliac artery demonstrates a small pseudoaneurysm of the right distal external iliac artery (R EIA) (arrow in b). Active extravasation was noted on dynamic imaging (not shown) The patient underwent successful thrombin injection of the PSA

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Fig. 12
figure 12

Erdheim–Chester disease in a middle-aged male. Axial post-contrast CT image obtained at the level of the kidneys shows bilateral perirenal and periaortic soft tissue encasement (arrows)

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The primary differential diagnosis for infiltrative, confluent retroperitoneal soft tissue is retroperitoneal fibrosis (RPF). Both RPF and lymphoma have a similar imaging appearance, and therefore these entities may be indistinguishable on imaging. Extension of a mass beyond the retroperitoneum favors a diagnosis of lymphoma, as such extension is not typical for RPF. Anterior displacement of the aorta by a confluent nodal mass is more characteristic of lymphoma rather than RPF, which usually encases the vasculature without displacing it (Fig. 13). Diffusion restriction on MRI is more pronounced in lymphoma compared to RPF [41].

Fig. 13
figure 13

Retroperitoneal fibrosis in a middle-aged woman. Pre-contrast (a) and dynamic post-contrast (b–d) T1W FS images show infiltrating soft tissue in the retroperitoneum (arrows) encasing the aorta (Ao). Progressive enhancement most pronounced on the delayed phase (d) indicates active inflammation

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Neurogenic tumors

Neurogenic tumors constitute 10–20% of all primary retroperitoneal malignancies. They are more likely to be benign and have a better prognosis. [42] In the retroperitoneum, they follow the distribution of the sympathetic ganglia and paraspinal areas, including the organs of Zuckerkandl. The main subcategories of neurogenic tumors in adults include nerve sheath tumors (schwannoma, neurofibroma), ganglionic cell tumors (ganglioneuroma), and paraganglionic cell tumors (paraganglioma).

Neurofibroma and Schwannoma

Neurofibroma (NF) and schwannoma are benign nerve sheath tumors. [43] NFs can be isolated (90%) or can occur in association with type 1 neurofibromatosis (NF1). On imaging, both tumors have a variable appearance, demonstrating solid and/or cystic components depending on the degree of degeneration within the tumor, with possible hemorrhage and calcification. Due to the presence of myxoid degeneration, these tumors tend to be centrally hyperintense on T1WI, peripherally hyperintense on T2WI, and hypoattenuating on CT. On post-contrast imaging, homogeneous or target-like enhancement can be seen. NFs are unencapsulated and result in expansion of the entire nerve, with nerve fibers coursing through the mass itself. Plexiform neurofibromas have an infiltrative appearance with multiple sites of disease that track along nerve roots (Fig. 8). Malignant transformation is more common with neurofibromas. Schwannomas are encapsulated and usually extend along the course of a nerve, resulting in nerve flattening. The entering and exiting nerve signs may be apparent. Highly cellular components demonstrate hypointense signal on both T1 and T2WI (Fig. 7) [43, 44].

Ganglioneuroma

Ganglioneuromas are benign tumors that arise from sympathetic ganglia and are most commonly located in the retroperitoneum, extending along the paravertebral sympathetic ganglia and sometimes the adrenal medulla. They carry a favorable prognosis. These tumors are most commonly asymptomatic, however, on occasion secrete catecholamines or androgenic hormones [45]. On imaging, ganglioneuromas are hypoattenuating masses that demonstrate hypointense T1 and hyperintense T2 signal, and occasionally present with a centrally located intact vessel (Fig. 9). Punctate calcifications are characteristic and seen in 20-30% of tumors [46]. Post-contrast enhancement is variable.

Paraganglioma (extra-adrenal pheochromocytoma)

Paragangliomas are tumors related to chromaffin cells that originate in an extra-adrenal location; most commonly in the organ of Zuckerkandl. Production/secretion of catecholamines and associated symptoms is a hallmark of these tumors, present in approximately 40% of cases [45]. They typically appear as a large mass with areas of hemorrhage and necrosis on cross-sectional imaging (Video 2). Large paragangliomas can be complicated by extensive retroperitoneal hemorrhage and rupture. Punctate calcifications and avid contrast enhancement are common. Metastases are seen in up to 50% of cases. [42]

Germ cell origin tumors (GCT)

Primary extragonadal GCT and sex cord stromal tumors (SCST) Primary extragonadal GCT (Video 3) are thought to arise from aberrant primordial germ cell rests, and account for approximately 1–2.5% of all germ cell tumors [47]. Although the retroperitoneum is the second most common site of presentation, the majority of retroperitoneal GCTs represent metastases from a primary gonadal lesion. Sometimes primary gonadal tumors may not be detected on imaging due to tumor regression [47]. Several subtypes of these tumors have been recognized, including seminomas and non-seminomatous GCT (embryonal carcinomas, yolk sac tumors, choriocarcinomas, teratomas, and mixed GCTs).

Retroperitoneal primary SCST arise from ectopically located sex cord stromal tissue or from sex cord-like differentiation of somatic cells. Extragonadal primary SCST are commonly seen in the pelvis, retroperitoneal location is less common. Several types of stromal tumors are recognized, including thecomas, Sertoli–Leydig cell tumors, and unclassified sex cord tumors. Elevated serum estrogen levels are typical in patients with granulosa cell tumors and thecomas.

Imaging findings of SCST are nonspecific: tumors are heterogeneous and enhancing on all cross-sectional imaging modalities. These tumors may demonstrate local invasion (Fig. 10) [48,49,50].

Teratoma

Retroperitoneal teratomas represent 10% of all teratomas and originate from ectopically located pluripotent germ cells [51, 52]. Teratomas are subdivided into three main categories: mature (cystic/solid, most are benign), immature (malignant) and monodermal (highly differentiated) [53]. Imaging may not reliably differentiate these types.

Mature teratomas (also called dermoid cysts) are predominately cystic. Calcifications and fat components are common. The large cystic components contain fatty sebaceous fluid and fat-fluid or fluid-fluid levels. Malignant transformation occurs in 2–3% of mature teratomas, specifically adenocarcinoma type [7].

Immature teratomas consist of 10% of undifferentiated tissue [50]. Elevated AFP levels may be present. On CT and MRI they appear as enhancing predominantly solid masses of variable CT attenuation and MR signal, with foci of fat, calcification, and simple cysts (Video 4). Local and vascular invasion is partly responsible for their poor prognosis. Treatment is surgical [5].

Primary non-neoplastic retroperitoneal conditions

Multiple non-neoplastic disease entities may be encountered in the retroperitoneum (Table 2), with more common entities including abscess, arteriovenous malformations (AVM), and hematoma, and less common ones including amyloidosis, retroperitoneal fibrosis, Erdheim–Chester disease, extramedullary hematopoiesis, and Castleman disease.

Retroperitoneal hemorrhage and AVM are the most common vascular lesions of the retroperitoneum. Ill-defined, infiltrative morphology, and high attenuation on unenhanced CT distinguish hemorrhage from neoplasms (Fig. 11). A blush of enhancement may be present from extravasation if there is active hemorrhage. AVM present as a tangle of enlarged vessels on CT and MRI, with prominent flow voids on T2WI. Further characteristics of these entities are listed in Table 2.

Retroperitoneal fibrosis and Erdheim–Chester disease

Retroperitoneal fibrosis (RPF) and Erdheim–Chester disease (ECD) may resemble a number of the retroperitoneal neoplasms such as lymphoma, multiple myeloma, metastatic disease and desmoid tumor. Involvement of other systems and organs or pattern of distribution may aid in differentiation of RPF and ECD from malignant processes. Both conditions demonstrate a mantle growth pattern with fibrotic changes encasing the RP structures and vessels and absence of invasion. ECD involves RP visceral organs (especially kidneys and perirenal space), long bones, skin, globes, lungs, brain, pituitary, and heart (Fig. 12). RPF develops around the aortic bifurcation and spreads upward, enveloping renal hila (Fig. 13, Video 5). Both processes may result in ureteral and vascular obstruction [54]. Tethering of vessels to adjacent vertebrae and medial displacement of the ureters are characteristic of nonmalignant RPF. Active inflammation seen in acute stage RPF shows avid enhancement on post-contrast CT and MRI, and T2 hyperintense signal on MRI. Chronic (acellular) RPF demonstrates T2 hypointense signal and delayed or no enhancement on post-contrast imaging. A malignant form of RPF also exists and carries a poor prognosis; it is characterized by large size, irregular lobular margins, variable enhancement, and significant mass effect resulting in anterior displacement of vessels and lateral displacement of the ureters [54].

Extramedullary hematopoiesis

The retroperitoneum is an uncommon site for extramedullary hematopoiesis (EMH), which on imaging presents as homogeneous bilateral soft tissue masses that extend along the paravertebral region or arise in the perirenal space, and may contain macroscopic fat, hemosiderin deposits and demonstrate variable enhancement (Fig. 14). Calcifications and bony destruction are rare [55]. Disease processes that affect marrow hematopoiesis result in EMH [7]. The presence of hepatosplenomegaly, anemia and skeletal changes may aid in diagnosis [56, 57].

Fig. 14
figure 14

Extramedullary hematopoiesis. Contrast-enhanced CT at the level of the kidneys shows bilateral mixed fat and soft tissue attenuation masses (arrows) representing hematopoetic tissue in the anterior pararenal and perirenal space in a patient with profound, chronic anemia. Although the masses are heterogeneous, calcifications and fluid density components are absent. RK = right kidney, LK = left kidney

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Castleman disease

Castleman disease (CD) is a rare disease characterized by the development of benign lymphadenopathy that may be localized or systemic, often mimicking malignancy [58, 59]. On imaging, CD presents as homogeneously hyperenhancing mass(es) some with calcifications (Fig. 15) [60, 61]. Diagnosis requires tissue sampling.

Fig. 15
figure 15

Castleman’s disease in a middle-aged male. Axial contrast-enhanced CT shows an avidly enhancing retroperitoneal soft tissue mass (M) near the left iliac vessels exerting mass effect on the left psoas muscle. Soft tissue attenuation (asterisk) around the mass representing lymphoid tissue infiltrates the retroperitoneal fat around the mass

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Amyloidosis

Amyloidosis is a heterogeneous group of diseases resulting from the extracellular deposition of abnormal protein either at a specific site (localized amyloidosis) or throughout the body (systemic amyloidosis) [62]. It may be a primary disease process or secondary to a variety of chronic infectious, inflammatory, and neoplastic conditions; accordingly, clinical presentation is widely variable. Isolated involvement of the retroperitoneum may occur in localized amyloidosis [63].

CT and MRI features include the replacement of retroperitoneal fat with infiltrative soft tissue representing extracellular deposits of abnormally folded protein, which encases the aorta and retroperitoneal organs (Fig. 16a) and may progressively calcify over time (Fig. 16b). This appearance is similar to RPF and lymphoma, although unlike those conditions amyloid deposits can demonstrate signal dropout on opposed phase T1WI due to the intermixing of lipid and water elements [63]. Protein deposits may coalesce into a discrete mass (amyloidoma) and mimic malignancies including lymphoma or plasmacytoma on cross-sectional imaging. Biopsy is often necessary for diagnosis [63].

Fig. 16
figure 16

a Retroperitoneal amyloidosis in a 64-year-old man. a Axial contrast-enhanced CT of the abdomen shows infiltrative soft tissue surrounding the aorta and replacing retroperitoneal fat (asterisk). b Axial contrast\-enhanced CT shows several coarse calcifications, usually seen in chronic amyloid deposits, throughout the infiltrative soft tissue (arrowheads). b Retroperitoneal amyloidosis in a 64-year-old man. a Axial contrast-enhanced CT of the abdomen shows infiltrative soft tissue surrounding the aorta and replacing retroperitoneal fat (asterisk). b Axial contrast-enhanced CT shows several coarse calcifications, usually seen in chronic amyloid deposits, throughout the infiltrative soft tissue (arrowheads)

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Conclusion

Primary retroperitoneal pathologies are a diverse group of benign and malignant conditions that arise within the retroperitoneal space independent of the organs located there. Although the exact diagnosis is often difficult to establish based on imaging alone, a few specific imaging findings on CT and MRI can help in narrowing the differential. Familiarity with the epidemiology, pathogenesis, imaging features, and treatment of these retroperitoneal entities can aid radiologic diagnoses and guide appropriate patient management.