The most common retroperitoneal sarcoma is liposarcoma [1]. On computed tomography (CT), a retroperitoneal tumor containing fat and soft tissue parts suggests a diagnosis of a retroperitoneal liposarcoma [1, 2]. The diagnosis of fat in a tumor on CT is based on its low density which is identical to that of subcutaneous, peritoneal, or retroperitoneal fat by visual inspection. A measured mean density of a region of interest less than −10 Hounsfield units on CT also provides a quantitative way for fat detection. On magnetic resonance image (MRI), fat appears as a high signal intensity region on T1-weighted images (T1WI) and has signal drop on T1-fat-suppressed (T1FS) images [2]. The detectability of gross fat on images of retroperitoneal liposarcomas is somewhat related with histological subtypes [1]. Well-differentiated liposarcoma often has detectable fat on CT or MRI, and other subtypes of liposarcomas may not have detectable fat [1]. On CT, myxoid liposarcomas are characterized by pseudocystic appearance on unenhanced images and increased reticular or swirl enhancement over time on enhanced images [1]. On MRI, myxoid liposarcomas appear as a homogeneous mass of low signal intensity on T1-weighted images and high signal intensity on T2-weighted images (T2WI) [1]. Other subtypes (i.e., pleomorphic and dedifferentiated) of liposarcomas without detectable fat could have similar appearance as other sarcomas [1].

Malignant fibrous histiocytoma (MFHS) is the third most common retroperitoneal sarcoma [3]. MFHS typically presents as a large well-defined tumor of soft tissue density similar to that of muscle on unenhanced CT and heterogeneous enhancement after contrast medium administration [3]. Necrosis is common in large MFHS [3]. Calcification is occasionally found (7–20 %) in MFHS with reported “ringlike” or “lumpy” appearance [3]. About 40 % of MFHS has close proximity to the kidney with renal invasion, compression, or displacement [3].

Leiomyosarcoma (MFHS) is the second most common retroperitoneal sarcoma [3]. Retroperitoneal leiomyosarcomas come from vessels, remnant of Wolffian or Mullerian duct, or spermatic cord [3]. A retroperitoneal tumor with contiguous extension to the vessel (most commonly IVC) on images suggests the diagnosis of a leiomyosarcoma [3]. This is because other sarcomas rarely have intravascular tumor extension [3]. Retroperitoneal leiomyosarcoma without intravascular extension (about 60 % of leiomyosarcomas) usually presents as a large tumor with obvious central necrosis [3] and thus heterogeneous density on CT as well as heterogeneous signal intensity on MRI [1]. On the other hand, leiomyosarcomas of intravascular location in the IVC are usually of smaller sizes without necrosis and have densities similar to muscle on CT [3] as well as low signal intensity on MRI.

About one-third of retroperitoneal malignant tumors are lymphomas [3]. Retroperitoneal lymphomas are usually distinguishable from primary retroperitoneal sarcomas [4]. The presence of enlarged lymph node in addition to a dominant retroperitoneal mass suggests the diagnosis of retroperitoneal lymphoma [5]. Para-aortic lymph node involvement is present in about 25 % of non-Hodgkin lymphomas and 55 % of Hodgkin lymphomas [3]. Non-nodal involvement of lymphoma could be depicted on computed tomography (CT) or magnetic resonance imaging (MRI), which is usually present in non-Hodgkin lymphoma and not found in Hodgkin lymphoma [3]. The involved lymph nodes or non-nodal sites of retroperitoneal lymphomas are typically homogeneous soft tissue masses on both unenhanced and enhanced images without necrosis, fatty density, cystic area, or calcifications on CT [3, 4]. Retroperitoneal lymphomas may sometimes have a dominant retroperitoneal mass which has a characteristic growth pattern of tumor extension between and surrounding existing structures (e.g., vessels) and has a homogeneous density as well [2, 3]. On MRI, retroperitoneal lymphomas always present as high signal intensity on diffusion-weighted images by water restriction effect of tumors with high cellularity [5]. The signal intensity on T2-weighted images and enhancement of retroperitoneal lymphomas however vary from homogeneous to heterogeneous appearances [5].

Lymph node metastasis from a primary malignant tumor could appear as a solitary or multiple discrete nodules along the lymphatic drainage, which are easier to be recognized as enlarged lymph nodes by their anatomical locations on computed tomography (CT) or magnetic resonance imaging. On the other hand, lymph node metastasis could present as a large retroperitoneal mass with or without coexisting enlarged discrete lymph nodes as well. Almost any primary malignant tumor could metastasize to the retroperitoneal lymph nodes [3]. Thus, lymph node metastasis should be listed in the differential diagnosis of any retroperitoneal mass, and a search for a primary malignant tumor on images should be done [3]. The testis should be examined and surveyed because retroperitoneal lymph node metastasis from testicular malignancy is common [3].

Retroperitoneal Castleman’s disease is rare [6]. Castleman’s disease is most commonly seen in the mediastinum although it could occur in any site of the body with lymphatic tissue [6]. Castleman’s disease is categorized as hyaline vascular type and plasma cell type by histology [6]. By involved solitary or multiple lymph node sites, Castleman’s disease is classified as unicentric and multicentric diseases [7]. Hyaline vascular type of retroperitoneal Castleman’s disease typically shows homogeneous density and homogeneous moderate to strong enhancement on computed tomography (CT), as well as hypointense on T1-weighted image and hyperintense on T2-weighted image on magnetic resonance imaging (MRI) [6, 7]. Hyaline vascular type of retroperitoneal Castleman’s disease typically presents with unicentric disease as a solitary mass [6]. When retroperitoneal Castleman’s disease of hyaline vascular type occurs in pararenal locations, it could have encasement, adhesion, or displacement of the ipsilateral renal pedicles with preservation of vascular patency [6]. Plasma cell type of retroperitoneal Castleman’s disease is even rarer than hyaline vascular type and usually presents with multicentric diseases [6]. Plasma cell type of Castleman’s disease is usually less enhanced than hyaline vascular type.

Schwannoma is also called neurilemoma [3]. Retroperitoneal schwannomas are usually located at paraspinal or presacral areas [3]. On images, retroperitoneal schwannoma usually presents as a round or oval mass with well-defined border without invasion to the adjacent structures by its benign nature [3]. The density, signal intensity, and enhancement of retroperitoneal schwannomas depend on the histological composition [1, 3]. Retroperitoneal schwannoma with myxoid stroma shows “pseudocystic” appearance on computed tomography (CT) and hyperintense on T2-weighted image of magnetic resonance imaging (MRI). Areas of cystic degeneration in retroperitoneal schwannomas show non-enhancing low density on CT, low signal intensity on T1-weighted image, and high signal intensity on T2-weighted images. The cellular tissue of retroperitoneal schwannomas show low signal intensity on both T1-weighted and T2-weighted images of MRI [1]. Retroperitoneal schwannomas with myxoid stroma or cystic degenerations have heterogeneous enhancement on CT and MRI. Nonetheless, small retroperitoneal schwannomas have more homogeneous enhancement than large ones [1].