Liposarcoma is the most common (33 %) sarcoma to occur in the retroperitoneum [22]. Retroperitoneal liposarcomas account for 10–15 % of all liposarcomas, and they are more common in the 50–70-year age group, with no sex predilection [1, 22]. They are classified histologically into four main subtypes: atypical lipomatous tumor/well differentiated, dedifferentiated, myxoid, and pleomorphic (Fig. 6.4) [15, 23]. Different histological subtypes may coexist in the same lesion. Such histological subtypes are usually classified on the basis of the most aggressive cellular component. The fourth edition of the World Health Organization (WHO) Classification of Tumors of Soft Tissue and Bone was published in February 2013 [16]. The changes for adipocytic tumors in the fourth edition were the removal of the terms “round cell liposarcoma” and “mixed-type liposarcoma” [16, 17]. While the section heading lists only “atypical lipomatous tumor” on the recent WHO classification, the authors point out that retention of “well-differentiated liposarcoma” is still appropriate for tumors in sites at which margin-negative resection is often impossible (e.g., retroperitoneum, mediastinum), since such tumors are associated with substantial mortality [17]. The terminology “atypical lipomatous tumor” and “well-differentiated liposarcoma” continues to need clarification [17].

Liposarcoma is usually large (average diameter, >20 cm) and is a slow-growing tumor [1]. In general, liposarcomas may be distinguished based on the presence of fat, which varies depending on the subtype [1, 24]. The well-differentiated subtype is low-grade tumor and the most common type of retroperitoneal liposarcoma [1, 22]. The fact that well-differentiated liposarcoma shows no potential for metastasis unless it undergoes dedifferentiation led, in the late 1970s, to the introduction of terms such as atypical lipoma or atypical lipomatous tumor, particularly for lesions arising at surgically amenable locations in the limbs and on the trunk since, at these sites, wide excision should usually be curative, and hence the designation “sarcoma” is not warranted [25]. However, in sites such as the retroperitoneum and mediastinum, it is commonly impossible to obtain a wide surgical excision margin, and, in such cases, local recurrence is almost inevitable and often leads to death, even in the absence of dedifferentiation and metastasis – hence, at these sites, retention of the term well-differentiated liposarcoma can readily be justified [17, 26].

Well-differentiated liposarcomas are predominantly hypoattenuating lesion with fat attenuation on CT images and typically round or lobulated with smooth margin and may displace or surround local structures (Fig. 6.4a). The fat content in the mass demonstrates high signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images, with loss of signal on fat-suppressed images. The macroscopic fat content may be greater than 75 %, and these lesions must be distinguished from other fat-containing neoplasms which may occur in the retroperitoneum, such as lipoma, adrenal myelolipoma, renal angiomyolipoma, and teratoma [27]. Retroperitoneal lipomas are fat-containing lesions that do not have any associated soft tissue component or nodularity. Although the appearance of well-differentiated liposarcoma may be similar to that of a lipoma, this subtype frequently has thicker, irregular, and nodular septa that show enhancement after contrast material administration [28]. Calcification or ossification is rare and considered a poor prognostic feature, which often indicates dedifferentiation [3]. Well-differentiated tumors can recur but do not metastasize [1].

Dedifferentiated liposarcomas are high-grade tumors with poor prognosis [29]. This subtype is defined by the presence of sharply demarcated regions of non-lipogenic sarcomatous tissue within a well-differentiated tumor [16]. At CT and MR imaging, these dedifferentiated tumors appear as more heterogeneous tumors with both fat and solid components (Figs. 6.4b and 6.5) [1]. There may be no evidence of fat in up to 20 %, which makes the diagnosis difficult based on imaging alone [28]. Calcification is seen in as many as 30 % of these tumors and is an important sign of dedifferentiation [1, 30]. Variable signal intensity and enhancement of the solid portion may be seen [30].

Myxoid liposarcoma is of intermediate-grade malignancy and the second most common subtype of liposarcoma, accounting for more than one-third of liposarcomas and representing about 10 % of all adult soft tissue sarcomas. The lesions formerly known as round cell liposarcoma are interpreted as histologic continuum with myxoid subtype and are included in this category. This subtype shows a heterogeneous hypoattenuating mass, with attenuation less than that of muscle at CT (Fig. 6.4c). Homogeneous distribution of fat and soft tissue within the mass may result in a “pseudocystic” appearance, near fluid density, on non-contrast scan [15]. They characteristically show gradual, heterogeneous enhancement [15]. There is no macroscopic fat in over 50 % and calcifications are uncommon [28]. At MR imaging, there is low signal intensity on T1-weighted images and high signal intensity on T2-weighted images because of the mucopolysaccharide contents in the myxoid matrix [1]. Lacy, linear, or amorphous areas of high signal intensity on T1-weighted images and intermediate signal intensity on T2-weighted images may be seen because of the intratumoral fat content [1]. They demonstrate slowly progressive, reticular contrast enhancement due to solid components, which enables differentiation from a cyst [31].

Pleomorphic liposarcomas are high-grade sarcomas and the least common histologic subtype. This subtype tends to occur in the extremities (lower > upper limbs), whereas the trunk and the retroperitoneum are less frequently affected. They are seen as heterogeneous soft tissue masses with little or no fat within and may contain areas of necrosis that are indistinguishable from other solid tumors (Fig. 6.4d) [1]. Calcifications are rare.

Local recurrence after resection is the most common cause of relapse and is usually seen within 2 years. Recurrent liposarcomas may be difficult to differentiate from retroperitoneal fat, but recurrent tumors frequently have a slightly higher attenuation than the surrounding fat [13]. Recurrent liposarcomas often have a different appearance from the primary tumor and in many cases may not contain any visible fat.

Leiomyosarcomas are the second most common primary retroperitoneal sarcoma, accounting for 28 % of all cases [22]. They arise from smooth muscle elements within the retroperitoneal muscle tissue, blood vessels, or Wolffian duct remnants [1, 19]. Leiomyosarcoma is more common in women, in the fifth to sixth decades of life [32]. They can grow to a large size (>10 cm) before compromising adjacent organs and precipitating clinical symptoms such as venous thrombosis [1]. Histopathologically, this tumor has large areas of necrosis and cystic degeneration, but calcification is uncommon [1]. They are large, well-circumscribed masses, which is isoattenuating to muscle on CT scan (Fig. 6.6). Contrast enhancement is often heterogeneous and predominantly peripheral. Small tumors may be homogeneously solid, but large tumors have extensive areas of low attenuation and represent necrosis and cystic degeneration. Rarely, leiomyosarcoma may appear as mostly cystic. At MR imaging, these tumors have intermediate to low signal intensity on T1-weighted images and intermediate to high signal intensity on T2-weighted images, depending on the amount of necrosis [1]. Mixed signal intensity and a fluid–debris level can be seen in hemorrhagic lesions [1]. The presence of extensive necrosis in a retroperitoneal mass, with contiguous involvement of a vessel, is highly suggestive of leiomyosarcoma [1]. Metastasis to the liver, lungs, or lymph nodes occurs late in the course of the disease [22, 32].

This new category in the 2013 WHO classification, which may comprise up to 20 % of all pleomorphic soft tissue sarcomas [33], encompasses tumors in which all recognizable lines of differentiation have been excluded. Many of these tumors would have been called MFH in the past. Since MFH was first described as a distinct histologic type of soft tissue sarcomas in 1964, large series of cases were reported [34, 35]. However, MFH has been plagued by controversy in terms of both its histogenesis and its validity as a clinicopathologic entity. Many pathologists recognized that most so-called MFH located in the retroperitoneum were dedifferentiated liposarcomas and only a few cases may be classified as undifferentiated pleomorphic sarcoma [35]. The latest WHO classification no longer includes MFH as a distinct diagnostic category but rather as subtypes of an undifferentiated/unclassified sarcoma [16, 17]. These tumors are typically high grade, show a wide range of morphological features, and are often associated with a poor prognosis. These tumors can be subclassified according to predominant morphological patterns: round cell, spindle cell, pleomorphic, and epithelioid. Undifferentiated sarcoma, formerly known as MFH, is the third most common sarcoma in the retroperitoneal space and has been regarded as the most common soft tissue sarcoma in the whole body [1].

CT and MRI findings of the MFH were nonspecific and demonstrated a large, infiltrating, and heterogeneously enhancing soft tissue mass with areas of necrosis and hemorrhage and with possible invasion of adjacent organs (Fig. 6.7). Variable patterns of calcification can be seen (7–20 % of cases) in the peripheral portions of these tumors [1]. The presence of calcification may help to distinguish MFH from leiomyosarcoma [22]. The changes or controversial application in terminology and classification has represented a source of potential diagnostic confusion. Imaging interpretation should be updated with the latest pathological classification to reach the more specific diagnosis in predicting the histological type or grade of retroperitoneal tumors.

Visceral pleura is the most common site of solitary fibrous tumor occurrence, but it can occur outside of the thoracic cavity. Retroperitoneal solitary fibrous tumors are rare, and their clinical manifestations depend on the location and size of the tumor. They grow slowly in an expansive way and displace adjacent structures by compression. For extrapleural solitary fibrous tumor, there is now complete omission of the prior synonym “hemangiopericytoma” according to the WHO classification of soft tissue tumors [16, 17].

The retroperitoneal solitary fibrous tumors were mostly lobulated and large, in round or oval shape (Fig. 6.8). Myxoid degeneration, hemorrhage, and necrosis may be visible, especially in huge masses; the tumor density or signal intensity became more heterogeneous on imaging. Rarely, some may have calcification. Most retroperitoneal solitary fibrous tumors show moderate or intense enhancement. Complete surgical resection is the standard therapy for solitary fibrous tumor, and pathologically negative resection margins are important to achieve a good prognosis.

Rhabdomyosarcoma is the most common soft tissue sarcoma in children. The tumor is believed to arise from primitive muscle cells, but tumors can occur anywhere in the body. The retroperitoneum is involved in 7 % of cases diagnosed with rhabdomyosarcoma [36]. Cross-sectional imaging shows a mass with heterogeneous attenuation or signal intensity, focal calcifications, necrosis, and significant contrast enhancement.

Extraskeletal chondrosarcoma in retroperitoneum is very rare. The histologic types of lesions that account for extraskeletal chondrosarcoma are myxoid, mesenchymal, and very rarely low grade [37]. On CT and MR images, both myxoid and mesenchymal chondrosarcomas have features similar to those described for tumors of these histologic types located in bone [37]. Imaging of these lesions demonstrates a nonspecific soft tissue mass with areas of chondroid matrix mineralization, which may be seen as ring- and arc-like, stippled, and highly opaque calcifications (Fig. 6.9).

Extraskeletal osteosarcoma is a rare malignant mesenchymal tumor characterized by the direct production of osteoid or bone by tumor cells without primary bone or periosteal involvement. The most common location of these tumors is the lower extremity, especially the thigh, followed by the upper extremity and the retroperitoneum [38]. Imaging shows a large soft tissue with areas of amorphous calcifications, necrosis, or old hemorrhage (Fig. 6.10). The imaging findings of extraskeletal osteosarcoma are nonspecific, with considerable overlap with other retroperitoneal masses. A wide range of differential diagnoses include several benign and malignant conditions that show mineralization, which include nontumoral benign conditions such as calcified hematoma or myositis ossificans and numerous benign and malignant retroperitoneal tumors.

Schwannoma, also known as neurilemmoma, is the most common retroperitoneal neurogenic tumor and accounts for 4 % of all retroperitoneal tumors [5]. It is a solitary, slow-growing benign tumor that arises from peripheral nerve sheath and is usually located in the paravertebral or presacral region. Pathologically, it appears as a rounded or oval mass and is well encapsulated and extends along the course of a nerve, with the nerve of origin flattened against the periphery of the tumor [1]. Microscopically, it consists of two different components (Antoni A and Antoni B). Antoni A area is highly cellular, while Antoni B area is less cellular, with cystic and edematous area. At MR imaging, highly cellular areas are hypointense on T1- and T2-weighted images, while cystic and edematous areas appear hyperintense on T2-weighted images. Small schwannomas may be seen as round, well-circumscribed, and homogeneous masses, but large schwannomas have a heterogeneous appearance and are more likely to undergo degenerative changes (hemorrhage, necrosis or cystic changes, calcification, and hyalinization) (Fig. 6.11). Contrast enhancement is heterogeneous, with nonenhancing cystic components and enhancing solid components [1]. Malignant transformation rarely occurs [1, 40].

Neurofibroma is a benign nerve sheath tumor that can occur as an isolated tumor (90 %) or as part of type 1 neurofibromatosis [1]. Approximately one-third of patients with a solitary neurofibroma are associated with type 1 neurofibromatosis, and almost every patient with multiple tumors or plexiform neurofibromas has type 1 neurofibromatosis [39]. Neurofibroma is more common in men, particularly in the 20–40-year age group [39]. Histopathologically, neurofibroma is an unencapsulated solid tumor that is composed of nerve sheath cells and collagen bundles with variable myxoid degeneration and causes expansion of the entire nerve, with nerve fibers traversing the tumor [1]. Cystic degeneration is rare. Plexiform neurofibroma is seen as a large extensive infiltrating mass and is commonly located in the lumbosacral plexus.

On imaging, neurofibroma is depicted as a well-defined, rounded mass with homogeneous contrast enhancement. Occasionally, it demonstrate target-like enhancement representing the center of nerve tissue and the periphery of myxoid degeneration. On T1-weighted images, the central portion of the tumor has higher signal intensity that is due to neural tissue, and on T2-weighted images, the periphery has higher signal intensity that is due to myxoid degeneration [1]. Tumors involving the neural foramina may show a “dumbbell” shape with widening of the bony foramina or vertebral body scalloping.

Malignant peripheral nerve sheath tumors include malignant schwannoma, neurogenic sarcoma, and neurofibrosarcoma [1]. Fifty percent of these tumors originate de novo, and the rest of them are derived from neurofibroma or ganglioneuroma or occur after exposure to radiation [39]. These are highly aggressive and infiltrative tumors, may present with distant metastases, and demonstrate high recurrence rate. They may be clinically asymptomatic, but progressive enlargement and pain can be suggestive of malignancy, especially when associated with type 1 neurofibromatosis. Although imaging is not reliable in differentiating between benign and malignant tumors, a heterogeneous nature, irregular or infiltrative borders with invasion into adjacent tissues and destruction of adjacent bones can be helpful for detecting malignancy (Fig. 6.12).