Peritoneal carcinomatosis and metastatic involvement of the retroperitoneum are manifestations of many organ-based malignancies and lymphoproliferative disorders. Primary malignancies of peritoneal and retroperitoneal origin occur much less frequently, and are difficult to distinguish from metastatic disease on imaging alone. However, the imaging features of these primary tumors, taken in concert with the clinical data, can be helpful in narrowing the scope of the differential diagnosis. This review presents the clinical and imaging features of primary peritoneal and retroperitoneal tumors arising from the various tissue components that comprise the ligaments, mesenteries, and connective tissues of the peritoneal and retroperitoneal spaces.
Key points
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Primary peritoneal and retroperitoneal malignancies are less common than metastatic disease.
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A variety of tumor types can occur in the peritoneum and retroperitoneum, and establishing a diagnosis from imaging alone can be difficult. Demographic and clinical information taken in concert with imaging findings can help narrow the differential diagnosis.
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Papillary serous carcinoma, the most common primary tumor of the peritoneum, occurs almost exclusively in women and is often densely calcified, which can aid in diagnosis.
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Liposarcoma is the most common primary tumor of the retroperitoneum, followed by leiomyosarcoma, each having imaging features that can suggest or even make the diagnosis.
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Imaging is useful in detection, staging, guiding biopsy (most often with computed tomography or ultrasonography), and evaluating response to therapy.
Introduction
Peritoneal carcinomatosis is a relatively common manifestation of many organ-based malignancies, particularly of the gastrointestinal tract and ovaries. Similarly, metastatic retroperitoneal lymphadenopathy and direct extension from an organ-based primary tumor are also common findings at imaging evaluation. Primary tumors of peritoneal and retroperitoneal origin occur much less frequently, but are often first identified on cross-sectional radiologic imaging studies, such as computed tomography (CT), ultrasonography, or magnetic resonance (MR) imaging.
Neoplastic involvement of the peritoneum and retroperitoneum generally manifest with an abnormal increase in soft tissue, which can appear infiltrative or tumorous and be associated with variable amounts of cystic change, calcification, macroscopic fat, enhancement, and surrounding fluid. However, because many nonneoplastic and metastatic processes demonstrate similar imaging findings, the appearance of many primary malignancies of the peritoneum and retroperitoneum is nonspecific. As a result, even in the absence of a known organ-based primary malignancy, metastatic disease is often the first consideration when confronted with an abnormal soft-tissue process arising within the peritoneal or retroperitoneal space. However, primary malignancies should also be considered in this setting.
This review presents the salient clinical and imaging features of several primary neoplasms ( Box 1 ) arising from the tissue components that comprise the ligaments, mesenteries, and connective tissues of the peritoneal and retroperitoneal spaces. Combining the imaging features with the patient’s relevant clinical information can often refine the differential diagnosis for peritoneal-based and retroperitoneal-based neoplasms. In addition to detection and characterization, cross-sectional imaging is useful for directing biopsy for tissue diagnosis.
Primary Peritoneal Malignancies
Mesothelioma
Papillary serous carcinoma
Desmoplastic small round cell tumor
Malignant fibrous histiocytoma
Liposarcoma
Other mesenchymal tumors
Primary Retroperitoneal Malignancies
Liposarcoma
Leiomyosarcoma
Malignant fibrous histiocytoma
Other mesenchymal tumors
Paraganglioma
Extragonadal germ cell tumors
Introduction
Peritoneal carcinomatosis is a relatively common manifestation of many organ-based malignancies, particularly of the gastrointestinal tract and ovaries. Similarly, metastatic retroperitoneal lymphadenopathy and direct extension from an organ-based primary tumor are also common findings at imaging evaluation. Primary tumors of peritoneal and retroperitoneal origin occur much less frequently, but are often first identified on cross-sectional radiologic imaging studies, such as computed tomography (CT), ultrasonography, or magnetic resonance (MR) imaging.
Neoplastic involvement of the peritoneum and retroperitoneum generally manifest with an abnormal increase in soft tissue, which can appear infiltrative or tumorous and be associated with variable amounts of cystic change, calcification, macroscopic fat, enhancement, and surrounding fluid. However, because many nonneoplastic and metastatic processes demonstrate similar imaging findings, the appearance of many primary malignancies of the peritoneum and retroperitoneum is nonspecific. As a result, even in the absence of a known organ-based primary malignancy, metastatic disease is often the first consideration when confronted with an abnormal soft-tissue process arising within the peritoneal or retroperitoneal space. However, primary malignancies should also be considered in this setting.
This review presents the salient clinical and imaging features of several primary neoplasms ( Box 1 ) arising from the tissue components that comprise the ligaments, mesenteries, and connective tissues of the peritoneal and retroperitoneal spaces. Combining the imaging features with the patient’s relevant clinical information can often refine the differential diagnosis for peritoneal-based and retroperitoneal-based neoplasms. In addition to detection and characterization, cross-sectional imaging is useful for directing biopsy for tissue diagnosis.
Primary Peritoneal Malignancies
Mesothelioma
Papillary serous carcinoma
Desmoplastic small round cell tumor
Malignant fibrous histiocytoma
Liposarcoma
Other mesenchymal tumors
Primary Retroperitoneal Malignancies
Liposarcoma
Leiomyosarcoma
Malignant fibrous histiocytoma
Other mesenchymal tumors
Paraganglioma
Extragonadal germ cell tumors
Anatomic considerations
The visceral and parietal peritoneum encloses the large potential space referred to as the peritoneal cavity. Pathologic processes that gain access to the peritoneal cavity can disseminate throughout this space via the relatively unrestricted movement of fluid and cells. Pathologic processes can also be disseminated within the subperitoneal space, which lies deep to the surface lining of the visceral and parietal peritoneum, omentum, and the various peritoneal ligaments and mesenteries. The subperitoneal space has both intraperitoneal and extraperitoneal components that bridge the peritoneum and retroperitoneum, which can result in bidirectional spread of disease processes. This concept helps to explain the involvement of both the peritoneal and retroperitoneal space that is sometimes encountered.
The retroperitoneal space is not defined by specific anatomic structures delineating its borders, but rather as the space posterior to the peritoneal cavity. Retroperitoneal structures may be defined as primary (ie, located within the retroperitoneum at the beginning of embryogenesis) or secondary (ie, initially suspended by a mesentery during early embryogenesis but subsequently migrated posteriorly and fused to become retroperitonealized). The extraperitoneal pelvis, including the presacral space, essentially represents the inferior continuation of the retroperitoneal space. The retroperitoneum and extraperitoneal pelvis represent a crossroads for several organ systems, containing portions of the gastrointestinal and genitourinary tracts, in addition to major vascular structures. The retroperitoneum, however, also contains connective tissues, fat, and neural elements.
This article focuses on primary malignancies arising directly from the supporting tissues of the peritoneal, subperitoneal, and retroperitoneal spaces, rather than tumors that arise from the organs contained within these spaces.
Primary peritoneal malignancies
Papillary Serous Carcinoma
Clinical features
Primary papillary serous carcinoma of the peritoneum is a relatively uncommon malignancy that predominantly affects postmenopausal women. Because this tumor is histologically identical to serous ovarian papillary carcinoma and is clearly distinguishable only when the ovaries are either not involved or superficially involved, its incidence is underestimated. Treatment generally consists of an abdominal hysterectomy, bilateral salpingo-oophorectomy, and debulking surgery, which are followed by combination chemotherapy. Despite these interventions, the prognosis is dismal.
Imaging features
Cross-sectional imaging often shows extensive, multifocal involvement of the peritoneum, with omental caking, ascites, and, importantly, no associated primary ovarian mass ( Fig. 1 ). Misclassification as primary ovarian cancer is common in the authors’ experience, and peritoneal origin may be first suggested by the radiologist. As with primary ovarian primary tumors there is often extensive calcification within the peritoneal implants, which can be a useful CT finding for differentiating this tumor from peritoneal mesothelioma.
Mesothelioma
Clinical features
Mesothelial cells line the body cavities, including the pleura, peritoneum, pericardium, and scrotum. Mesothelioma is a rare tumor, which arises from these cells and most frequently involves the pleural space. However, approximately 30% arise solely from the peritoneum. There are benign, borderline, and malignant variants, but benign cystic mesothelioma is not related to malignant mesothelioma. Compared with the pleural form, malignant mesothelioma of the peritoneum is less often associated with asbestos exposure. However, cases with both pleural and peritoneal involvement are usually asbestos-related. In general, malignant mesothelioma of the peritoneum is an aggressive tumor with a rapidly progressive clinical course and a universally poor prognosis. For untreated cases, median survival ranges from 5 to 12 months, with little improvement seen in patients receiving multimodality therapy.
Imaging features
The imaging features of peritoneal mesothelioma are variable. The “dry” appearance consists of single or multiple peritoneal-based soft-tissue masses that may be large or confluent ( Fig. 2 ). The “wet” appearance consists of peritoneal thickening that may be nodular and/or diffuse and is associated with peritoneal fluid (ascites) ( Fig. 3 ). The tumor can be identified in any location within the omentum, mesentery, or peritoneal folds, and spreads along serosal surfaces. Scalloping, mass effect, or direct invasion of adjacent abdominal organs can be seen (see Fig. 3 A). The most commonly involved organs are the colon and liver. Calcification, either within the mass or associated with peritoneal plaques, is uncommon, and one should consider other causes in the setting of extensive calcification in a peritoneal-based tumor.
Desmoplastic Small Round Cell Tumor
Clinical features
Desmoplastic small round cell tumor is a rare, highly aggressive malignancy that has been relatively recently described. It is a member of the family of lesions classified as small round blue cell tumors (lymphoma, neuroblastoma, rhabdomyosarcoma, Ewing sarcoma, neuroectodermic tumors), and is characterized by a recurrent specific chromosomal translocation and distinct immunohistochemical pattern (epithelial, mesenchymal, and neural markers). It generally behaves like a soft-tissue sarcoma and has a predilection for primary peritoneal involvement, particularly in adolescent and young adult males. The disease tends to be rapidly progressive, and metastatic disease to the liver, lungs, lymph nodes, and bones are often present at diagnosis. Treatment is relatively ineffective, but attempted therapy often includes combination surgical debulking, chemotherapy, and radiation therapy.
Imaging features
The most common imaging appearance is that of multiple, bulky, rounded peritoneal-based masses ( Fig. 4 ). There can be associated ascites, and heterogeneous enhancement of the masses with areas of central necrosis is common. Contrast enhancement is often weak, likely because of the fibrous nature of the tumor. The omentum and paravesical regions are often involved. Although the lesions are usually discrete, an infiltrative appearance is sometimes seen. Calcifications and lymphadenopathy are not usually present. These tumors are often 18 F-fluorodeoxyglucose avid, and positron emission tomography/CT can sometimes provide additional staging information.
Malignant Fibrous Histiocytoma
Clinical features
Primary sarcomas of the peritoneal/subperitoneal space, such as malignant fibrous histiocytoma (MFH) and liposarcoma, occur less frequently than their retroperitoneal counterparts. These tumors are most frequently seen in adults and typically are fairly large at diagnosis.
MFH accounts for 20% to 24% of all soft-tissue sarcomas, most commonly arising in the extremities and retroperitoneum. However, MFH is also reported to be the single most common primary peritoneal sarcoma. It occurs more frequently in men, with a peak incidence in the fifth and sixth decades of life. The mass is often clinically silent until it is large, as it is usually painless. Constitutional symptoms such as fever, malaise, and weight loss can occur but are nonspecific. The only treatment is complete surgical resection, if possible. Metastatic disease most often involves the lungs, bone, and liver. Prognosis is related to tumor grade, size, and presence or absence of metastatic disease. Specifically, high-grade tumors and tumors larger than 10 cm have a poor outcome, with 10-year survival rates of less than 50%.
Imaging features
Radiographically, MFH typically manifests as a large heterogeneous soft-tissue mass, as do most sarcomas. Biopsy is required to make a specific diagnosis. The mass is frequently lobulated with peripheral nodular enhancement, can have associated calcifications (in approximately 10%), and may demonstrate heterogeneity from central necrosis, hemorrhage, or myxoid degeneration ( Fig. 5 ). Fatty components are not seen in MFH, and its presence would suggest liposarcoma. The tumor may directly invade the abdominal musculature, but vascular invasion is rare.
Liposarcoma
Clinical features
Fat-containing tumors are common in general, and account for approximately half of all soft-tissue tumors in most surgical series. However, most of these represent benign lipomas, and differentiating these tumors from liposarcoma is not a trivial matter. Although liposarcoma is one of the most common primary retroperitoneal malignancies, primary peritoneal liposarcoma is relatively rare. The clinical presentation is usually delayed because of the lack of associated symptoms. Ultimately the mass may become palpable, create symptoms related to mass effect on adjacent structures, be incidentally identified at the time of imaging. Treatment is surgical resection, with or without chemotherapy and radiotherapy. Prognosis is inversely related to differentiation of the tumor and directly related to completeness of resection.
Imaging features
When macroscopic fat is present, it is easily and confidently recognized on CT and MR imaging, and this observation significantly limits the differential diagnosis. If the mass is homogeneous, well defined, and consists almost entirely of fat with only minimal if any soft-tissue component, the diagnosis of a benign lipoma is almost certain. Liposarcomas are typically less well defined, have indistinct borders, and contain variable but increased amounts of soft tissue. In fact, some poorly differentiated liposarcomas have no demonstrable fat on cross-sectional imaging and are therefore indistinguishable from other sarcomas ( Fig. 6 ).
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