Louis P. Dehner
John D. Pfeifer
I. NORMAL ANATOMY. The retroperitoneal space (in some respects a virtual space) is located between the posterior parietal peritoneum and the fascia that covers the muscles of the lumbar region. It extends upward to the diaphragm, downward to the base of the sacrum and iliac crests, and laterally to the external borders of the lumbar muscles and the ascending and descending colon. The retroperitoneum contains loose connective tissue surrounding lymph nodes, the aorta and inferior vena cava with their vascular branches, the adrenal glands, the kidneys and ureters, the pancreas, and portions of the duodenum. This chapter will focus on the entities that arise from the tissues of the retroperitoneal space; disorders arising from the organs that are completely or partially retroperitoneal are covered in the respective chapters.
II. NONNEOPLASTIC DISEASES
1. Idiopathic fibroinflammatory lesions constitute a morphologically similar category of tumefactive processes, and a variety of diagnostic terms such as sclerosing mesenteritis and/or panniculitis have been applied. The usual presentation is a mass in the retroperitoneum and/or root of the mesentery; sclerosing changes may extend into the inferior retroperitoneum with similar features, if not identical, to idiopathic retroperitoneal fibrosis. A biopsy reveals a variably intense mixed lymphocytic and plasmacellular infiltrate in a dense, relatively hypocellular collagenous background. Residual adipose tissue may have the features of fat necrosis with dystrophic calcification and panniculitis. In the presence of lymphadenopathy, the possibility of lymphoma should be excluded. Metastatic disease including poorly differentiated adenocarcinoma of the pancreas, carcinoid, lobular carcinoma of the breast, and acinar carcinoma of the prostate may be a accompanied by disproportionate fibroin-flammatory reaction to the actual volume of tumor, and thus also need to be excluded. If the biopsy has a prominently spindle cell component, inflammatory pseudotumor, inflammatory myofibroblastic tumor (IMT), sarcomatoid carcinoma, pleomorphic spindle cell sarcoma, and IgG4-related sclerosing disorder are also in the differential diagnosis (Nat Rev Gastroenterol Hepatol. 2010;7:401; J Clin Pathol. 2008;61:1093). Since the histologic findings of idiopathic fibroinflammatory lesions are nonspecific, even after the other possibilities have been excluded, the diagnostic line in the report may simply read “chronic inflammation and fibrosis.”
2. Malakoplakia occasionally affects the retroperitoneal soft tissues. The process is a response to infection and grossly appears as a yellow plaque-like lesion. Microscopically, it is composed of abundant inflammatory cells and sheets of granular and vacuolated histiocytes that contain lamellated, PAS-positive diastase-resistant inclusions known as Michaelis-Gutmann bodies. These distinctive inclusions (e-Fig. 47.1)* are thought to represent the remnants of bacteria within phagosomes that have been mineralized by calcium and iron.
3. Retroperitoneal abscesses are generally secondary to infectious processes of adjacent organs, most commonly of the kidneys. Less frequently, abscesses originate from distant septic foci that propagate via a hematogenous route.
4. Other nonneoplastic conditions including hemorrhage, bile collection, and extravasation of urine can be occasionally encountered in the retroperitoneal space. Endometriosis may also involve the retroperitoneum.
B. Idiopathic retroperitoneal fibrosis (sclerosing retroperitoneal fibrosis, Ormond disease) is an uncommon inflammatory process characterized by sclerosing fibrosis of the retroperitoneum that can ultimately cause constriction and obliteration of the ureters.
Grossly, there is poorly circumscribed fibrosis, usually at the level of the lower abdominal aorta and its bifurcation. Microscopically, the process is characterized by dense fibrosis with collagen entrapment, with associated alternating areas of prominent inflammation primarily consisting of plasma cells, histiocytes, eosinophils, and lymphocytes (e-Fig. 47.2). The plasma cells may be IgG4 positive in which case the disorder may be a member of the IgG4-related sclerosing disorders, especially in affected males (Am J Surg Pathol. 2009;33:1833); periaortitis in association with retroperitoneal fibrosis may be part of the same sclerosing disease spectrum (Am J Surg Pathol. 2008;32:197), as are sclerosing inflammatory diseases in the thyroid, mediastinum, and bile ducts. In the remaining cases, the etiology is undetermined or associated with methysergide and other drugs (and often regresses after cessation of the drug).
C. Cystic lesions. While most cystic lesions of the retroperitoneum represent a secondary or degenerative change within a benign or malignant neoplasm, several benign primary cystic lesions also occur.
1. Cystic lymphangioma or cystic lymphatic malformation of the retroperitoneum accounts for 5% of all retroperitoneal cystic lesions and can occur at any age. Involvement of the mesentery is an associated feature since this malformation is not well circumscribed (J Surg Oncol. 1996;61:234). Pathologically, multi- or unilocular cysts contain clear or milky fluid, and are lined with a single layer of flattened endothelium, which is D2-40 immunopositive (J Pediatr Surg. 1999:34;1164).
2. Multicystic peritoneal inclusion cyst occurs predominantly in women of reproductive age and, even though rarely seen in the retroperitoneum, is seen in the differential diagnosis of cystic lesions (especially of multilocular cystic lymphangioma). Prior abdominal surgery is common. Histopathologically, the lesion consists of mesothelial-lined cystic spaces usually containing watery secretions, separated by a delicate fibromuscular stroma. The lining epithelium is usually immunoreactive for calretinin. As discussed in some detail in Chapter 11 (Section IV.B), some confusion exists regarding the proper classification of multicystic peritoneal inclusion cyst, as demonstrated by the fact that the lesion is also known as multicystic mesothelioma (Ann Diagn Pathol. 2000;4:308).
3. Bronchogenic cysts can rarely occur in a subdiaphragmatic location, including the retroperitoneum, as part of the morphologic spectrum of bronchopulmonary foregut malformations. The related anomaly, extralobar sequestration with features of congenital cystic adenomatoid malformation type 2 presents a solid mass in the same site, usually in infants and young children; its suprarenal location can lead to confusion with neuroblastoma (J Pediatr Surg. 2007;42:1627). The cysts are lined by respiratory-type, pseudostratified, ciliated columnar epithelium that can focally contain seromucous glands and nodules of hyaline cartilage.
4. Müllerian cysts of the retroperitoneum are usually in excess of 10 cm in greatest dimension and are lined by cuboidal to columnar epithelium that often contains ciliated cells; there is no atypia, although stratification and epithelial
tufting can be present as in tubal epithelium. Beneath the lining of amüllerian cyst, loose fibrous tissue, dilated vessels, and incomplete smooth muscle bundles can be seen (Hum Pathol. 2003;34:194). Mucinous cystadenoma with ovarian type stroma (which is inhibin immunopositive) occurs rarely as an extrapancreatic cyst in the retroperitoneum.
5. Enteric duplication cyst (EDC) is an extremely uncommon congenital lesion and may be detected prenatally (Pediatr Radiol. 2000;30:671). Although usually intra-abdominal and associated with the small intestine, it has sporadically been described in the retroperitoneum. A spherical, tubular, or dumbbellshaped cyst with a smooth lining and viscous contents is the gross appearance. Microscopically, the cyst wall has a variably differentiated enteric mucosa consisting of a simplified cuboidal or cylindrical epithelium, to a more normal appearing mucosa with a muscularis mucosa and a muscularis propria. Focal areas of squamous metaplasia and gastric-type mucosa have also been described in retroperitoneal EDCs (JOP. 2006;7:492). Heterotopic pancreas is another infrequent finding.
III. NEOPLASMS. Primary retroperitoneal tumors arise from the extravisceral tissues that comprise the retroperitoneum; the neoplasms for the most part are malignant in adults and originate from lymphoid and various soft tissue elements including fat and neural structures. The histogenesis of some of the high-grade sarcomas is not apparent, and the immunophenotype often does provide information that is specific in terms of lineage.
In children and young adults, germ cell neoplasms can present in the retroperitoneum, but the most common neoplasms of the retroperitoneum in children include the neuroblastic tumors (neuroblastoma and its variants) and high-grade lymphomas, in particular Burkitt lymphoma, large cell lymphoma, and Hodgkin lymphoma. The most commonly encountered primary retroperitoneal tumors in adults are sarcomas, followed by lymphomas and germ cell tumors (Sarcoma. 2001;5:5). Overall, 70% to 80% of retroperitoneal tumors are malignant (and thus the retroperitoneum is the only body site where the frequency of malignant neoplasms exceeds that of benign tumors).
The anatomy of the retroperitoneal space makes it possible for primary (as well as metastatic) tumors to attain substantial dimensions before clinical manifestations become evident. Malignant retroperitoneal tumors have a generally poor prognosis since they are often found at an advanced stage of disease, and even resectable tumors often recur.
A. Soft tissue tumors. Primary sarcomas arising in the retroperitoneum constitute 10% to 20% of all soft tissue sarcomas in adults: liposarcoma (40% of cases), leiomyosarcoma (30%), and pleomorphic sarcoma—malignant fibrous histiocytoma (25%) (Expert Rev Anticancer Ther. 2009;9:1145). As a group, retroperitoneal sarcomas are often in excess of 10 cm and weigh in excess of 400 to 500 g. Complete resection is associated with the most favorable outcome, but size and involvement of other structures often limits the extent and completeness of the resection; local recurrence is guaranteed in the latter cases. With complete resection and no local recurrences, the 5-year survival is 40% to 50%. As for soft tissue sarcomas arising in other anatomic sites, the American Joint Committed on Cancer (AJCC) staging system is utilized for staging of retroperitoneal sarcomas (see Chap. 46).
1. Adipocytic tumors
i. Myolipoma, one of the more common benign retroperitoneal lipomatous tumors, is more common than pure retroperitoneal lipomas. This encapsulated lesion has a lobulated fatty appearance with bands and nodules of white to gray-white tissue (World J Surg Oncol. 2005;3:72); the large size, usually 10 cm or greater, often raises the question of
liposarcoma. Microscopically, the tumor contains an admixture of mature adipocytes and bundles of smooth muscle (which usually predominate), but there is an absence of mitotic activity and atypia. The tumor is diffusely and strongly immunopositive for smooth muscle actin (SMA) and desmin.
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