Gross appearance of peritoneal cysts. They have a thin, translucent wall and contain a clear fluid.

So-called multicystic benign mesothelioma. A, Gross appearance.

So-called multicystic benign mesothelioma. B, Microscopic appearance. The flat shape of the mesothelium lining the cyst simulates the appearance of a lymphangioma.

Twisted appendix epiploica. A, Gross appearance.

Twisted appendix epiploica. B, Microscopic appearance, showing inflammatory reaction to fat necrosis.

A and B, Florid mesothelial hyperplasia in hernia sac. The complex papillary architecture may simulate a mesothelioma.

Reactive mesothelial hyperplasia in a patient with granulosa cell tumor of the ovary. This should not be overinterpreted as a tumor implant.

A and B, Florid mesothelial hyperplasia with a pattern resembling adenomatoid tumor.

Malignant mesothelioma with papillary formations and desmoplastic stromal reaction.

Malignant mesothelioma combining hyper- and hypocellular areas.

So-called deciduoid mesothelioma combining hyper- and hypocellular areas.

So-called lymphohistiocytoid mesothelioma. The tumor cells are admixed with numerous reactive lymphocytes and histiocytes.

Malignant mesothelioma cells show immunoreactivity for keratin 5/6

Malignant mesothelioma cells show immunoreactivity for calretinin.

Primary papillary serous carcinoma of peritoneum. The tumor is morphologically and immunohistochemically indistinguishable from its ovarian counterpart.

Gross appearance of desmoplastic small cell tumor. There are multiple nodules, one of them of considerable size. Note the large areas of fibrosis.

Microscopic appearance of intra-abdominal desmoplastic small cell tumor. Low-power view showing well-defined tumor nests surrounded by cellular stroma.

Desmoplastic small cell tumor showing glandular formations at the periphery of the tumor nests.

Desmoplastic small cell tumor showing an unusual degree of pleomorphism. It was also unusual in the sense of occurring in a 76-year-old female and involving the outer uterine wall. The diagnosis was confirmed by demonstrating the EWS WT1 gene fusion by PCR.

Desmoplastic small cell tumor showing typical polyphenotypic reactivity for keratin.

Desmoplastic small cell tumor showing typical polyphenotypic reactivity for desmin.

Desmoplastic small cell tumor showing typical polyphenotypic reactivity for neuron-specific enolase.

Gross appearance of solitary fibrous tumor attached to the peritoneal side of the diaphragm.

Peritoneal angiosarcoma. The pattern of growth greatly simulates that of malignant mesothelioma.

Epithelioid hemangioendothelioma of peritoneum. A, Hematoxylin eosin (from another case of the same entity).

Epithelioid hemangioendothelioma of peritoneum. B, CD31 (from another case of the same entity).

Epithelioid hemangioendothelioma of peritoneum. C, FLI-1 (from another case of the same entity).

Low-power view of leiomyomatosis peritonealis disseminata.

Gross appearance of pseudomyxoma peritonei. The entire peritoneal cavity is occupied by a multinodular mucinous mass.

Microscopic appearance of pseudomyxoma peritonei. Clusters of well-differentiated mucin-producing glandular cells are seen floating in a sea of mucin.

So-called peritoneal gliomatosis resulting from rupture of an ovarian teratoma.

Positive peritoneal cytology in a patient with serous carcinoma of the ovary. Note the psammomatous bodies.

Positive peritoneal cytology in a patient with pancreatic adenocarcinoma.

Cytologic appearance of malignant mesothelioma. The clustering of tumor cells into morula-like structures is particularly characteristic.

Sclerosing mesenteritis. There is fibrosis with hyalinization, chronic inflammation, and fat necrosis surrounded by clusters of foamy macrophages.

Heterotopic mesenteric ossification. This section, which corresponds to the peripheral portion of the lesion, shows well-developed bone trabeculae.

Inflammatory pseudotumor showing an admixture of inflammatory cells in a fibrosed background.

Typical golden yellow color of chylous cyst.

Fibromatosis of mesentery involving the bowel wall. Cases like this are likely to be misinterpreted as GIST.

Fibromatosis of mesentery. A, Entrapment of muscle fibers from bowel wall.

Fibromatosis of mesentery. B, Typical growth of myofibroblasts in a heavily collagenized background. Note the sharply etched thick-walled vessel.

Gross appearance of paraganglioma of the mesentery. This is a most unusual location for this tumor type. The associated hemorrhage due to the high degree of tumor vascularization is a common event.

Castleman s disease located in the root of the mesentery. This is a relatively common location for this entity.

Umbilical polyp partially lined by glandular epithelium derived from the omphalomesenteric duct.

Malakoplakia of retroperitoneum. A, The H&E appearance simulates malignant fibrous histiocytoma.

Malakoplakia of retroperitoneum. B, von Kossa stain, showing numerous Michaelis Gutmann bodies.

Cut surface of well-differentiated liposarcoma (atypical lipomatous tumor) of the sclerosing sub type.

Typical perirenal location of well-differentiated liposarcoma (atypical lipomatous tumor).

Gross appearance of retroperitoneal well-differentiated liposarcoma (atypical lipomatous tumor) accompanied by foci of dedifferentiation, manifested by the solid whitish areas.

Angiolipoma of retroperitoneum. This tumor differs from angiomyolipoma by virtue of the absence of a vascular component and its lack of reactivity to HMB-45.

Gross appearance of retroperitoneal leiomyosarcoma, removed in continuity with the spleen. The whorling appearance of the cut surface is characteristic.

Retroperitoneal tumor having morphologic and immunohistochemical features analogous to GIST.

Retroperitoneal smooth muscle tumor in pelvic region morphologically similar to uterine leiomyoma. Note the prominent hyalinization.

Solitary fibrous tumor/hemangiopericytoma of the pelvic region. This is one of the most common locations for this tumor type.

Retroperitoneal schwannoma. The tumor is encapsulated and shows marked secondary hemorrhagic and cystic changes.

Partially calcified retroperitoneal malignant peripheral nerve sheath tumor.

Mature retroperitoneal teratoma. Gross appearance, showing multiple cystic spaces.

Typical microscopic appearance of yolk sac tumor of sacrococcygeal region.

Ganglioneuroma of retroperitoneum. The gross appearance is similar to that of neurofibroma.

A and B, Low-power and high-power appearance of müllerian-type cystadenocarcinoma located in the retroperitoneal region.

Tailgut cyst lined by pseudostratified epithelium surrounded by a muscle wall.

Tailgut cyst lined by mucin-secreting welldifferentiated epithelium with goblet cells.

Sacrococcygeal teratoma protruding as a polypoid, partially ulcerated mass.