Again, when there is known ovarian, gastrointestinal tract (stomach, colon, appendix, gallbladder, and pancreas), or other primary malignancies (breast, lung, and uterus), then metastatic disease causing peritoneal thickening should be considered. Metastatic disease to the peritoneum can present as diffuse spread of disease along the extensive surface area of the peritoneum [3]. The ligaments, mesenteries, and compartments formed from the folding of the peritoneum along with the normal circulation of peritoneal fluid dictate the spread of disease [2].

Peritoneal carcinomatosis is defined as widespread metastatic disease in the peritoneal cavity (Fig. 16.4). It is commonly seen as small tumor nodules studding the peritoneal surface, which can later grow and invade the subperitoneal tissues. Omental caking results when tumor nodules have invaded the omental fat and there is an associated fibrotic response (Fig. 16.5). Furthermore, metastatic disease to the ovaries (Krukenberg tumors) forms from the transperitoneal spread of disease. When there is involvement of the mesentery, a sheetlike growth pattern can be seen, similar to malignant mesothelioma. Ascites can be present with peritoneal carcinomatosis. A known complication of peritoneal metastatic disease is small bowel obstruction (Fig. 16.6).

CT imaging is widely used for imaging of peritoneal metastasis. However, Low et al. has found that delayed gadolinium-enhanced MRI is accurate for detecting small volume peritoneal tumors and carcinomatosis [3]. Metastatic disease is suspected when peritoneal enhancement is greater than that of the liver on delayed images (5–10 min) with associated peritoneal nodules, thickening, and masses. Also, adding DWI improves the sensitivity and specificity for depicting peritoneal metastasis [3].

The term pseudomyxoma peritonei is not a pathologic diagnosis, but a clinical and radiographic description. It is described when there is abundant, thick, and gelatinous material on the surface of the peritoneum and associated viscera, without invasion of underlying tissues. This is usually due to low-grade mucinous carcinomas arising from the appendix (Fig. 16.7). On the other hand, mucinous carcinomatosis usually arises from a mucinous carcinoma from other parts of the gastrointestinal tract, gallbladder, pancreas, or ovary. In contrast to pseudomyxoma peritonei, mucinous carcinomatosis often has larger soft tissue and fibrotic components and is characterized by a more aggressive course, such as invasion into the adjacent organs, causing omental carcinomatosis, containing mesenteric and retroperitoneal lymphadenopathy, and producing pleural effusions and hematogenous metastasis elsewhere (Fig. 16.8). Occasionally, it will be difficult to distinguish pseudomyxoma peritonei from mucinous carcinomatosis, as their imaging features can overlap.

CT will show mucinous ascites as low in attenuation with scalloping of the liver, spleen, and gastric surfaces. Unlike regular ascites, mucin will displace bowel and mesentery and causing them to not float.

When there is diffuse peritoneal thickening, peritoneal lymphomatosis can be included in the differential diagnosis. Gastrointestinal lymphoma can present with omental caking, peritoneal thickening, and masses as seen on CT (Fig. 16.9). The classic appearance has been described as the “sandwich sign,” in which confluent masses encase the superior mesenteric artery and vein (Fig. 16.10). A thickened bowel wall with aneurismal dilatation and splenomegaly can help in differentiating peritoneal lymphomatosis from peritoneal carcinomatosis [4].

Gastrointestinal stromal tumor (GIST) is a mesenchymal tumor that typically arises from the gastrointestinal tract. They are rarely seen in the omentum and mesentery. Their presentation as seen on CT imaging can be diffuse peritoneal thickening and seeding, similar to peritoneal carcinomatosis. They have been found to characteristically present as large masses with hemorrhage or necrosis. Furthermore, a differentiating feature that has been described is their hypervascularity (Fig. 16.11) [4].

In the absence of a visceral or malignant tumor elsewhere, primary peritoneal tumors can present with peritoneal thickening and masses. Primary peritoneal tumors are rare. Examples include malignant mesothelioma, primary peritoneal serous carcinoma, multicystic mesothelioma, leiomyomatosis peritonealis, and desmoplastic small round cell tumor. They arise from mesothelial, submesothelial, and uncommitted stem cells [1].

Malignant mesotheliomas can arise in the pleura, pericardium, and peritoneum, where the cavities are lined by a serosal membrane. They have also been found in the tunica vaginalis, since it is a continuation of the peritoneal membrane. The majority of malignant mesotheliomas involve the pleura. 6–10 % involves the peritoneum [1]. Malignant mesothelioma is known to be associated with asbestos exposure, particularly in men where the majority is found.

Two patterns of peritoneal involvement have been described on cross-sectional imaging by malignant mesothelioma. They can be seen as focal intraperitoneal masses or diffuse involvement of the peritoneum. The focal pattern presents as solid, heterogenous masses with irregular margins. Invasion of the adjacent visceral organs can occur.

The diffuse pattern encases the peritoneal cavity and the viscera in a rind. The peritoneum is thickened and nodular. Involvement of the mesentery has been described as pleated or stellate. Omental caking and ascites can occur, sometimes overlapping and being indistinguishable from peritoneal carcinomatosis. Nevertheless, when there are imaging findings of prior asbestos exposure and sheetlike thickening of the peritoneum, malignant mesothelioma should be highly considered (Fig. 16.12).

Primary peritoneal serous carcinoma is also another entity that should be considered in the differential diagnosis of primary peritoneal tumors presenting as peritoneal thickening, especially in a female patient with no visceral primary or ovarian mass. Primary peritoneal serous carcinoma spawns from the peritoneal epithelium, resembling malignant ovarian surface epithelial tumor. It is also indistinguishable from metastatic serous ovarian carcinoma at gross, histologic, and immunohistochemical examination.

Nonspecific cross-sectional imaging characteristics include ascites and peritoneal and omental nodules and masses (Fig. 16.13). Nevertheless, a few criteria have been established for the diagnosis of primary peritoneal serous carcinoma:

Besides cancer, an inflammatory or infectious process can also cause peritoneal thickening (Fig. 16.14). Granulomatous peritonitis has been used to describe a spectrum of infection and inflammation in the peritoneum, including tuberculosis, histoplasmosis, and pneumocystic infection; foreign materials such as talc and barium; bowel contents; ruptured ovarian cysts; bile; gallstones; Crohn disease, sarcoidosis, and Whipple disease. Splenic or lymph node calcification along with thickening of the cecum and terminal ileum and low attenuation lymphadenopathy may help in identifying tuberculosis causing peritoneal thickening (Fig. 16.15).

Peritoneal thickening found in a peritoneal dialysis patient could suggest sclerosing encapsulating peritonitis, a rare entity. It is also referred as abdominal cocoon. It is a rare complication observed in long-term dialysis patients and other forms of peritoneal inflammation such as peritoneovenous or ventriculoperitoneal shunts, tuberculosis, and fibrogenic foreign material. The disease manifests in the form of peritoneal thickening with signs of inflammation. Additionally, peritoneal thickening, peritoneal calcifications, loculated fluids, small bowel feces sign, small bowel obstruction, and signs of bowel ischemia have been described [5]. Classically, an abdominal cocoon manifests as a collection of obstructed bowel loops surrounded by a thick fibrocollagenous membrane/sac as seen on CT imaging (Fig. 16.16).

Miscellaneous etiologies such as endometriosis, splenosis, gliomatosis peritonei, and osseous metaplasia can also cause peritoneal masses and thickening.

Splenosis is the ectopic implantation of splenic tissue after disruption of the normal splenic capsule. It can be located anywhere in the peritoneal cavity. Splenosis should be considered when a peritoneal mass or nodule demonstrates similar echogenicity to the spleen on ultrasound and similar attenuation, signal intensity, and enhancement on CT or MRI, respectively (Fig. 16.17). Equivocal lesions can be further investigated with sulfur colloid or technetium 99 m heat-damaged tagged red blood cell scintigraphy. The ectopic splenule will take up the radiotracer.

Another example of peritoneal thickening and mass formation in the peritoneum is from endometriosis. Endometriosis occurs when there is functional endometrial glands and stroma outside the uterus. Not only the pelvis but the remainder of the abdominal peritoneum can be involved. These ectopic endometrial implants occur commonly in women.

The ultrasound and CT findings of endometriosis are nonspecific. They can be solid, cystic, or complex cystic and solid changes as seen on CT imaging. MRI is more specific for the diagnosis of endometriosis, appearing hyperintense on T1-weighted and hypointense on T2- weighted images due to blood products (Fig. 16.18). Other features of MRI findings that have been described include low signal intensity rim on T1- and T2-weighted images due to surrounding fibrosis and hemosiderin macrophages [2].