Computed Tomography (CT) plays an important role in peritoneal lesions detection, assessment of their extent and identification of peculiar aspect of each entity.
Primary tumors
-Peritoneal malignant mesothelioma (PMM): highly aggressive malignancy, usually occurring in older men exposed to higher levels of asbestos. Diffuse PMM shows sheet-like peritoneal thickening or disseminated peritoneal nodules, resulting in tumor irregularly infiltrating the peritoneal surfaces and encasing intraperitoneal organs. It may spread into the retroperitoneum, abdominal wall and pleural cavity, growing through the diaphragm. Additional manifestations are omental cake and ascites (Fig1). Localized PMM is characterized by moderate/large-sized soft-tissue masses, often coalescing, with associated peritoneal studding. Focal masses show heterogeneous density and enhancement –especially in case of cystic degeneration or necrosis-, irregular margins, and may invade adjacent visceral structures; manifestations of diffuse PMM, such as ascites, omental cake, and nodal involvement do not typically occur.
-Multicystic mesothelioma (MM): is a rare, indolent proliferative process, usually occurring in women. CT reveals a multiloculated fluid-filled cystic mass, arising from the pelvic peritoneum, with grapelike pattern, located in the cul-de-sac and along the peritoneal surfaces of the uterus, rectum, and bladder. Cysts may contain a variable number and pattern of septations, from few and incomplete septa to multiple complex and thick septations, until nodular or polypoid growth of internal walls, which significant enhancement (Fig2). Further cysts can coexist away from the dominant cyst in the abdomen.
-Leiomyomatosis peritonealis disseminata (LPD): is a rare, benign lesion, occurring in women with history of uterine leiomyomas, usually incidentally detected. LPD is characterized by numerous smooth muscle nodules throughout the peritoneal cavity. CT shows multiple well-circumscribed solid masses, with lobulated margins, heterogeneous attenuation and late enhancement, similar to uterine leiomyomas, mostly located in the pelvic peritoneum and greater omentum. Ancillary features to confirm the diagnosis are: coexistence of uterine leiomyomas, absence of omental caking, lymphadenopathies or ascites, and signal intensity patterns on MR images (as uterine leiomyomas).
-Desmoplastic small round cell tumor (DSRCT): highly aggressive, rare malignancy of unknown histogenesis, predominantly occurring in young males. DSRCT arises as a solitary peritoneal mass, generally large and bulky, and may spread throughout the peritoneum, appearing as diffuse peritoneal thickening and disseminated nodules. CT reveals heterogeneous masses, with mixed attenuation and central hypodensity, due to intratumoral necrosis or hemorrhage; punctate calcifications are often detected within the masses and ascites is frequently present (Fig3). Metastases to the liver, lung, and bones are common.
-Solitary fibrous tumor (SFT): most commonly arising in the pleura or in the dura and, in very rare cases, in the peritoneal cavity and in the retroperitoneal space. CT reveals a well-defined, smooth, and lobulated soft tissue mass with scattered calcifications. Smaller tumors enhance homogeneously; larger lesions show cystic or necrotic change and intratumoral hemorrhage. Large masses can compress the adjacent organs and deviate abdominal vessels (Fig4). Lymphadenopathies or distant metastases are uncommon.
Secondary tumors
Tumor cells may disseminate into the peritoneum and tend to pool in gravity-dependent recesses and to implant in regions where stasis of peritoneal fluid occurs: the pouch of Douglas, the root of the small bowel mesentery, paracolic gutters, right subphrenic space, retrovescical space, ileocecal region, the hepatorenal fossa and the greater omentum.
-Peritoneal Carcinomatosis (PC): the most frequent peritoneal secondary lesion, primary arising from gastrointestinal carcinomas, ovarian, breast and uterus malignancies. CT shows disseminate peritoneal nodules, solid, cystic or mixed, with different patterns:-plaque-like growth: solid lesions, often confluent, with “sheet-like” aspect; -micronodular growth: 1-5mm implants on the serosa, omentum and mesentery; -nodular growth: >5mm implants, with smooth, lobulated or spiculated margins; -omental caking: diffuse micro-reticolonodular involvement of the omentum, with omental fat replaced by tumor; -mass-like growth: coalescence of nodules in solid masses of bigger size; -malignant ascites, due to obstructed lymphatic vessels (Fig5). PC may infiltrate the mesenteric fat and the perivascular spaces of the small bowel mesentery which becomes rigid, appearing as pleated at CT with denser vessel within (“stellate mesentery”). When the small bowel is diffusely infiltrated, the CT appearance is defined as “teca aspect” and may lead to small bowel obstruction.
-Pseudomixoma peritonei (PP): also known as “jelly belly”, defined as the presence of gelatinous material covering the peritoneal surface, associated with mucinous ascites. According to microscopic classification, PP is divided into two categories: “classic” PP, and Peritoneal Mucinous Carcinomatosis (PMC). CT findings are mucinous deposits in gravity-dependent recesses, with low attenuation, often accompanied by soft-tissue nodules or masses and sporadic curvilinear or amorphous calcifications within. A typical feature is the scalloping of the intraperitoneal organs: indentations of the capsular surfaces of abdominal viscera -mostly liver and spleen- caused by the extrinsic pressure applied from the mucinous and solid implants (Fig6-7).
Vascular pathologies
-Omental Infarction (OI): generally occurs in the greater omentum, due to spontaneous vascular impairment (primary OI), or due to trauma, thrombosis, surgery, and inflammation (secondary OI). The characteristic imaging feature consists in a cake-like focal area of heterogeneous fat stranding, usually larger than 5 cm, with hyperattenuating peripheral halo (Fig8); when torsion is the cause, it may be identifiable at CT through the swirling of the omental vessels (“whirl sign”), consisting in a concentric distribution of fibrous and fatty linear strands converging toward the infarct.
-Epiploic Appendagitis (EA): consists in spontaneous torsion or thrombosis of the venous vessels of the appendage (primary EA), or in the inflammation of the adjacent organs (secondary EA), resulting in ischemia. CT imaging features consist of a fat-density ovoid structure, mass-like, usually <5 cm, with thin high-density rim, also known as “hyperattenuating ring sign”, peripheric fat stranding and focal thickening of the parietal peritoneum (Fig9). The pathognomonic feature is the “central dot sign”, due to the thrombosed vascular pedicle within the inflamed epiploic appendage. Chronically, EA transforms into a fibrotic or calcified mass with popcorn or eggshell-like calcifications and, sometimes, it may detach from the colonic wall, forming an intraperitoneal loose body, called “peritoneal mice”.
Infective pathologies
-Granulomatous Peritonitis (GP): is a peritoneal inflammation, secondary to disseminated infection, irritation from foreign material, or inflammatory diseases. The most common condition is tuberculosis infection, characterized by diffuse nodular thickening of the peritoneal surfaces, due to caseating granulomas; nodules show soft-tissue attenuation on CT images and enhance after contrast medium administration (Fig10). Tuberculous peritonitis is usually classified in three forms: -wet (variable amount of ascites, diffuse or loculated, with increased density, related to the presence of protein and cells); -fibrotic fixed (peritoneal nodules and/or masses adhering to the walls of gastrointestinal tract; small amount of ascites); -dry plastic (less common; fibrotic reaction of the peritoneum, no ascites). The latter two pattern may result in fibrotic fixation of the small bowel. Ancillary features to the diagnosis are: splenomegaly and splenic calcifications, miliary microabscesses in the liver or spleen, lymphadenopathies with peripheral ring enhancement and central hypodensity and calcifications, and involvement of terminal ileum and cecum showing regular and smooth wall thickening.
-Inflammatory Pseudotumor (IP): benign inflammatory lesion, indolent in behavior, with unspecific imaging features, which may occur in the mesentery and in the peritoneum. CT shows a soft-tissue mass, with prominent vascularity, well-defined or infiltrating margins, often localized within the mesentery or adjacent to bowel segments. When a central hypodense area can be identified into the mass, internal necrosis is likely.
Others
-Endometriosis: presence of functional endometrial glands outside the uterus, most commonly in the peritoneal cavity. CT findings are non-specific: endometrioma may appear as a solid, cystic, or mixed mass; long-standing lesions may become fibrotic, causing peritoneal adhesions with obliteration of pelvic recesses and distortion of fallopian tubes and pelvic segments of bowel, most commonly rectosigmoid and distal ileum. MR imaging is crucial to confirm the diagnosis, detecting blood products within the lesions.
-Splenosis: heterotopic implantation of splenic tissue after traumatic or iatrogenic disruption of the splenic capsule; splenic cells may disseminate and proliferate into the peritoneal cavity, resulting in well-circumscribed, round or lobulated masses, hysodense to the liver, enhancing as the normal splenic parenchyma, with heterogeneous pattern in the arterial phase and homogeneous appearance in portal venous phase (Fig11).
