primary and secondary disease of the peritoneum and mesentery: review of anatomy and imaging...

Primary and secondary diseaseof the peritoneum and mesentery: reviewof anatomy and imaging features

Ashish P. Wasnik, Katherine E. Maturen, Ravi K. Kaza, Mahmoud M. Al-Hawary,

Isaac R. Francis

Division of Abdominal Imaging, Department of Radiology, University of Michigan Health System, 1500 East Medical Center

Drive, Ann Arbor, MI 48109, USA

Abstract

The largest and most complex serosal membrane in thebody, the peritoneum, lines the abdominal cavity, andthe abdominopelvic viscera. It is frequently involved in avariety of benign and malignant processes. While sec-ondary involvement of the peritoneum is more common,primary tumors can be a diagnostic challenge. Knowl-edge of the anatomy is crucial in understanding thevarious pathologic processes. Cross-sectional imagingplays an important role in diagnosing and evaluating theextent of the disease processes. This article reviews theimaging anatomy of the peritoneum and mesentery andthe common pathologies involving it.

Key words: Peritoneum—Mesentery—Omentum—Tumors—Computedtomography—Magnetic resonance imaging

The peritoneum is a serosal membrane lining theabdominal cavity with a complex anatomical relation-ship with the intra-abdominal viscera: forming potentialspaces, ligaments, mesenteries, and omentum. Patho-logical processes involving the peritoneum can broadlybe categorized as non-neoplastic (inflammatory, infec-tious, reactive); tumor-like or tumor mimics, and neo-plastic (primary and secondary). While some of theentities can be confidently diagnosed on imaging (e.g.,peritoneal inclusion cysts, splenosis, omental infarction,endometriosis)—many other pathologies may pose adiagnostic challenge (e.g., inflammatory pseudotumor,solitary fibrous tumor, granulomatous peritonitis, peri-toneal carcinomatosis). In general, focal diseases remain

asymptomatic, diffuse disease may present with non-specific features such as abdominal distension, abdomi-nal pain, or weight loss prompting imaging evaluation.Knowledge of etiopathogenesis and imaging features ofthese entities may help providing the correct imagingdiagnosis in the appropriate clinical setting. In this arti-cle, we review the imaging-based anatomy as well as thevarious more common primary and secondary patholo-gies involving the peritoneum and mesentery.

Anatomical considerations

The peritoneum is a large serosal membrane lining theabdominal cavity and abdominal viscera. The purpose ofthe peritoneum is twofold; first to provide a lubricatedsurface (by secreting small amount of sterile fluid thatalso serves as local bacterial defense) over which theviscera can move in a frictionless manner, and secondto serve as a conduit of fluid transport [1, 2]. The clear-ance of the fluid from the peritoneal cavity is via con-tinued cephalic circulation (thought to be due tocontinuous change in intra-abdominal pressure duringrespiration and from the peristalsis of the bowel) to thesubdiaphragmatic space from where the majority of thelymphatic clearance occurs via the subphrenic submeso-thelial lymphatics [2, 3]. This mechanism also predisposescertain locations for tumor deposition such as the hepaticdome (subdiaphragmatic region), paracolic gutters, anddependent portions of the pelvis. These areas should bescrutinized in patients with a known malignancy orunexplained ascites. The peritoneal cavity is a potentialspace between the visceral peritoneum which envelopsthe abdominal viscera (organs), and the parietal perito-neum and lines the abdominal wall (anterior, lateral,posterior abdominal wall, undersurface of diaphragm,anterior surface of the retroperitoneal viscera, and

Correspondence to: Ashish P. Wasnik; email: [email protected]

ª Springer Science+Business Media New York 2014

AbdominalImaging

Abdom Imaging (2014)

DOI: 10.1007/s00261-014-0232-8

Fig. 1. A, B Illustrations reviewing anatomy of the peritoneum.

A. P. Wasnik et al.: Review of anatomy and imaging features

pelvis). The peritoneal cavity is a closed sac in males,while in females it remains open at the ostia of the fal-lopian tube (oviduct), which allows communication be-tween the intraperitoneal and extraperitoneal pelvis [4].The peritoneal ligaments, mesenteries, and omenta areformed by two layers of the peritoneum containing bloodvessels, lymphatics, lymph nodes, and adipose tissuetraversing through. The ligaments and mesentery serve asboundaries as well as conduits for disease processes [4].The peritoneal cavity consists of two major spaces, thegreater sac and the lesser sac (potential space behind thestomach and anterior to pancreas) which communicatevia the epiploic foramen also known as the foramen ofWinslow [5] (Fig. 1A). The small bowel mesentery, adouble peritoneal fold, extends from the duodenojejunalflexure to the ileocecal valve, encasing, and suspendingthe bowel. The greater omentum is the large peritonealfold anterior to the small bowel that hangs from the

stomach and proximal duodenum coursing caudally andthen again ascends to insert into the anterior superioraspect of the transverse colon, forming a four-layeredgastrocolic ligament [6, 7]. The lesser omentum extendsfrom the liver to the lesser curvature of the stomach andproximal duodenum and is formed by hepaticoduodenaland hepaticogastric ligaments and forms the anteriormedial boundary of the lesser sac [4, 8, 9] (Fig. 1B).

The peritoneal cavity can potentially be divided intotwo major compartments by the transverse colon and itsmesentery: supramesocolic and inframesocolic compart-ments [4, 10] (Fig. 1A).

The supramesocolic compartment consists of multiplesubspaces [11]. (a) Subphrenic space with the right andleft subphrenic spaces separated by falciform ligament:the right subphrenic space bounded by diaphragmaticsurface of the right hepatic lobe, right coronary ligamentposteriorly, and falciform ligament medially; and the leftsubphrenic space (subdivided into anterior and posteriorcomponent) which lies between the left hemidiaphragm,anterior wall of the stomach, and superior surface of thespleen posteriorly. (b) Right subhepatic space, as thename suggests, is located inferior to the right hepatic lobeand bounded anteriorly by the transverse colon andposteriorly by the parietal peritoneum overlying the rightkidney. The right paracolic gutter is a space lateral toascending colon and serves as a conduit between rightsubphrenic and subhepatic spaces [4]. (c) Left perihepaticspace, has anterior and posterior subspaces which com-municates with the left subphrenic space. The falciformligament and diaphragm forms the medial and anteriorboundary of the left perihepatic space, respectively, andthe left hepatic lobe forms the posteromedial boundary[4]. The phrenicocolic ligament (between left hemidia-phragm and left transverse mesocolon) separates the left

Fig. 2. A 65-year-old man with mesenteric panniculitis. AGray scale ultrasound of the mid-abdomen shows an ill-de-fined echogenic area (arrow) and B corresponding misty(hazy) mesenteric fat with tiny nodes on axial contrast-en-hanced CT (CECT).

Fig. 3. A 52-year-old man with sclerosing mesenteritis. AxialCECT shows a lobulated soft tissue mass (arrow) in the smallbowel mesentery with tiny calcific foci (arrowhead), confirmedon image guided biopsy as chronic inflammation.


perihepatic (supramesocolic) space from left paracolicgutter [12].

The inframesocolic compartment consists of a largerleft infracolic space and a small right infracolonic spacesurrounded inferiorly by the small bowel mesentery.

Classification of peritoneal disease

Given the complexity of the differential diagnosis forperitoneal disease, an organized approach may behelpful to evaluate the diagnostic possibilities. The

pathologic process involving the peritoneum can bebroadly categorized as non-neoplastic (inflammatory,infectious, reactive); tumor-like or tumor mimics, andneoplastic (primary and secondary). Table 1 presents aschematic.

Non-neoplastic

Sclerosing mesenteritis

It is an idiopathic disorder of the mesentery charac-terized by chronic non-specific inflammation and man-ifesting as tumor-like masses. An association withIgG4-related sclerosing disease has been recently de-scribed [13]. Sclerosing mesenteritis can be subcatego-rized based on the histologic features and predominanttissue type, with mesenteric panniculitis demonstratingfatty proliferation being the milder form of chronicinflammation, and more chronic retractile mesenteritishaving a dominant component of soft tissue andfibrosis [14, 15].

Ultrasound may demonstrate focal area of increasedechogenicity with posterior acoustic shadowing(Fig. 2A). Mesenteric panniculitis involves the smallbowel mesentery and is seen as an encapsulated area offat with haziness with multiple non-enlarged lymphnodes on CT (Fig. 2B). Retractile/sclerosing mesenteritismanifests as an ill-defined or well-defined mesenteric softtissue attenuation mass (rarely low attenuation areasfrom necrosis) with scattered internal calcifications(Fig. 3). There may be encasement of mesenteric vesselswith small lucent halos of non-infiltrated fat—‘‘fat ringsign’’ [15, 16]. The mesenteric mass may cause retractionand shortening of the small bowel mesentery and maypresent with partial small bowel obstruction [14]. OnMRI, sclerosing mesenteritis is seen as low to interme-diate signal intensity on T1-weighted images, and vari-able signal intensity on T2-weighted images relative tothe adjacent normal mesenteric fat. The appearance isdependent on the degree of inflammation and myxoma-tous change [17].

Pearl: Presence of ‘‘fat ring sign’’ may help in dif-ferentiating from lymphoma or carcinoid. Somatostatin-receptor scintigraphy (OctreoScan) can be used to dis-tinguish between the two with uptake seen in carcinoid[14].

Omental infarction

Omental infarction is a rare cause of acute right lowerquadrant abdominal pain, which at times is indistin-guishable from acute appendicitis or cholecystitis.Omental infarction occurs due to vascular compromisewhich can be due to torsion, abdominal trauma, or priorsurgery. A longer and mobile omentum on the right sidemay be attributed to right-sided predilection [18].

Fig. 4. A 48-year-old man with omental infarction. Contrast-enhanced CT performed for acute RUQ pain shows a focalarea of increased omental fat stranding, vascularity andwhorled appearance (arrow).

Fig. 5. A 50-year-old man with biopsy-proven inflammatorypseudotumor. CT shows a heterogenous mass in the rightlower abdomen (arrow) and an additional soft tissue nodules(thick arrow) in the right posterior pararenal space.


On CT, omental infarct is seen as ill-defined or well-defined focal area with fat density adjacent to thetransverse or ascending colon in the right abdomen withhyperdense peripheral halo. A whorled appearance fromconcentric linear strands may be evident when the etiol-ogy is torsion [18, 19] (Fig. 4). Pearl: Self-limiting pro-cess and mostly managed conservatively. The differentialdiagnosis is acute epiploic appendagitis that is usuallysmaller in size than omental infarct and is seen as ovoidfat density lesion abutting the adjacent colon (often leftsided and involves the sigmoid colon), surrounded by ahyperdense rim and a central hyperdense dot (throm-bosed vascular pedicle) [20].

Inflammatory pseudotumor

It is a rare benign inflammatory process predominantlyseen in children and young adults, histologically consistingof lymphocytes, plasma cell, fibroblastic spindle cells, and

collagen. The suggested causative factors range fromtrauma or post surgical inflammation to immunemediatedprocess (including IgG4-related sclerosing disease) [21]. CTdemonstrates awell-circumscribedor infiltrative ill-definedmass with tiny calcification demonstrating variable post-contrast enhancement [14] (Fig. 5). Treatment usuallyconsists of surgical excision as percutaneous biopsy is oftennot helpful [22]. Pearl: This is a diagnosis of exclusion.

Lymphangioma

Cystic lymphangioma is a benign proliferation of lym-phatic channels. Histologically, these are thin-walledintercommunicating cysts containing chylous, protein-aceous, serous, or hemorrhagic fluid [23]. Although com-monly seen in children and young adults in the neck oraxilla (referred to as cystic hygroma), it can be seen in themesentery [24]. Ultrasound demonstrates anechoic tohypoechoic multiloculated or multiseptated cystic masses,

Fig. 6. A 44-year-old man with abdominal lymphangioma. ATransverse mid-abdominal US with color doppler and B axialCECT show a large complex multiseptated/multiloculatedcystic mass in the mid-abdomen (arrows) with some

enhancing septae (arrowhead). C Coronal T2WI and Dcoronal post-contrast T1-WI show numerous cystic foci withthin enhancing septae. Surgical resection confirmed benignlymphangioma.


sometimes with calcifications [23] (Fig. 6A). CT demon-strates cystic masses which can be relatively simple or mayhave internal debris or hemorrhage (Fig. 6B). On MRI,while lymphangiomas typically demonstrate high signalintensity onT2-weighted images heterogeneitymay be seenwith internal layering debris or hemorrhage (Fig. 6C, D).Pearl: Mostly asymptomatic, usually manifesting whenvery large in size orwith underlying hemorrhage or torsion.

Granulomatous peritonitis

This includes a range of inflammatory and infectiousprocesses often encountered in immunosuppressed pa-tients, with the common infectious agents being tuber-culosis, histoplasmosis, or pneumocystis. It may alsooccur as a reactive process secondary to foreign materi-als, ruptured ovarian cyst, or dropped gallstones [2]. Attimes imaging features may resemble peritoneal carci-nomatosis, making definitive diagnosis a challenge [25].

Tuberculous peritonitis is an extrapulmonary infec-tion, commonly a result of hematogenous spread fromactive or miliary pulmonary tuberculosis. In the absenceof clinical features of pulmonary tuberculosis, the diag-nosis can be difficult. On imaging, tuberculous peritonitismay manifest as ‘‘wet type’’ presenting with significantdiffuse or loculated ascites and or ‘‘dry plastic type’’lacking ascites [26]. Enlarged abdominal lymph nodes,ileocecal thickening, peritoneal, or omental nodules ormasses with fibrotic scarring are noted (Fig. 7A, B).Laparoscopy and biopsy may be needed to confirm thediagnosis in some cases. Similar findings are seen withhistoplasmosis and pneumocystis and remain indistin-guishable by imaging [27]. Pearl: Presence of abdominaland retroperitoneal lymph nodes enlargement with cen-tral areas of necrosis and hepatic or splenic abnormalities

Fig. 7. A 48-year-old woman with tuberculosis. A AxialHRCT of the chest shows numerous tiny nodules (arrows)consistent with miliary TB. B Axial CECT through the lowerabdomen shows diffuse peritoneal and omental thickening(thick arrow), mesenteric edema/stranding (arrows), and mildascites (arrowhead) consistent with TB peritonitis.

Fig. 8. A 60-year-old man with sclerosing peritonitis relatedto long-term peritoneal dialysis. A Abdominal radiographshows curvilinear calcifications in the right upper abdomen(arrows), as confirmed on NECT (B) lining the visceral peri-toneum. Also note failed calcified right lower quadrant renaltransplant (A) (arrowhead).


(enlargement, focal lesions, or calcifications) may behelpful in differentiating from peritoneal carcinomatosis[25].

Sclerosing encapsulating peritonitis (also knownas Sclerosing peritonitis, abdominal cocoonsyndrome)

It is a chronic inflammatory reaction of the peritoneumcommonly seen in patients on long-term continuousambulatory peritoneal dialysis (CAPD) [28]. It can alsobe seen in patients with ventriculoperitoneal shunts,

tuberculosis, and as reactive process to foreign materialin peritoneal cavity [2]. On CT, there is ascites with dif-fuse thickening of the peritoneum which shows post-contrast enhancement. The thickened peritoneum mayencase the small bowel and lead to obstruction [29].Widespread peritoneal calcification, easily detected onplain abdominal radiograph, is a feature of advancedsclerosing peritonitis (Fig. 8). Pearl: In patients withCAPD, loss of ultrafiltration capacity during peritonealdialysis should raise a suspicion for sclerosing peritonitis.Recognition by imaging can prompt cessation of CAPDand avoid progression.

Fig. 9. A 55-year-old man with splenosis (prior splenectomyfor splenic injury). A, B Axial unenhanced CT shows multiplesoft tissue nodules (arrows) in the left upper quadrant (sple-

nectomy bed) and in the anterior peritoneum (arrowheads).Damaged RBC-tagged SPECT–CT (C, D) shows intenseradiotracer uptake in these nodules.


Tumor mimics

Splenosis

Splenosis is intraperitoneal dissemination of splenic tis-sue after trauma or surgery secondary to disruption ofthe splenic capsule [30]. By contrast, an accessorysplenule is a congenital incorrect migration of primitivesplenic tissue, often seen adjacent to intact spleen. Ineither case, splenic implants function as normal splenictissue and may proliferate depending on the response tostressors, and may remain of clinical significance inpatients who undergo splenectomy for hematopoietic

disorders [2]. While splenosis commonly occurs in theperitoneal cavity, it can also be seen in the thorax alongthe diaphragmatic surface.

On imaging, splenosis is seen as one or more well-defined round or lobulated soft tissue masses demon-strating similar attenuation pattern as that of the nativespleen or expected attenuation of the spleen if there issplenectomy. Tc-99m sulfur colloid nuclear scintigraphyshows increased uptake in splenic tissue but there areoccasional false negatives and in such instance, techne-tium (Tc)99m-tagged heat-damaged red blood cell scanscan be used and are considered the most sensitive test toconfirm the diagnosis of accessory splenic tissue [31](Fig. 9). Pearl: Isoattenuating to spleen on imaging.Nuclear scan may be used in indeterminate cases.

Endometriosis

Endometriosis is ectopic implantation of functioningendometrial tissue outside of the uterus. Typically,endometrial implants are noted within the pelvic organs(ovaries), although the visceral and parietal peritoneum

Fig. 10. A 46-year-old woman with confirmed endometriosison laparoscopy, presenting with pelvic pain and altered bowelhabits. A Barium enema with coned-down view of the sigmoidcolon shows mild narrowing (arrowhead), corresponding toserosal implants (arrow) seen on axial CECT through thepelvis (B).

Fig. 11. A 43-year-old woman with gliomatosis peritoneifollowing resection of left ovarian mature teratoma. A, B AxialCECT through the abdomen show mass-like thickening of theperitoneum (arrows) and complex intraperitoneal collectionscausing scalloping of liver margin (arrowheads).


may be involved. Endometriosis is commonly seen in thechildbearing years. Patients may either be asymptomaticor may present with pelvic pain, dysmenorrhea, dysfunc-tional uterine bleeding, and/or dyspareunia. In patientswith long-standing endometriosis, the endometrioticimplant may undergo fibrotic change and scarring withadhesions of adjoining bowel loops that can lead to bowelobstruction.

CT shows nodular or spiculated soft tissue implantson the serosal surface of the bowel, but is non-specific tothe etiology (Fig. 10). MRI remains the imagingmodality of choice for evaluating endometriosis becauseof its ability to detect blood products. Focal endome-triomas typically exhibit high signal intensity on pre-contrast T1-weighted images and low signal intensity on

T2-weighted images (described as T2 shading, secondaryto chronic degraded blood products like deoxyhemo-globin and methemoglobin). However, these featuresmay not be seen with small peritoneal deposits, as theydo not accumulate substantial blood products [2, 32].Fibrotic components demonstrate low signal intensity onT1 and T2-weighted images and may demonstrate de-layed post-contrast enhancement [33]. Laparoscopy maybe required in indeterminate cases. Pearl: Presence ofmultifocal T1 hyperintense deposits and T2-shading if anassociated endometrioma is present. Imaging diagnosis ispotentially very important, as endometriosis is hormon-ally sensitive and can be successfully managed non-sur-gically in some patients.

Gliomatosis peritonei

It is a rare condition characterized by peritonealimplantation of mature glial tissue. The theories postu-lated for implantation of the neuroglial tissue includerupture of immature ovarian teratoma, migration ofneuroglial tissue with ventriculoperitoneal shunt cathe-ters, and inherent metaplastic change in the peritonealtissue [34–36]. Presence of mature glial tissue on histol-ogy suggests the benign nature of the tissue with afavorable outcome; although occasional case reports ofmalignant transformation have been described [37].

Imaging features of gliomatosis peritonei vary fromsoft tissue thickening, masses, and nodules scatteredthroughout the peritoneum with associated omentalcaking and ascites (Fig. 11). On MRI, the glial tissuedeposits are seen as lobulated moderately high signalintensity foci on T2-weighted images without post-con-trast enhancement [38]. Pearl: Presence of an ovarianteratoma or past history of teratoma resection, may helpin suggesting a diagnosis. It is difficult to differentiatebetween benign and malignant forms.

Fig. 12. A 38-year-old woman with peritoneal inclusion cyst.A Transvaginal pelvic ultrasound with color Doppler shows amultiloculated avascular cyst (arrows); the ovary was adja-cent and separate from the cystic structure. B Axial CECTshows a corresponding lobulated fluid attenuation structure(arrow) in the pelvis. Pelvic aspiration revealed simple serousfluid. UT, uterus.

Fig. 13. A 62-year-old man with peritoneal mesothelioma.CT showed omental nodules (arrowheads) in a patient with noknown primary cancer; biopsy confirmed mesothelioma.


Peritoneal inclusion cysts (also known as benigncystic mesothelioma)

These are seen in premenopausal women in the setting ofprior pelvic surgery, pelvic inflammatory disease, orpelvic adhesions and represent non-neoplastic reactivemesothelial proliferation causing impaired absorption ofthe peritoneal fluid leading to the formation of multi-locular fluid-filled cysts usually seen encasing the ovaries[39].

On ultrasound the peritoneal inclusion cyst is ananechoic cystic mass with multiple septation/loculationswith posterior acoustic enhancement, often seen trappingthe ovary or interdigitating with pelvic sidewall vessels(Fig. 12A). Color Doppler evaluation may show low

Fig. 14. A 49-year-old woman with primary peritoneal ser-ous carcinoma. A–C Axial CECT images show abdominaldistention due to large volume ascites, with peritoneal softtissue nodules (arrow in A) and omental thickening (arrow-heads in B). The ovaries were unremarkable (C). UT, uterus;LO, left ovary. Omental biopsy confirmed primary peritonealserous carcinoma.

Fig. 15. A 48-year-old woman with familial adenomatosispolyposis syndrome and mesenteric desmoid. A, B AxialCECT through the mid-abdomen shows a soft tissue heter-ogenous mass in the small bowel mesentery (arrow). Alsonote small bowel polyp (arrowhead in A) and surgicalabsence of colon.


resistance flow within the septation [39]. CT and MRIdemonstrate multiloculated cystic mass with simple fluidattenuation without evidence of solid components orenhancement (Fig. 12B). Sometimes hemorrhage mayoccur within this cyst which can appear as areas of highdensity on CT and slightly heterogeneous signal intensityon T1- and T2-weighted images [40]. Pearl: Fluid filledmasses which conform to the shape of adjacent structuresand invaginate between them, rather than displacingthem.

Neoplasms

Primary peritoneal neoplasm

Primary peritoneal tumors are much less common thansecondary peritoneal neoplasms. Primary peritoneal tu-mors can be classified histologically based on the celllineage of origin into a) mesothelial cell origin (meso-thelioma-focal or diffuse), b) epithelial cell origin (peri-toneal serous neoplasm), c) smooth muscle origin(leiomyomatosis peritonealis disseminate), and d) tumorsof uncertain origin (desmoplastic round cell tumor orsolitary fibrous tumor) [1].

While the majority of malignant mesotheliomas arisein the pleura, peritoneal primary mesothelioma accountsfor approximately 6%–10% [41]. Asbestos exposure hasan established association with malignant mesothelioma.On CT, diffuse peritoneal malignant mesothelioma canbe seen as sheet-like nodular irregular thickening orenhancing soft tissue masses and is usually associatedwith ascites [42] (Fig. 13). Malignant mesothelioma mayinfiltrate the small bowel mesentery causing adhesionsand bowel obstruction. Focal or localized peritonealmesothelioma usually presents as large heterogeneoussolid intraperitoneal mass with localized or small-volume

ascites [1]. Pearl: Lymph node enlargement and calcifi-cation in primary diffuse peritoneal mesothelioma is rare(unlike pleural mesothelioma) and its presence shouldraise the possibility of secondary peritoneal disease [1,43].

Similar imaging findings are noted with primary per-itoneal serous carcinoma (epithelial tumors), almost al-ways seen in women, except that calcifications may beseen in the up to 30% of cases representing psammo-matous calcification [44] (Fig. 14). Pearl: Absence of anovarian mass is important for excluding metastaticpapillary serous ovarian carcinoma. However, surgicaland chemotherapeutic regimens are similar.

Desmoid

Also known as intra-abdominal fibromatosis, desmoid is alocally aggressive fibrous tumor with the ability toinfiltrate or recur but not metastasize [14, 45]. Based onthe location, they are termed mesenteric, retroperitoneal,pelvic, or abdominal wall fibromatosis. The small bowelmesentery is the most common site for intra-abdominalfibromatosis. While most cases are sporadic, associationwith familial adenomatosis polyposis has been describedin 13% of patients with mesenteric fibromatosis, withprior surgery being a risk factor [46]. Histologicallydesmoids are composed of spindle-shaped fibroblastswith scant cytoplasm, although in some tumors themyxoid component predominates [14]. CT shows a softtissue mass with variable attenuation and enhancement,and can demonstrate heterogeneous low attenuationareas due to necrosis (Fig. 15). On MRI, desmoids usu-ally have low or intermediate signal on T1-WI, inter-mediate to high signal on T2-WI, and heterogenous post-contrast enhancement. Recurrent or aggressive desmoidstend to have higher T2 signal intensity probably due toincreased cellularity and myxoid components. Pearl:Aggressive or recurrent tumor have higher T2 signalintensity of MRI.

Fibrous tumor

Solitary fibrous tumor of the peritoneum (also known asextrapleural solitary fibrous tumor) is a rare neoplasm ofsubmesothelial origin. Although solitary fibrous tumor iscommonly seen arising from the pleura, it can beencountered in an extrapleural location within themesentery, peritoneum, and pericardium [47]. Solitaryfibrous tumor of the peritoneum has been shown to havea slight male predilection. Histologically, they are com-posed of spindle-shaped fibroblasts with mixed collagencomposition and surrounded by an intact layer ofmesothelium [48]. Imaging features are similar to otherfibrous tumors, although solitary fibrous tumors tend tobe more well-defined than desmoids and are sometimesstrikingly hypervascular (Fig. 16). Some tumor may

Fig. 16. A 55-year-old man with mesenteric solitary fibroustumor. Axial portal venous phase CT shows a well-defined,lobulated, and briskly enhancing mass (arrow) centered in thesmall bowel mesentery.


show aggressive behavior with infiltration into adjoiningstructures [14]. Pearl: These tumors should potentially beincluded in the imaging differential whenever an intra-peritoneal sarcoma is considered.

Desmoplastic small round cell tumor (DSRCT)

This is a rare malignancy with very poor prognosis,commonly seen in adolescents and young adults. Thetumor is characterized histologically by the presence of‘‘small round blue cells’’ and lies in the gamut of Wilm’stumor, Ewing sarcoma, peripheral primitive neuroecto-dermal tumor, and Askin tumor [1]. Imaging featuresinclude diffuse peritoneal thickening, nodules, and mas-ses which demonstrate tiny calcification, heterogeneous

attenuation and post-contrast enhancement with areas ofcentral necrosis or even internal hemorrhage and tinypunctate calcifications [49] (Fig. 17). Ascites is usuallypresent. Occasionally, a solitary peritoneal mass may beseen as an initial presentation. MRI demonstrates lowsignal intensity on T1WI and high signal intensity withvarying degree of heterogeneity on T2WI [50]. The tumoris known to metastasize via hematogenous and lymphaticroutes. Pearl: Should be considered in young adults withno apparent organ based primary site.

Secondary peritoneal carcinomatosis

Spread of malignancy into the peritoneum from hema-togenous or direct peritoneal spread and seeding results

Fig. 17. A 21-year-old man with peritoneal round cell tumor.Axial CECT through lower abdomen (A) and pelvis (B), showmultifocal heterogeneous soft tissue masses in the abdomen

and pelvis (arrows). C 18-FDG- PET shows avid FDG uptakewithin peritoneal implants (arrow) and also in the right sub-phrenic space (arrowhead).


in secondary peritoneal carcinomatosis, commonlyencountered with gastrointestinal tract, ovarian, breast,and lung malignancies. Unexplained diffuse peritonealthickening and ascites may commonly be due to sec-ondary carcinomatosis from an occult or known primaryneoplasm [51, 52].

Contrast-enhanced CT remains the primary modalityfor preoperative staging of the peritoneal metastases,with sensitivities and specificities ranging from 25% to100% and 78% to 100%, respectively; however, CT sen-sitivity is significantly reduced for tumor deposits lessthan 5 mm [53–55]. CT may demonstrate soft tissuenodularity or sheet-like masses involving the peritoneum[2] (Fig. 18). Infiltration of the tumor into small bowelmesentery may result in small bowel obstruction. MRIwith diffusion-weighted imaging has shown promisingresults for the detection of peritoneal carcinomatosiswith high sensitivity and specificity of 90%–95.5% [56,57]. Fat-suppressed post-gadolinium contrast helps indetecting smaller peritoneal deposits given their charac-teristic higher enhancement relative to normal liver[58]. Combined 18-fluorodeoxyglucose positron emission

tomography CT (18-FDG PET CT) has shown sensi-tivity ranging from 78% to 100% for tumor implants,especially in patients with clinically suspected recurrenttumor [59]. Pearl: Attention to areas of fluid stasis(paracolic gutters, subdiaphragmatic, subhepatic spaces,and pelvic cul-de sac) is important.

Pseudomyxoma peritonei

It is a clinical entity characterized by large amounts ofthick mucinous or gelatinous ascites typically arisingfrom perforation or rupture of low-grade mucinousneoplasm of the appendix (and thought by some to bedue to ovarian mucinous tumors) [2, 60]. This entity isthought to be different from the peritoneal mucinouscarcinomatosis which is secondary to invasive high-grademucinous carcinoma commonly arising from the GItract, gallbladder, pancreas, or ovary.

Fig. 18. A 55-year-old woman with peritoneal carcinomato-sis from ovarian cancer. Coronal reformat CECT shows alarge heterogenous mass arising from the right adnexa/ovary(OM), with linear peritoneal thickening (arrowhead) and mildascites. Also noted is a mesenteric calcified mass (arrow)which had been present on an older CT prior to developmentof ovarian malignancy, most likely sclerosing mesenteritis. Fig. 19. A 59-year-old man with pseudomyxoma peritonei

from ruptured appendiceal mucinous cystadenoma. A, BAxial CECT images illustrate scalloping of liver margin bygelatinous mucin (arrows A), which also extends into in inci-sional hernia (arrowhead B).


CT shows multifocal areas of low attenuation causingscalloping and indentation of the solid upper abdominalviscera, typically along the hepatic and splenic surface,which may have curvilinear foci of calcification. How-ever, no direct infiltration of the parenchymal or visceralorgans is noted (Fig. 19). These findings warrant carefulevaluation for any primary appendiceal tumor or ovariantumors. Pearl: While imaging differentiation betweenpseudomyxoma peritonei and secondary mucinous car-cinomatosis can be very difficult, the presence of pleuralmasses or pleural effusion, omental caking and infiltra-

Fig. 20. A 49-year-old man with peritoneal lymphomatosis.A Axial and B coronal CECT show an ill-defined soft tissuemesenteric mass in the right abdomen (arrow) with peritonealthickening (arrowhead).

Fig. 21. A 89-year-old man with metastatic carcinoid tumor.Axial CECT through the upper (A) and mid-abdomen (B)shows a right lower quadrant heterogenous avidly enhancingmass (arrowhead in B), with a tiny hypervascular lesion inright hepatic lobe, possibly a metastasis (arrow in A). Thepatient had an elevated serum 5-HIAA level. C In111 octreo-tide nuclear scintigraphy shows intense focal radiotracer up-take in the mesenteric mass.


tion of the abdominal viscera should favor the latterdiagnosis [61, 62].

Lymphomatosis

Lymphomatous involvement of the peritoneum can beseen as a primary or secondary process. Primary lym-phoma of the peritoneum is predominantly seen inimmunocompromised patients, mostly HIV, and hasbeen attributed to Human Herpes Virus (HHV 8) andEpstein–Barr Virus (EBV). Primary lymphoma of theperitoneum is usually confined to the peritoneal cavity,and has a poor prognosis relative to secondary peritonealinvolvement. Secondary involvement of the peritoneumcan be seen with nodal or extranodal non-Hodgkin’slymphoma, or Burkitt lymphoma and may arise inimmunocompetent patients [2].

Imaging features of secondary peritoneal lympho-matosis resemble peritoneal carcinomatosis, with diffusemass-like thickening of the peritoneal surface, multifocaldiscrete nodules, infiltration of the small bowel mesen-tery, and omental caking [2] (Fig. 20). However, thepresence of significant abdominal and retrocrural lymphnode enlargement and splenomegaly may favor second-ary lymphomatosis. The presence of predominant largevolume ascites without significant lymph node enlarge-ment in an HIV-positive patient favors primary lym-phomatosis [63]. Pearl: Should be included in theimaging differential list of unidentified cause of perito-neal thickening, ascites, and hepatosplenic lesions.

Carcinoid tumor

Carcinoid tumor of the mesentery occurs secondary todirect or lymphomatous spread of gastrointestinal car-cinoid tumors. Carcinoid tumors arise from the neuro-endocrine cells of the intestine, most commonly involvingthe distal ileum with approximately 40%–80% involvingthe mesentery [64]. Many patients may be asymptomaticin the absence of liver metastasis, with hepatic metastasis

usually manifest with carcinoid syndrome due to releaseof vasoactive amines (serotonin, 5-hydroxytryptophan)into the circulation—clinically presenting as flushing anddiarrhea, episodes of wheezing, and heart failure [65].

While these are intensely arterially enhancing masses,heterogeneous attenuation may be seen depending ontumor size and presence of internal degeneration(Fig. 21A, B). Calcifications are seen in approximately70% of mesenteric masses [66]. Radiating strands of softtissue into adjoining mesenteric fat, could either repre-sent tumor infiltration or desmoplastic reaction and orfibrotic proliferation. There is often encasement of mes-enteric vessels. Carcinoid tumors typically cause adher-ence and infiltration of the adjoining small bowel loopswhich can demonstrate wall thickening. Often the pri-mary small bowel neoplasm itself is very small and maynot be discretely identified, on CT or MRI. In this situ-ation, 111In-labeled somatostatin analog (pentetreotide),nuclear scintigraphy has been shown to have high sen-sitively and specificity in identifying carcinoid tumors[67] (Fig. 21C).

Pearl: Biochemical markers serum/urine 5-hydrox-yindoleacteic acid (5-HIAA) and serum chromogranin Aare elevated in functioning carcinoid tumors and also pro-vide the basis for functional imaging with octreotide scan.

Conclusion

Primary and secondary diseases of the peritoneum andmesentery include a diverse group of pathologies andwhile secondary peritoneal disease remains much morecommon than primary neoplasm, differentiating primaryfrom secondary diseases remains important in patientmanagement. Although peritoneal diseases demonstratemany overlapping imaging features, some features maybe helpful in differentiating primary from secondaryinvolvement. While the clinical history, laboratoryparameters and imaging features may aid in narrowingthe diagnostic considerations, tissue sampling will berequired in most instances for confirmation and appro-priate management.

Table 1. Classification of peritoneal diseases

Non-neoplastic Tumor mimics/tumor-like Neoplastic

Primary:Sclerosing mesenteritisOmental infarctionInflammatory pseudotumorLymphangioma

SplenosisEndometriosisGliomatosis peritoneiPeritoneal inclusioncyst (some consider it as benign primaryneoplasm ‘‘benign cystic mesothelioma’’)

Primary:Primary peritoneal mesotheliomaPrimary peritoneal serous tumorDesmoid tumorSolitary fibrous tumorDesmoplastic (round cell) tumor

Secondary:Granulomatous peritonitisSclerosing encapsulating peritonitis

Secondary:Peritoneal carcinomatosisPseudomyxoma peritoniiLymphomatosisCarcinoid


Acknowledgments. The authors acknowledge and thank Sarah Abateand Vanessa Allen, Radiology Media Division-University of MichiganHealth System, for help with illustrations and image annotations.

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