Peritoneal disease: key imaging findings that help in the differential diagnosis

Abstract

The peritoneum is a unique serosal membrane, which can be the site of primary tumors and, more commonly, secondary pathologic processes. Peritoneal carcinomatosis is the most common malignant condition to affect the peritoneal cavity, and the radiologist plays an important role in making the diagnosis and assessing the extent of disease, especially in sites that may hinder surgery. In this review, we address the role of the radiologist in the setting of peritoneal pathology, focusing on peritoneal carcinomatosis as this is the predominant malignant process, followed by revising typical imaging findings that can guide the differential diagnosis.

We review the most frequent primary and secondary peritoneal tumor and tumor-like lesions, proposing a systemic approach based on clinical history and morphological appearance, namely distinguishing predominantly cystic from solid lesions, both solitary and multiple.

Introduction

The peritoneum is a unique serosal membrane, which can be the site of primary tumors and, more commonly, secondary pathologic conditions. The peritoneum has a complex anatomy, which dictates the distribution and flow of fluid within the abdomen and pelvis and consequently the anatomic location and distribution of pathology. When evaluating peritoneal masses, metastatic disease should be accounted as the most common neoplastic process that involves the peritoneal cavity. However, the peritoneum is frequently the site of secondary benign processes and other tumor-like lesions, or more rarely of primary peritoneal tumors, and therefore the radiologist should be aware of their main imaging features, thus allowing a discussion of the most likely differential diagnoses. The purpose of this article is to review the spectrum of peritoneal pathologic conditions, focusing on imaging characteristics and possible differential diagnoses.

Peritoneal anatomy

The peritoneum is a membrane which consists of a single layer of simple low-cuboidal epithelium called a mesothelium. The peritoneum that lines the abdominal wall is the parietal peritoneum, whereas the peritoneum that covers the organs is the visceral peritoneum. The peritoneal cavity is a potential space between the parietal peritoneum and the visceral peritoneum.¹ The peritoneal ligaments and mesenteries are double folds of peritoneum that suspend and support the intraperitoneal organs and divide the peritoneal cavity into interconnected compartments. The peritoneal cavity normally contains only a small amount of sterile fluid similar to plasma, and the patterns of circulation and clearance of this fluid from the peritoneum is one of the major factors that dictate the location of disease.¹ The peritoneal fluid usually circulates upward to the subdiaphragmatic spaces where the subphrenic submesothelial lymphatics provide most of the lymphatic clearance from the peritoneal cavity. The right subphrenic submesothelial lymphatics provide most of the lymphatic clearance from the peritoneal cavity, making this a common site of disease.^2,3 The upward movement of peritoneal fluid to reach the undersurface of the hemidiaphragm is due to the fluctuations in intra-abdominal pressure during respiration and the intestinal peristalsis. In pathologic conditions that result in ascites, relative stasis of fluid at specific sites promotes seeding of malignant cells or development of infection at these locations: the right paracolic gutter; the ileocolic region; the root of the sigmoid mesocolon; and the peritoneal recesses of the pelvis^2,3 (Figure 1).

Figure 1.

CT of the abdomen in a patient with voluminous ascitis. The phrenicocolic ligament (arrow in a and c) partially separates the left paracolic gutter from the left subdiaphragmatic space, limiting the spread of peritoneal disease in the left upper quadrant. The root of the small bowel mesentery (arrowhead in b) divides the inframesocolic compartment into the right and left infracolic spaces. The predominant sites of ascitic fluid accumulation (* in c) are prone to seeding of malignant cells or development of infection.

Imaging in peritoneal disease

On CT, the normal peritoneum can occasionally be recognized as a very fine uniform linear structure, although usually it is not visible. Thus, visualization of peritoneal lining on CT scans should raise the suspicion of peritoneal thickening. Benign conditions such as infectious or inflammatory peritonitis commonly cause smooth peritoneal thickening, compared to more irregular, nodular and often discontinuous peritoneal thickening in malignancy.

Although CT is still the imaging modality most used for evaluating peritoneal pathology, MRI and positron-emission tomography (PET)-CT have some advantages over the former technique. Therefore, at present, MRI could be regarded as the primary imaging tool in the evaluation of patients with peritoneal disease, especially suspected carcinomatosis, providing the highest sensitivity for the detection of peritoneal tumors. It is, however, less available, with many centers using it only for selected patients, particularly those with indeterminate or equivocal involvement of mesentery and small bowel on CT.⁴

The main advantage of PET-CT is the fact that it is a whole body imaging technique that can detect distant metastases elsewhere. Its major limitations include radiation exposure, higher cost, limited depiction of small tumors (current spatial resolution: 4 mm) and lower detection rate of mucinous carcinomas by ¹⁸F-FDG-PET.^5,6

The role of ultrasound imaging is limited in this setting. However, this imaging modality is often the first used when peritoneal disease is discovered incidentally, and it remains one of the diagnostic techniques for image-guided biopsy to obtain a histological diagnosis.⁷

Peritoneal pathology

The peritoneum is the site of both neoplastic and non-neoplastic pathology. The most frequent involvement occurs secondarily, either in an acute setting (surgery complications, bowel perforation) or more chronic conditions (carcinomatosis, ascites secondary to portal hypertension). Nonetheless, it is important to be aware that although uncommon, the peritoneum can be the primary site of pathology, namely tumor and tumor-like lesions.

Peritoneal pathology is a heterogeneous group of conditions characterized by intraperitoneal spread, and its classification is often challenging (Table 1). A frequent classification of peritoneal disease distinguishes primary from metastatic/secondary pathological conditions.^3,4,8 Another useful approach to guide the reasoning about differential diagnosis is to separate lesions according to their predominant imaging characteristics (Figure 2). Even if the border between peritoneal and mesenteric conditions (which are beyond the scope of this review) may be indistinct and their differentiation challenging on imaging alone – furthermore, some diseases may involve these two compartments simultaneously – a condition that causes peritoneal thickening and ascites has a higher likelihood of being of primary peritoneal origin.

Table 1.

Classification of primary and secondary peritoneal pathologies

Malignant lesions	Benign tumor/tumor-like lesions	Infectious/inflammatory processes	Iatrogenic/traumatic
Primary Malignant peritoneal mesothelioma Primary peritoneal serous carcinoma Desmoplastic small round cell tumor Solitary fibrous tumor Secondary Peritoneal carcinomatosis Pseudomyxoma peritonei Lymphomatosis	Multicystic mesothelioma Well-differentiated papillary mesothelioma Adenomatoid tumor Primary peritoneal serous borderline tumor Leiomyomatosis peritonealis disseminata Lymphangioma Enteric duplication cyst Endometriosis Gliomatosis peritonei Splenosis	Peritonitis Hemoperitoneum Peritoneal abscess Granulomatous peritonitis Sclerosing encapsulating peritonitis Peritoneal hydatid disease Inflammatory pseudotumor Non-pancreatic pseudocysts	Seroma Biloma Urinoma Lymphocele Gossypiboma

Figure 2.

Proposed diagnostic algorithm for the differential diagnosis of peritoneal conditions according to the predominant characteristics (and number) of lesions.

Therefore, in this review, we address the fundamental role of the radiologist in the diagnostic approach of predominantly solid and predominantly cystic lesions of the peritoneum, both solitary and multiple. Due to its importance and frequency, carcinomatosis is discussed separately.

Peritoneal carcinomatosis

Peritoneal carcinomatosis (PC) designates seeding of metastatic implants in the visceral peritoneal lining from a variety of malignant primary neoplasms – most frequently, colon, ovary, and stomach⁴ (Figure 3).

Figure 3.

Peritoneal carcinomatosis associated with gastric adenocarcinoma (arrows in a). Ascites and omental caking are present (arrowheads in b).

The location of implants is extremely important for the presurgical and pretreatment evaluation of carcinomatosis. Radiologists should specify every site of PC to stage it as accurately as possible. The presence of pathology involving the right subphrenic space, Treitz ligament, small bowel, and mesenteric root is crucial for pre-operative staging because it can be an exclusion criterion for surgical resection.⁹

Even in the absence of a known organ-based malignancy, findings that are suspicious for PC should mandate a thorough search for an occult gastrointestinal, ovarian, or other organ-based cancer. Special attention should be granted to the evaluation of the ovaries, as their enlargement may be due to either primary ovarian neoplasms or Krukenberg tumors, and these are very common in the setting of peritoneal dissemination of disease. Approximately, 10% of ovarian tumors are metastatic, from which almost 50% are Krukenberg tumors, more frequently causing bilateral ovarian involvement.¹⁰

Imaging features of peritoneal carcinomatosis

Ascites is a common feature in advanced peritoneal malignancy, and loculation of ascitic fluid is one of the most helpful findings pointing to a malignant etiology. When ascites is not present, peritoneal tumors can be subtle and difficult to identify.

Peritoneal deposits can be either solid masses (varying from multifocal discrete nodules to infiltrative masses) or, when mucinous, low-density lesions, which can sometimes be obscured by ascites. Metastasis from mucinous neoplasms of ovary, colon, appendix, stomach, pancreas, gallbladder and the urachus, can exhibit low density and specks of calcification⁴ (Figure 4). A specific focus on identification of the appendix in all patients with cystic peritoneal lesions is therefore warranted, particularly when other more common primary mucinous cancers (ovary and colon) are excluded.

Figure 4.

Three patients presenting with peritoneal carcinomatosis from mucinous adenocarcinomas. Enhanced CT shows omental caking with low density masses (* in a, arrows in b and c). Patient in image (c) presented with bulky hypodense implants, characteristic of mucinous metastasis.

The use of intravenous contrast is crucial for the assessment of PC. Lesions usually enhance slowly, and therefore are best seen on venous or delayed phases, both on CT and MRI. Abnormal enhancement should be suspected when the peritoneum enhances more than the liver or has associated thickening, nodularity, or mass.

On MRI, cystic nodules appear hyperintense on T₂WI and low-b value DWI, but hypointense on high-b value DWI. Conversely, solid nodules are hypointense/of intermediate signal on T₂WI, show restricted diffusion and enhance following intravenous contrast administration.¹¹

Detection of peritoneal implants on CT is strongly influenced by lesion size, location and presence of ascites, with studies reporting a sensitivity of detection of individual peritoneal implants <1.0 cm ranging from 9 to 50% and of nodules <0.5 cm of only 11%.^12,13 Sensitivity for detecting peritoneal thickening or enhancement is lower in the right subdiaphragmatic space, in the paracolic gutters, in the omentum, and in the mesentery and bowel serosa.¹² The use of postive oral contrast may improve the sensitivity of CT in detecting small bowel and mesenteric implants, which may potentially hamper surgery.^14,15 Nevertheless, the diagnostic performance of PET-CT, and particularly of MRI, surpasses that of CT, and both methods should be preferred in patients with equivocal disease.

In a recent meta-analysis evaluating the diagnostic performance of CT, PET-CT, and MRI in detecting peritoneal metastasis, the latter, especially with the addition of DWI, provided the highest sensitivity for the detection of peritoneal metastases (sensitivity and specificity respectively of 68 and 88% for CT, 80 and 90% for PET-CT and 92 and 85% for DW-MRI).¹⁶

In the first prospective study evaluating the diagnostic value of whole-body (WB) DW-MRI compared with CT and PET-CT in assessing peritoneal staging in ovarian cancer patients, Michielsen et al reported accuracies of 91%, 75%, and 71%, respectively. Mesenteric and serosal deposits and subcentimetric lesions were better described by DW-MRI than by CT.¹⁷

However, it is crucial that the MRI protocol for peritoneal imaging is optimized for depicting small peritoneal implants. All patients comply with 4 h fasting prior to their examination, and the study protocol should include T₂ WI and T₁ WI (in axial and coronal planes), axial DWI (with at least one low-b value – 0–100 s/mm² – and one high-b value – 700–1000 s/mm² – acquisitions) and non-enhanced followed by gadolinium-enhanced fat-suppressed T₁ WI of the pelvis and abdomen (in the axial and coronal planes). Fat suppression improves depiction of small tumors, and, with the same aim, high-resolution imaging is mandatory. Oral intake of contrast as well as intravenous administration of spasmolytics is optional and largely dependent on the clinical condition of patients.

Quantifying peritoneal carcinomatosis

The information provided by imaging studies guides clinical management and determines prognosis, being of particular importance in patient selection for peritoneal cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC).^4,16 In cases of peritoneal malignancy, the radiologist should focus on assessing the extent of disease, especially in sites that may complicate surgery, rather than to classify disease as resectable or not.

The Peritoneal Cancer Index (PCI) is the most widely used quantitative descriptor of peritoneal disease with validated prognostic value.^18–22 It was initially described and validated at laparotomy, later being adopted to estimate peritoneal malignancy on imaging, and has the advantage of combining information on lesion size and distribution of intra-abdominal disease.^23,24 Figure 5 provides further insights on the methodology to quantify the PCI. The PCI cut-off for survival benefit of CRS and HIPEC is controversial, largely depending on the biology and aggressiveness of the primary tumor and individual case analysis.^18,19,25 Although PCI can be used in different types of peritoneal malignancies, it has been strongly validated only in the setting of colorectal and ovarian cancers, failing as a prognostic tool in other malignancies such as pseudomyxoma peritonei and cystic mesothelioma.^26,27 Another limitation is that a low PCI score does not automatically translate into a favorable prognosis, because it does not consider disease involvement in crucial sites.^18,28

Figure 5.

PCI. Determination of the index requires division of the abdomen into nine regions, plus four small bowel segments: a central compartment, the four quadrants, the two paracolic gutters, the epigastrium and the pelvis, combined with four regions in the small bowel and its mesentery. The anatomic boundaries of these regions are defined by anatomic landmarks: two transverse planes and two sagittal planes are used to divide the abdomen into nine abdominopelvic regions. The upper transverse plane is the lowest aspect of the costal margin, and the lower transverse plane is the anterior superior iliac spine. The sagittal planes divide the abdomen into three equal sectors. The small bowel is divided in upper jejunum, located in the left upper abdomen; lower jejunum, located in the left lower abdomen; upper ileum located in the right upper abdomen just below the liver; and lower ileum, including the terminal ileum located in the right lower abdomen. A lesion size score, ranging from 0 to 3 and representing the size of the largest lesion in each region (<0,5 cm: one point; 0,5–5 cm: two points; >5 cm: three points), is allocated per region, yielding a PCI score which may range from 0 to 39. PCI, Peritoneal Cancer Index

Predominantly solid peritoneal lesions

Malignant peritoneal mesothelioma

Malignant peritoneal mesothelioma (MPM) is an uncommon malignant neoplasm that arises from mesothelial cells, most commonly from the pleura, with peritoneal MPM accounting for only 7–30% of cases.²⁹ While most pleural mesotheliomas are attributable to asbestos exposure (70%–90%), for peritoneal mesothelioma the proportion is lower (58%), particularly in females (10–40%).³⁰ It is important to differentiate between diffuse and localized subtypes, with significant prognostic value: diffuse MPM is usually highly aggressive, while localized MPM can have a good prognosis with complete surgical excision.^8,31 Morphologically, three main subtypes are described: epithelial, sarcomatous, and biphasic. Patients with pure epithelial mesotheliomas usually have a better prognosis than those with sarcomatous or mixed tumors.⁸

The diffusely infiltrative type is characterized by plaque-like diffuse peritoneal thickening, peritoneal and mesenteric nodules, omental disease in the form of omental caking and visceral peritoneal thickening seen as a rim of soft tissue around the small bowel (Figure 6). Ascites, though often present, does not necessarily occur.⁸ It can have an invasive nature, extending into the abdominal wall, retroperitoneum, and diaphragm into the pleural cavity (Figure 7).

Figure 6.

Malignant peritoneal mesothelioma, epithelial type. Ultrasound images (a, b) show peritoneal fluid surrounding the liver with clearly defined peritoneal nodules (arrows). Contrast-enhanced CT (c, d) demonstrates voluminous ascites with peritoneal enhancing nodules (white arrows) and involvement of falciform ligament (orange arrow).

Figure 7.

Malignant peritoneal mesothelioma, sarcomatous type. Ultrasound (a) reveals a large heterogeneous mass involving the liver and right diaphragm. CT shows a heterogeneously enhancing lesion involving the liver, diaphragm, and thoracic wall (white arrows in (b) and (c). Calcified pleural plaques attributed to prior asbestos exposure (orange arrows in b) are also seen, as well as peritoneal thickening with calcifications (arrows in d), which is uncommon in peritoneal mesothelioma.

Unlike in pleural mesothelioma, calcification is uncommon. In the presence of a peritoneal-based mass with extensive calcification, other diagnoses should be considered. However, other asbestos-related changes, including calcified pleural plaques, may be present within the chest⁸ (Figure 7).

The peritoneal nodules typically enhance in a non-specific fashion on dynamic cross-sectional studies, with a heterogeneous pattern of enhancement occurring when there is intratumoral degeneration, necrosis, or cystic areas. MRI features of MPM are based on a few case reports, describing intermediate to low-signal intensity on T₁WI, intermediate to high-signal on T₂WI and diffusion restriction.^16,32 The ¹⁸F-FDG uptake on PET/CT is typically high, with a diffuse, focal, or mixed distribution pattern.³³

The major differential diagnosis is carcinomatosis, from which is often indistinguishable. Lymphomatosis and peritoneal infections such as tuberculosis and histoplasmosis may have similar appearances, but they are typically associated with lymphadenopathy. MPM should be considered when the predominant imaging findings are: 1) sheet-like thickening of the peritoneum; 2) imaging evidence of asbestos exposure; 3) absence of primary malignancy or metastasis elsewhere and 4) absence of lymphadenopathy.

Primary peritoneal serous carcinoma

Histologically, this tumor is indistinguishable from metastatic serous ovarian carcinoma, which makes the true prevalence of primary peritoneal serous carcinoma (PPSC) likely underestimated.

CT and MRI frequently disclose ascites, peritoneal nodules, and omental nodularity or caking.^8,34,35 Additional findings include large solid pelvic masses, lymphadenopathy, and pleural effusions. Multiple calcified masses, representing underlying psammomatous bodies, have been reported to occur in as many as 30% of patients with primary peritoneal serous carcinoma^8,34–36 (Figure 8). Peritoneal and omental thickening or nodularity demonstrate low signal intensity on T₁WI, high signal intensity on T₂WI and diffusion restriction (Figure 9). After intravenous contrast administration, heterogeneous enhancement of the peritoneal nodules and thickening is usual.

Figure 8.

Primary peritoneal serous carcinoma. Multiplanar CT shows ascites and omental caking with multiple calcified nodules (orange arrows in (a) and (b). Subtle peritoneal nodularity with fine calcification is also present (arrow in c).

Figure 9.

Primary peritoneal serous carcinoma. ADC map (a), axial high b-value (b), and T₂WI (c) show peritoneal thickening with restriction to diffusion in the rectouterine pouch. ADC, apparent diffusion coefficient; T₂WI, T₂ weighted imaging.

The cross-sectional imaging features of PPSC overlap with those of other diffuse peritoneal malignancies, such as PC, metastatic ovarian carcinoma, and malignant mesothelioma. However, as previously emphasized, PPSC should be suggested in the differential diagnosis when the findings of ascites and peritoneal and omental nodules are identified in a female patient with no evidence of a visceral primary or ovarian mass.⁸

Desmoplastic small round cell tumor

Desmoplastic small round cell tumor (DSRCT) is a rare malignancy of unknown etiology that occurs predominantly in adolescent and young adult males, with a universally poor prognosis The typical feature is the presence of a dominant mass usually located in the pelvic cavity, often with discrete and confluent peritoneal and omental nodules or diffuse peritoneal thickening of the abdomen and pelvis.³⁷

Focal areas of non-enhancement or low attenuation on contrast-enhanced CT possibly represent high fibrotic content, in addition to necrosis and intratumoral hemorrhage.^38,39

On MRI, the tumors typically show heterogeneous signal intensity but are predominantly hypointense on T₁WI and hyperintense on T₂WI. However, the relatively low signal of solid components on T₂WI was also reported and related to densely packed cells and the desmoplastic response.^38,40

The ¹⁸F-FDG uptake on PET/CT is high to very high, focal, and heterogeneous, due to zones of necrosis within the tumor.⁴¹

DSRCT should be considered in the differential diagnosis of regional tumors in adolescents and young adults.^39,40 Malignant mesothelioma, rhabdomyosarcoma, splenosis and lymphoma can produce a similar appearance on imaging. Gastrointestinal stromal tumor (GIST) also can have a similar appearance, but it rarely disseminates to lymph nodes.

Lymphomatosis

Lymphomas affecting the peritoneum generally cause peritoneal thickening on cross-sectional imaging. Mild to moderate non-loculated and non-septated ascites is also usual. Retroperitoneal lymphadenopathy, bulky-infiltrative mesenteric and solid organ involvement are common imaging findings as well⁴² (Figure 10).

Figure 10.

Peritoneal lymphomatosis. CT depicts bulky masses involving both ovaries and pelvic peritoneum (* in (a); white arrows in (b) and (c). Retroperitoneal lymphadenopathy with homogenous enhancement is also present (blue arrows).

The FDG avidity in lymphoma is variable, and indolent lymphomas can have low-grade or no FDG uptake because of their low metabolic activity.⁴³

Imaging findings of omental caking with homogeneous bulky masses rather than a nodular pattern, in addition to a diffuse distribution of enlarged lymph nodes, are helpful signs of peritoneal lymphomatosis, although benign and malignant mesenteric malignancies, gynecologic and gastrointestinal malignant diseases can all cause similar appearances.^42,44

Endometriosis

Extrapelvic endometriosis occurrs in about 1% of all cases, and bowel segments located closest to the pelvis (rectosigmoid and distal ileum) are the most affected.⁴⁵

CT findings of endometriosis are non-specific, and the lesions may appear solid, cystic, or mixed solid and cystic.³Typical MRI findings of deep endometriosis include fibrotic nodular or retractile masses of low signal intensity on T₂WI with or without hyperintense hemorrhagic foci on T₁WI with fat suppression, abutting the bowel wall (Figure 11). Hyperintensity on T₁WI and hypointensity on T₂WI arises from the presence of deoxyhemoglobin and methemoglobin, reflecting chronic and recurrent hemorrhage within the lesion. Use of contrast material does not add specificity to the diagnosis, because the enhancement patterns are non-specific.^3,45 Within solid endometriotic masses, hyperintense foci on T₂WI, representing dilated ectopic endometrial glands, may be seen.⁴⁶ Differential diagnosis includes epithelial gynecologic neoplasms: DWI may be useful in this setting demonstrating no restricted diffusion within the endometriotic mass, thus avoiding the need for invasive surgery.⁴⁶

Figure 11.

Peritoneal endometriosis. MRI displays a lesion in the left paracolic gutter between the left psoas muscle and the left colon. On T₁WI (black arrow in a) it shows intermediate SI, whereas on T₂WI it exhibits low SI (white arrow in b); CT scan shows a heterogeneously enhancing lesion with irregular margins (orange arrow in c). SI, signal intensity; T₁WI, T₁ weighted imaging; T₂WI, T₂ weighted imaging

Splenosis

Patients with splenosis are typically asymptomatic, and the condition is detected incidentally by imaging. Splenic implants can be solitary or multiple and usually remain small (<3 cm).

On CT, these nodules can be rounded, oval, sessile, or pedunculated and possess density and enhancement pattern like that of spleen. On MRI, they demonstrate intermediate to low signal intensity on T₁WI, intermediate to high signal intensity on T₂WI, and restricted diffusion, identical to normal splenic tissue.⁴⁷ Scintigraphic studies using ^99mTc heat-damaged erythrocytes or Indium 111-labeled platelets are highly sensitive and specific for splenic uptake, making these tests the current diagnostic tools of choice^48,49 (Figure 12).

Figure 12.

Peritoneal splenosis in a middle-aged male with a history of splenectomy due to traumatic splenic rupture. Large omental nodules are seen on CT (blue arrows in a). PET-CT with ¹⁸F-FDG did not show any abnormal uptake (orange arrows in b). Scintigraphy using Tc-99m heat-damaged erythrocytes was diagnostic for splenic ectopic tissue (white arrows in c). FDG, fludeoxyglucose; PET, positron emission tomography.

Gliomatosis peritonei

Gliomatosis peritonei (GP) designates the presence of benign, mature glial implants throughout the peritoneum. The condition occurs almost exclusively in association with solid or immature ovarian teratomas, but it has also been reported to occur in association with ventriculoperitoneal shunts.⁵⁰

On CT, GP presents as soft-tissue peritoneal nodules and masses, with omental caking, and with associated ascites in the setting of an adnexal or pelvic mass suggestive of a teratoma³ (Figure 13; Figure 14). Deposits of glial tissue are of moderately high signal on T₂WI. Lesions tend to be homogeneous although may be multilobulated.⁵¹ Cases with cystic components within the lesions have been reported.⁵² The imaging findings of GP can be indistinguishable from those of PC.

Figure 13.

Ovarian immature teratoma in a young female. CT depicts a large well-defined, lobulated mass, arising from the pelvis and extending superiorly to the abdomen (*). It shows heterogeneous enhancement with fat density areas and multiple calcifications.

Figure 14.

Peritoneal gliomatosis in the same patient shown in Figure 13, after surgical removal of the ovarian teratoma. Follow-up CT reveals multiple peritoneal large nodules in the right upper quadrant, with macroscopic fat (arrow in a), as well as small nodules (arrowheads in b). Some nodules present with calcifications (arrow in c). Restaging surgery disclosed mature teratoma implants.

Leiomyomatosis peritonealis disseminata

Leiomyomatosis peritonealis disseminata (LPD) is characterized by dissemination and proliferation of peritoneal and subperitoneal lesions primarily originating from smooth muscle cells. It is more common in premenopausal females, and although benign in nature, it may degenerate into peritoneal leiomyosarcoma.^53,54

MRI features include multiple solid masses with signal intensity similar to that of skeletal muscle and smooth muscle on both T₁- and T₂WI and with homogeneous enhancement.^3,55,56 DWI also provides useful information, as LPD does not show restricted diffusion, in contrast with peritoneal malignancies.⁵³

Solitary fibrous tumor

A solitary fibrous tumor is a rare mesenchymal neoplasm of the peritoneum, since most cases arise in the pleura.^57,58 Reports of extrapleural solitary fibrous tumors describe large, well-circumscribed, encapsulated, and non-infiltrating solitary lesions with solid and cystic components (Figure 15). Contrast-enhanced scans show marked and heterogeneous enhancement during the early portal venous phase.^58–60

Figure 15.

Solitary fibrous tumor arising from the pelvic peritoneum. Enhanced CT scan (a, d); T₁WI (b); Contrast enhanced T₂ fat-suppressed WI (c, f); T₂WI (e) demonstrate a large, encapsulated, predominantly cystic lesion in the rectovesical pouch (*). The lesion shows a solid enhancing component (arrows in a, c and f). T₁WI, T₁ weighted imaging; T₂WI, T₂ weighted imaging.

Based on clinical presentation and radiological appearances, the main differential diagnosis is GIST.

Predominantly cystic peritoneal lesions

When evaluating cystic lesions, radiologists should try to establish its true peritoneal origin since some loculated fluid collections due to infection or inflammation (abscess, pancreatitis,...) may simulate a cystic peritoneal lesion. Also, it should be again stressed the importance of evaluating the ovaries and the appendix in all patients with cystic peritoneal lesions.

Pseudomyxoma peritonei

Pseudomyxoma peritonei (PMP) is a clinical or radiologic description rather than a pathologic diagnosis, describing copious, thick mucinous or gelatinous material on the surfaces of the peritoneal cavity, often from a primary mucinous malignancy of the appendix.³ Generally, PMP produces mass effect on the mesentery and small bowel rather than direct involvement of these structures.⁴

On ultrasound, PMP may present as highly echogenic intraperitoneal fluid, with internal echoes that do not move with body changes, and multiple echogenic septations in a laminated concentric appearance.⁶¹ These septations represent the edges of the mucinous nodules (Figure 16).

Figure 16.

Pseudomyxoma peritonei in two patients with low-grade mucinous malignancy of the appendix. Ultrasound images show a cystic lesion with peripheral calcifications in the right lower quadrant (a) and highly echoic intraperitoneal fluid that did not move with changes in body position with multiple echoic septations (* in b). CT reveals a ruptured appendix with wall calcification (white arrow in c), with large ascites and scalloping of the liver margins (blue arrow in d). In a second patient (e), the peritoneal cavity is filled with voluminous multiseptated ascites and scalloping of the liver and spleen due to extrinsic pressure from the mucinous implants (blue arrows in e) is readily apparent.

CT findings of PMP include hypodense or cystic masses (<20 HU), with mass effect and scalloping of peritoneal surfaces and visceral organs, which indicates extrinsic pressure from the mucinous implants.¹⁶ Curvilinear or amorphous calcifications may be present.⁶²

On MRI, signal intensity varies with mucin concentration but hypointensity on T₁WI and hyperintensity on T₂WI are characteristic. Delayed-enhancing fat-suppressed sequences allow detection of cellular components within the pool of ascites and mucin. Merging conventional MRI protocols with DWI improves the sensitivity and specificity for detecting PMP when compared to conventional MRI protocols alone.^63,64

The major differential is peritoneal mucinous carcinomatosis, which tends to have: 1) larger soft-tissue components; 2) involvement of the chest more frequently with effusions and/or pleural masses; and 3) mesenteric or retroperitoneal lymphadenopathy and invasion into parenchymal organs.

Multicystic mesothelioma

In contrast to MPM, multicystic peritoneal mesothelioma (MCPM) has no association with asbestos exposure, but rather to chronic inflammation.⁶⁵

MCPM presents as a multilocular cystic tumor, commonly arising from the pelvic surfaces of the peritoneum (but potentially found throughout the peritoneal cavity) and has a benign or indolent biologic behavior in most patients. Nonetheless, given reports of recurrence and malignant transformation, most are resected.¹⁶ It occurs mainly in young to middle-aged females (average age of 37 years), with males representing only 16% of cases (average age of 47 years).⁸

Ultrasound reveals multiple clusters of anechoic cysts divided by thin-walled septations. One or both ovaries entrapped in the cysts are characteristic of this disease.¹⁶ The fluid within the cysts is generally anechoic, but the cysts may contain echoes from debris or hemorrhage.⁸ MRI is helpful by showing lesions with the typical multicystic appearance, hypointense on T₁WI and with high to intermediate signal on T₂WI, showing mild enhancement of the wall and septations.^8,66

Lymphangioma is the most important differential imaging diagnosis, but it occurs mainly in the pediatric population.⁶⁵ Other differential diagnoses to be considered are loculated ascites, other mesenteric cysts such as duplication cysts and cystic ovarian neoplasms.⁸

Lymphangioma

Lymphangiomas are uncommon benign lymphatic lesions that may occur in many anatomic locations. They are more frequent in children.¹³ Most occur in the neck (95%) with other locations accounting for only 5%. The cysts may contain chylous, serous, hemorrhagic, or mixed fluid.⁶⁷

On CT, lymphangiomas are cystic masses, and can exhibit negative attenuation content if filled with chylous fluid. Hemorrhagic contents with high attenuation on CT and calcifications are uncommon. Lymphangiomas may show enhancement of the wall and septa. On MRI, they are most often purely cystic lesions. In the presence of chyle, a signal drop may be seen in the opposed-phase chemical shift images.^67,68

Large mesenteric lymphangiomas can be differentiated from ascites by the presence of septa, compression on adjacent intestinal loops, and lack of fluid in the dependent recesses of the peritoneum.^67,68

Granulomatous peritonitis

The term granulomatous peritonitis encompasses a wide range of unusual pathological conditions that cause granulomatous inflammation of the peritoneum, with tuberculous peritonitis being the hallmark.³

Classically, tuberculous peritonitis has been described as having three imaging patterns (wet, fibrotic fixed, and dry plastic), depending on the relative amount of ascites and soft-tissue component, with the “wet-type” being responsible for 90% of cases and characterized by large amounts of viscous ascitic fluid that is diffusely distributed or loculated.³ Although tuberculous involvement limited to the peritoneum only is rare, peritoneal involvement is frequently seen in association with widespread abdominal disease⁶⁹ (Figure 17; Figure 18).

Figure 17.

Peritoneal tuberculosis. Hypoechoic peritoneal-based nodules are seen in the ultrasound image (calipers in a). CT reveals heterogeneous peritoneal nodules with central low-density areas (white arrows in b and c). Low density ascites is also present (orange arrow in c). Tuberculous lymphadenitis (d), with CT depicting multiple, enlarged mesenteric lymph nodes with typical central low-attenuation due to caseous necrosis, and peripheral rim enhancement.

Figure 18.

“Wet-type” granulomatous peritonitis in a patient with abdominal tuberculosis. Diffuse peritoneal thickening (white arrows in a and b) is accompanied by low attenuation ascites. Clustering of small bowel loops with tethered appearance (* in b) is also seen, corresponding to encapsulating peritonitis due to chronic inflammatory response.

Peritoneal thickening and tiny nodules are the most frequent findings in peritoneal tuberculosis. On CT, the fluid typically has high attenuation values (25–45 HU), which probably reflects its high protein and cellular content. Tuberculous chylous ascites is rare, and when present a fat-fluid level can be demonstrated.⁷⁰

The thickened peritoneum shows pronounced enhancement after contrast, with the nodules showing restricted diffusion on DWI.¹⁶

Peritoneal hydatid disease

Hydatid disease (HD) of the peritoneum, or peritoneal echinococcosis, generally results from rupture of hepatic or splenic involvement. Cross-sectional imaging shows well-defined thick-walled cystic lesions. Mural calcification is a distinguishing feature and should suggest the diagnosis, particularly in the presence of hepatic disease.^68,71,72

In this setting, ultrasound may help detecting internal daughter cysts, floating membranes, and matrix accurately. CT is the best technique to show cyst calcification, infection, and peritoneal seeding. The most characteristic feature with MRI is the low-signal intensity rim of the hydatid cyst on T₂WI due to abundant collagen content of the pericyst that may aid in making the correct diagnosis when other features are absent^68,71 (Figure 19).

Figure 19.

Hydatid disease of the liver and peritoneum. MRI depicts several well-defined thick-walled cystic lesions, both within the liver (white arrows in a) and the omentum (black arrows in b). They show heterogeneous contents and low-signal intensity of the wall on T₂WI (a, b, c) with slight enhancement on T₁W fat-suppressed image (d). T₁WI, T₁ weighted imaging; T₂W, T₂ weighted.

Sometimes, detached membranes can be seen floating in the cyst cavity and produce a “water lily” sign. This sign is considered a pathognomonic finding of HD seen on ultrasound, CT, and even MRI.

Conclusion

The peritoneum can be the site of multiple pathologic disorders, sometimes with overlapping imaging features, despite distinctly different from a pathological point of view. Metastatic disease should be the initial concern in a patient with ascites and peritoneal nodularity at imaging, and the radiologist must focus on assessing the extent of disease, especially in sites that may complicate surgery, as this guides the management of patients.

In the absence of a known primary malignancy, other differentials besides PC should be considered, namely benign tumor-like lesions and primary peritoneal tumors. The predominant structure (solid or cystic) and multiplicity (single or multiple) may provide additional clues to narrow the list of differential diagnoses.

MRI is emerging as the primary and most important imaging tool in the evaluation of patients with peritoneal disease, especially suspected carcinomatosis, providing the highest sensitivity for the detection of peritoneal implants. CT, however, due to its greater availability, is still the workhorse for evaluation of peritoneal pathology in many circumstances.

Radiologists are of utmost importance in the management of patients with known or suspected peritoneal disease, starting from establishing the diagnosis, either by interpreting imaging features or by performing directed biopsies, and by participating in disease staging, thus guiding many of the treatment decisions in these patients.