Non-neoplastic conditions mimicking peritoneal carcinomatosis at CT imaging

Abstract

The general appearance of peritoneal carcinomatosis at abdominal CT and other cross-sectional imaging modalities consists of varying amounts of peritoneal-based soft tissue implants (mass-forming or infiltrative), peritoneal fluid, and occasionally calcification. However, a wide variety of common and uncommon neoplastic and non-neoplastic conditions can closely mimic peritoneal carcinomatosis at imaging. Neoplastic mimics of peritoneal carcinomatosis include primary peritoneal and sub peritoneal tumors, as well as peritoneal lymphomatosis and sarcomatosis.

Broad categories of non-neoplastic mimics of peritoneal carcinomatosis include tumor-like conditions, systemic processes, atypical infections, and fat-based conditions. For many entities, suggestive or specific patient information and/or CT imaging findings exist that may allow the radiologist to narrow the differential diagnosis. In this article, we review the salient clinical and cross-sectional imaging features of non-neoplastic mimics of peritoneal carcinomatosis and malignancy, with emphasis on the CT findings and the additional clues that may suggest the correct benign diagnosis.

Introduction

Peritoneal carcinomatosis is often generically defined as the intraperitoneal dissemination of any tumor that does not originate from the peritoneum or subperitoneal space,¹ although the reference to “carcinoma” specifically implies cancers of epithelial origin. The term was initially introduced in 1931 to describe peritoneal metastatic implants from ovarian adenocarcinoma.² Although intraperitoneal dissemination of ovarian tumors is commonly encountered, peritoneal carcinomatosis can result from nearly any organ-based epithelial malignancy, especially those arising from the gastrointestinal tract. In addition to carcinomatosis, neoplastic involvement of the peritoneum can be seen with peritoneal lymphomatosis and sarcomatosis, as well as from a wide variety of primary neoplasms of peritoneal and subperitoneal origin.³ To further complicate matters, a wide array of uncommon and unusual non-neoplastic conditions exist that may involve the peritoneal and sub peritoneal spaces.⁴

CT with intravenous (i.v.) contrast is often considered the initial imaging modality of choice and general workhorse for non-invasive peritoneal evaluation.^5,6 To optimize the distinction between bowel and peritoneal-based lesions, many experts would also advocate for the use of positive oral contrast material.⁷ The main cross-sectional imaging features of peritoneal carcinomatosis consist of nodular or infiltrative peritoneal-based soft tissue, associated with free or loculated peritoneal fluid. However, these non-specific features may also be seen in a variety of common and uncommon neoplastic and non-neoplastic conditions. When available, the addition of FDG PET to CT may improve overall evaluation,⁸ although important pitfalls exist.⁹ Similarly, magnetic resonance (MR) evaluation of the peritoneal cavity is fraught with potential pitfalls, although inclusion of diffusion-weighted imaging (DWI) improves overall performance.^8,10 Despite major advances in cross-sectional and functional imaging, non-invasive imaging assessment for peritoneal tumor involvement still cannot replace the reference standard of exploratory surgery for confirmation of early or limited disease involvement. Nonetheless, cross-sectional imaging often provides complementary assessment, as incomplete peritoneal assessment is relatively common at laparoscopy.

In this article, we review the imaging features of the many uncommon non-neoplastic conditions that can mimic the much more common scenario of peritoneal carcinomatosis, with emphasis on CT findings. Broad categories of non-neoplastic entities include tumor-like conditions, systemic conditions, atypical infections, and fat-based conditions (Figure 1). The more common, everyday non-neoplastic entities with peritoneal manifestations, such as portal hypertension, peritonitis from usual causes, and post-operative collections, will not be discussed.

Figure 1.

Non-neoplastic conditions that may mimic peritoneal carcinomatosis at CT.

Tumor-like conditions

A wide spectrum of benign, non-neoplastic tumor-like conditions can present at CT with peritoneal soft tissue thickening, nodularity, or masses, often with associated ascites (Table 1). As such, these conditions are often misinterpreted as peritoneal carcinomatosis at initial presentation based on their cross-sectional imaging appearance. However, there are often additional clues from the patient’s medical history or additional imaging features that allow for the correct diagnosis.

Table 1.

Clinical and imaging clues to the diagnosis of tumor-like conditions mimicking peritoneal carcinomatosis.

Disease entity	Clinical clues	Imaging clues
Inflammatory pseudotumor	More common in pediatric and young adult population. Associated with IgG4-related disease.	Central areas of necrosis. Central calcifications.
Retractile mesenteritis	Mesenteric and bowel ischemia in advanced cases.	Spiculated mesenteric mass with variable calcifications. Fibrosis and shortening of the mesentery in advanced cases.
Endometriosis	Previously diagnosed endometriosis. Cyclical abdominal pain.	Areas of high T1 weighted signal and low T2 weighted signal from hemorrhagic and fibrotic foci, respectively.
Foreign-body reactions:
Fishbone perforation	Bowel obstruction.	High attenuation needle-like structure (fishbone).
Dropped gallstones	Previous laparoscopic cholecystectomy.	Surgical absence of the gallbladder. Ectopic gallstones.
Retained surgical sponges	Previous abdominal surgery.	Dense ribbon-like structure on CT scout or plain abdominal radiograph.
Extramedullary hematopoiesis	Anemia, infections, and/or bleeding from decreased blood cell lines.	Associated medullary expansion, osteosclerosis or other typical sites of involvement.
Splenosis	Previous splenic surgery or trauma.	Well-defined rounded soft tissues nodules with CT attenuation and MR signal intensity characteristics similar to the normal splenic tissue. Surgical absence of the normal orthotopic spleen.
Castleman disease	Lymphadenopathy.	Well-defined rounded soft tissue masses with marked enhancement on contrast-enhanced phases. Central calcifications.
Encapsulating peritoneal sclerosis	History of tuberculosis or ongoing peritoneal dialysis.	Diffuse peritoneal thickening with associated calcifications.

Fibromatosis (desmoid tumor)

Mesenteric fibromatosis, also known as aggressive fibromatosis, desmoid tumor, or desmoid-type fibromatosis, is a rare non-metastasizing benign tumor-like condition of the small bowel mesentery that arises from the proliferation of well-differentiated fibroblasts.¹¹ Some now consider fibromatosis to be a true benign neoplasm, making its inclusion in this non-neoplastic review somewhat controversial. Fibromatosis is most commonly idiopathic but can be associated with familial adenomatous polyposis, sometimes referred to as Gardner syndrome. CT findings of prior proctocolectomy provides an important clue to the diagnosis in the setting of familial adenomatous polyposis (Figure 2).⁴ Aggressive fibromatosis is known for its rapid growth, local infiltration, and tendency to recur after resection.¹² The appearance on cross-sectional imaging is variable and can range from a well-defined mass to an infiltrative soft tissue process (Figure 2).¹³ Contrast enhancement is typically noted, but with variable timing. On MR, most lesions typically show relatively low signal intensity on T₁ weighted images and high signal intensity on T₂ weighted images.

Figure 2.

Tumor-like non-neoplastic conditions mimicking peritoneal carcinomatosis. (A, B) Fibromatosis (desmoid) in FAP (Gardner syndrome). Contrast-enhanced CT in a 46-year-old male (A) shows a well-defined ovoid mass in the central mesentery. He had previously had a larger pelvic desmoid resected 4 years earlier (not shown). Contrast-enhanced CT in a 28-year-old male (B) shows a more infiltrative mesenteric desmoid tumor, which continued to locally progress, leading to the patient’s demise at age 30. Note the post-colectomy appearance in both patients – an important clue to etiology. (C) Inflammatory pseudotumor. Contrast-enhanced CT in a young girl shows a large heterogeneously enhancing mesenteric mass, which proved to be a peritoneal inflammatory pseudotumor (myofibroblastic tumor). (D, E) Retractile mesenteritis. Coronal contrast-enhanced CT images in an 80-year-old male (D) and a 65-year-old male (E) show spiculated masses in the central mesentery, with varying degrees of dystrophic calcification. This was an incidental finding at CT angiography for iliac artery aneurysm in D, but was symptomatic in E. The appearance can loosely simulate metastatic carcinoid. (F) Endometriosis. CT colonography in 36-year-old female with episodic rectal bleeding shows serosal-based soft tissue implants (arrows) involving the sigmoid colon. FAP, familial adenomatous polyposis.

Inflammatory pseudotumor

Inflammatory pseudotumor, also referred to as inflammatory myofibroblastic tumor, is an uncommon benign process of unknown etiology, with characteristic myofibroblastic proliferation associated with an abundant inflammatory reaction.¹⁴ The histologic appearance can closely mimic inflammatory fibrosarcoma, which may lead to misdiagnosis.⁴ Although IgG4-related disease is a known association, at least one study suggests that IgG4 does not play an important role in the pathogenesis of these pseudotumors.¹⁵ Most cases have been reported in pediatric and young adult populations, whereas inflammatory pseudotumors are rare in adults.¹⁶ Although the lungs are the most common site of involvement, extra pulmonary sites of involvement include the orbits and peritoneum.¹⁷ On cross-sectional imaging, mesenteric inflammatory pseudotumors usually appear as infiltrative or well-defined peritoneal-based heterogeneous soft tissue masses with variable enhancement (Figure 2).⁴ Central areas of necrosis are common, and central calcifications have also been described.¹⁷ The younger age of onset may help suggest the diagnosis.

Retractile mesenteritis

Retractile mesenteritis is considered a subtype of sclerosing mesenteritis, a broad term used to describe a spectrum of idiopathic inflammatory conditions affecting the mesentery.^4,18 Mesenteric panniculitis (discussed later on under fat-based conditions), mesenteric lipodystrophy, and retractile mesenteritis are the main subtypes of sclerosing mesenteritis. Retractile mesenteritis is the term applied to more chronic cases where fibrosis predominates over inflammation.¹⁹ However, the common incidental CT finding of mesenteric panniculitis rarely progresses to the fibrotic retractile form, which may actually represent a discrete condition. Some cases may be related to IgG4-related disease. At CT imaging, a spiculated mesenteric mass with variable amount of calcification can be seen (Figure 2). The imaging appearance can simulate the findings of metastatic mesenteric spread of ileal carcinoid tumor. In advanced cases, fibrosis and shortening of the mesentery can be noted with compromise of the mesenteric vessels and subsequent bowel ischemia.^4,19

Endometriosis

Endometriosis is a chronic debilitating multifocal inflammatory condition, almost exclusively affecting females of reproductive age, which is characterized by the presence of functional endometrial tissue outside its normal location.²⁰ Endometrial implants are most commonly found on the serosal surfaces of pelvic organs and along the pelvic peritoneum. Extra pelvic peritoneal extension of endometriosis to involve the serosal surfaces of the bowel wall and other areas is less common.²¹ Endometriotic implants may also occur within the abdominal wall musculature, particularly associated with a Cesarean section scar. Given the more non-specific CT appearance of the peritoneal-based soft tissue implants related to endometriosis (Figure 2), MR is considered the gold-standard for non-invasive radiological evaluation.²² A wide spectrum of imaging appearances ranges from subtle peritoneal deposits to larger tumor-like implants with tissue invasion. In cases of deep infiltrating or mass-forming endometriosis, serosal-based soft tissue endometriotic implants can closely mimic peritoneal carcinomatosis (Figure 2).²³ Signal intensity on MR imaging is variable, but areas of high T₁ weighted signal from hemorrhagic foci and low T₂ weighted signal from fibrosis are more specific, with heterogeneous enhancement on contrast-enhanced sequences.²³ Classic imaging features, history of previously diagnosed endometriosis, and cyclical abdominal pain can be helpful clues when extra pelvic endometriosis is suspected.

Foreign-body reactions

Intra-abdominal foreign bodies may induce benign reactive changes that can mimic a malignant process such as peritoneal carcinomatosis. The reaction to a peritoneal foreign body includes a granulomatous inflammatory reaction with giant cell formation, followed by fibrosis or fibrous capsule formation.²⁴ Many causative agents have been reported to cause such a foreign body reaction, including fishbones, toothpicks, dental crowns, gallstones, and retained surgical material.^25–27 Ingestion of foreign bodies with subsequent gastrointestinal perforation, and complications related to abdominal surgical procedures represent the two main etiologies of peritoneal foreign bodies. Imaging clues at abdominal CT typically allow for a specific diagnosis.

Foreign body perforation, most commonly related to fishbone ingestion (Figure 3), can occur anywhere along the gastrointestinal tract, but the terminal ileum is reportedly the most common site.²⁶ The CT imaging features vary somewhat depending on the specific foreign body, but usually include localized bowel wall thickening at the site of perforation, and phlegmonous peritoneal-based inflammation that can mimic tumor. Extraluminal gas, fluid, and signs of bowel obstruction may also be seen. In the case of fishbone perforation, the presence of a high attenuation needle-like structure should allow for a specific diagnosis (Figure 3).

Figure 3.

Peritoneal-based foreign body reactions. (A, B) Fishbone perforation. Transverse (A) and coronal (B) CT images in a 46-year-old male with 2 week history of abdominal pain shows a phlegmonous soft tissue mass (arrows) centered in the jejunal mesentery. Note the thin linear density centrally, corresponding to the extraluminal fishbone. There is also eccentric thickening of an adjacent jejunal loop inferiorly on the coronal view, representing the site of perforation. (C, D) Dropped gallstones. Two different patients show somewhat ill-defined perihepatic masses, both of which were initially mistaken for malignancy. However, clues to the benign diagnosis include prior cholecystectomy in both cases, a peripheral calcified ectopic gallstone in C (arrow), and a central calcific focus in the otherwise low-attenuation dropped gallstone in D (arrow). (E, F) Retained lap sponge (gossypiboma). Contrast-enhanced CT (E) shows a complex peritoneal collection (arrow) containing mottled lucencies that suggest an abscess. Note, however, the high-density peripheral focus with streak artifact, which is better appreciated as a ribbon-like structure on the abdominal radiograph (F) to represent a retained surgical sponge (arrow).

Complications stemming from abdominal surgery that can result in tumor-like peritoneal-based foreign body reactions include dropped gallstones and retained surgical materials. Dropped gallstones from prior laparoscopic cholecystectomy demonstrate variable CT imaging appearances.²⁵ If the stones serve as a nidus of infection or inflammation, signs of peritoneal irritation will be visible at CT, often mimicking malignancy (Figure 3). Variability in the composition of gallstones leads to a variety of attenuation appearances at CT, but the key additional observation is surgical absence of the gallbladder (Figure 3).

The most common type of unintended retained operative material is the surgical sponge, which can result in a chronic foreign body reaction referred to as a gossypiboma or textiloma.²⁷ The CT imaging appearance is variable, but the constant feature is the radiopaque portion of the sponge, which is better appreciated as a dense ribbon-like structure on the CT scout or abdominal radiograph (Figure 3). The associated foreign body reaction often consists of a well-defined collection, but with variable internal appearances ranging from mottled gas, whorled texture, soft tissue, or fluid. The mottled, spongiform pattern of scattered gas foci can closely mimic an abscess (Figure 3), even though it is typically found as a chronic, sterile process.

Extramedullary hematopoiesis

Extramedullary hematopoiesis (EMH) represents the presence of normal yet extramedullary hematopoietic marrow elements as a compensatory response to inadequate bone marrow function or ineffective hematopoiesis.²⁸ Although EMH can take place in any tissue of mesenchymal origin, it most commonly involves the reticuloendothelial system within the spleen, liver, and lymph nodes.²⁹ Peritoneal involvement in EMH is relatively rare and may be focal or diffuse.³⁰ The diffuse form manifests as more widespread non-obstructive peritoneal thickening, sometimes with ascites, whereas the focal form appears more mass-like (Figure 4).^29,30 Depending upon the underlying cause, important diagnostic clues may include the presence of medullary expansion, osteosclerosis (in the setting of myelofibrosis), or other typical sites of EMH involvement (Figure 4).

Figure 4.

Tumor-like non-neoplastic conditions that mimic carcinomatosis. (A, B) EMH from myelofibrosis. Coronal (A) and transverse (B) CT images in a 72-year-old female show diffuse peritoneal-based infiltration with little or no mass effect. Note surgical absence of the spleen, which was enlarged by EMH. Note also diffuse osteosclerosis, with markedly increased vertebral trabecular attenuation of 326 HU. Peritoneal biopsy revealed EMH. (C) Splenosis. Contrast-enhanced CT in 47-year-old male shows eccentric cecal wall thickening from acute diverticulitis. Incidentally noted were multiple homogeneous soft tissue nodules (arrows) throughout the peritoneal cavity representing ectopic splenic tissue (splenosis). Note absence of the orthotopic spleen in the left upper quadrant. (D) Castleman disease. Coronal CT in 31-year-old male with unicentric Castleman disease (hyaline vascular variant) show a solitary enhancing ovoid soft tissue mass (asterisk). (E, F) EPS. CT image in 42-year-old male (E) with diabetic nephropathy shows diffuse curvilinear calcifications along parietal and visceral peritoneal surfaces, including protrusion into an umbilical hernia. The characteristic coiling of a peritoneal dialysis catheter suggests the underlying cause of EPS. CT image in a 57-year-old male (F) with biopsy-proven EPS shows more non-specific diffuse serosal-based bowel wall thickening and clustered appearance. EMH, extramedullary hematopoiesis; EPS, encapsulating sclerosing peritonitis.

Splenosis

Splenosis in an acquired condition of ectopic splenic tissue observed after splenic trauma or surgery involving the native spleen.³¹ Ectopic splenic implants have been described in essentially all intraperitoneal sites, most often as an incidental imaging findings (Figure 4).³² The soft tissue nodules in splenosis vary in location, shape, number, and size (usually <3 cm). On cross-sectional imaging, splenosis typically manifests as rounded homogeneous peritoneal-based soft tissue nodules or masses in the absence of the normal orthotopic spleen (Figure 4).³² On multiphasic studies, the CT attenuation and MR signal intensity characteristics of the lesions will be reminiscent of normal splenic tissue. For confirmation, heat-damaged RBC or sulfur colloid scintigraphy or ferumoxytol-enhanced MR can be performed if desired.

Castleman disease

Castleman disease is a rare benign lymphoproliferative disorder characterized by hyperplasia of the lymphoid follicles.³³ It can be classified clinically into unicentric and multicentric, or histologically into the more common hyaline-vascular (or angiofollicular) type and the plasma cell type.³⁴ Hyaline-vascular Castleman is localized in 75% of cases, whereas the plasma cell type is multicentric in more than 90% of cases.³⁴ Castleman disease most often manifests as lymphadenopathy involving the neck and thorax. Mesenteric and retroperitoneal involvement typically presents at CT as well-defined, rounded soft tissue masses that demonstrate marked enhancement after i.v. contrast administration (Figure 4).³⁵ Central dystrophic calcification may be present within the mass lesions.

Encapsulating peritoneal sclerosis

Encapsulating peritoneal sclerosis (EPS) is a rare but debilitating condition characterized by peritoneal inflammation and fibrosis causing encapsulation of bowel loops, associated with recurrent obstruction.³⁶ EPS can be primary (idiopathic), also referred to as abdominal cocoon syndrome, or secondary to peritoneal dialysis (the most common etiology) or inflammatory conditions such as tuberculosis.^36,37 The main cross-sectional imaging features include diffuse peritoneal thickening, sometimes with associated calcifications, causing clustering of thickened bowel. (Figure 4).^38–40 Associated findings of bowel obstruction and ascites are common. CT findings of ongoing peritoneal dialysis can be a very useful clue to the diagnosis (Figure 4).

Systemic and multiorgan conditions

A number of systemic or multiorgan conditions can manifest with peritoneal findings, which may mimic carcinomatosis on CT (Table 2). Clinical history and extra peritoneal findings of the systemic disease can often suggest the underlying pathology.

Table 2.

Clinical and imaging clues to the diagnosis of systemic and multiorgan conditions mimicking peritoneal carcinomatosis.

Disease entity	Clinical clues	Imaging clues
Amyloidosis	History of plasma cell dyscrasia or chronic inflammatory conditions.
IgG4-related disease	Elevated IgG4 levels	Concurrent pancreatic, renal, biliary or retroperitoneal fibrosis.
Sarcoidosis		Concurrent pulmonary findings.
Eosinophilic peritonitis	Peripheral eosinophilia. Associated with atopic and allergic conditions.
Erdheim-Chester disease	Middle-age or older adults	Concurrent skeletal, perirenal and pericardial findings.
Celiac disease (mesenteric cavitary lymph node syndrome)	Previously diagnosed celiac disease.	Enlarged mesenteric lymph nodes with fat-fluid level. Associated splenic atrophy.

Amyloidosis

Amyloidosis is a constellation of rare diseases characterized by the extracellular deposition of insoluble fibrillar proteins.⁴¹ The pathological deposition of these amyloid proteins can be primary, as seen with immunoglobulin light chain [AL] amyloidosis, or secondary to chronic disorders (reactive [AA] amyloidosis).⁴² Involvement of the gastrointestinal tract in amyloidosis is relatively common, but peritoneal localization is rare.⁴³ Whether localized or diffuse, peritoneal amyloidosis is usually clinically silent. CT findings include nodular or infiltrative peritoneal-based soft tissue, often with associated coarse dystrophic calcification and ascites (Figure 5).^44,45 The diagnosis can be confirmed by demonstrating the pathognomonic apple-green birefringence under polarized microscopy of biopsied tissues with Congo red staining.⁴²

Figure 5.

Systemic conditions that can mimic peritoneal carcinomatosis. (A, B) Amyloidosis. Contrast-enhanced CT in an asymptomatic 73-year-old male (A) with primary systemic amyloidosis shows multifocal mesenteric masses containing areas of dystrophic calcification. The size of the lesions had increased only slightly over a period of 5 years. Contrast-enhanced in 55-year-old male (B) also with light chain [AL] amyloidosis shows more infiltrative peritoneal-based soft tissue, without calcification. (C) IgG4-related disease. Contrast-enhanced CT in 60-year-old female who presented to ED with abdominal pain shows mesenteric soft tissue infiltration along an SMA branch, which was initially felt to represent mesenteric hemorrhage from SAM. However, subsequent pre- and post-contrast imaging demonstrated enhancement (not shown) and IgG4 levels were found to be elevated. (D) Sarcoidosis. Contrast-enhanced CT in 33-year-old female with increased abdominal distention, nausea, and vomiting shows non-specific ascites and diffuse peritoneal soft-tissue infiltration, most notably involving the omentum. SAM, segmental arterial mediolysis

IgG4-related disease

Immunoglobulin G4 (IgG4)-related disease is a recently recognized systemic sclerosing condition characterized by a dense lymphoplasmacytic fibrotic infiltrate that is rich in IgG4-producing plasma cells.⁴⁶ IgG4-related disease now ties together a number of seemingly heterogeneous fibrosing condition with similar pathogenesis such as autoimmune pancreatitis, Riedel thyroiditis, retroperitoneal fibrosis, and sclerosing cholangitis.⁴⁷ As noted above, IgG4-related disease is also associated with inflammatory pseudotumors. Peritoneal or mesenteric involvement with IgG4-related disease is rare, manifesting at CT with non-specific soft tissue infiltration or nodularity (Figure 5).^48,49 Concurrent pancreatic, renal, biliary, or retroperitoneal findings on CT could provide a clue to the diagnosis.

Sarcoidosis

Sarcoidosis is a relatively common multisystem inflammatory disease characterized by the formation of non-caseating granulomas in the absence of other autoimmune conditions, toxic agents, or infection.⁵⁰ The lungs are most frequently affected (90% of cases), followed by the lymphoid system (30% of cases), but sarcoidosis can affect virtually any organ. Extrapulmonary involvement has been reported in nearly one-third of patients, with the abdomen being the most common site. Abdominal sarcoidosis usually affects the liver, spleen, and abdominal lymph nodes, in the presence or absence of pulmonary disease, whereas peritoneal involvement is rare.⁵¹ Imaging findings of peritoneal sarcoidosis are non-specific and include ascites, and peritoneal thickening, most typically involving the omentum (Figure 5). Increased activity on¹⁸F-fludeoxyglycose positron emission tomography (FDG-PET) makes exclusion of malignancy challenging.⁵² Imaging findings can also mimic tuberculosis. The diagnosis of peritoneal sarcoidosis can be suggested in the presence of typical pulmonary (and other abdominal) findings. Tissue biopsy is generally required for confirmation, and for exclusion of other pathology.⁵⁰

Eosinophilic peritonitis

Eosinophilic gastrointestinal disorders represent a spectrum of uncommon inflammatory diseases with characteristic eosinophilic infiltration of the bowel wall, in the absence of other causes of tissue eosinophilia.⁵³ Peripheral eosinophilia is common, along with a strong association of atopic and allergic conditions.⁵⁴ Eosinophilic gastroenteritis can be classified based on the degree of infiltration into the mucosal, muscular, and serosal layers. Predominant or isolated serosal involvement, termed eosinophilic peritonitis, is the least common subtype and typically presents with isolated ascites assoicated with marked eosinophilia (Figure 6).⁵³ However, additional imaging findings can include peritoneal-based serosal bowel wall thickening, omental and mesenteric infiltration, and lymphadenopathy (Figure 6).⁵⁵ Eosinophilia of the fluid at paracentesis or peritoneal soft-tissue biopsy, along with peripheral eosinophilia or atopic history, are the main clues to diagnosis.

Figure 6.

Systemic conditions that can mimic peritoneal carcinomatosis. (A, B) Eosinophilic peritonitis. Contrast-enhanced CT images in symptomatic 55-year-old male with unexplained intractable ascites show diffuse peritoneal-based soft tissue infiltration predominantly involving the omentum. No mucosal abnormality was seen at panendoscopy, but peripheral eosinophilia was present. The diagnosis was confirmed by omental biopsy. (C) Erdheim-Chester disease. Contrast-enhanced CT in 41-year-old male shows diffuse omental soft tissue infiltration, in addition to the more characteristic perirenal soft tissue infiltration (arrows). (D) Celiac disease (mesenteric cavitary lymph node syndrome). Contrast-enhanced CT in 83-year-old female with a 3 month history of diarrhea and weight loss show enlarged mesenteric lymph nodes with characteristic fat-fluid levels (arrows). (D from Pickhardt Bhallaand,⁴ with permission)

Erdheim-Chester disease

Erdheim-Chester disease (ECD) is a rare subtype of non-Langerhans histiocytosis, typically affecting middle-age and older adults.⁵⁶ ECD typically affects the skeletal system, presenting as symmetrical osteosclerosis of long bones and focal pseudotumoral lesions. Findings of extra skeletal involvement at CT are common, including perirenal and pericardial manifestations, which are each involved in about 70% of cases.⁵⁷ Peritoneal disease is much less frequent, demonstrating non-specific peritoneal-based soft tissue infiltration or thickening of the mesentery, peritoneum and omentum (Figure 6).³⁴ This unusual diagnosis can be favored at abdominal CT in adults when associated with the characteristic perirenal and skeletal findings.

Celiac disease

Celiac disease or gluten-sensitive enteropathy, previously referred to as celiac sprue, is a common autoimmune disorder causing malabsorption with predominant intestinal findings, but also extra intestinal manifestations.⁵⁸ Common radiological findings of untreated disease include jejunal fold loss and luminal dilation; additional intestinal findings may include excess luminal fluid, slow transit, transient intussusception, and intramural fat deposition.⁵⁹ Prominent reactive mesenteric lymph nodes are a common but non-specific finding at abdominal CT. In contrast, the mesenteric cavitary lymph node syndrome is a rare but ominous complication of celiac disease, with a high mortality rate. Fat attenuation of enlarged mesenteric lymph nodes is readily identified on CT, and a pathognomonic fat-fluid level may even be appreciated (Figure 6).⁶⁰ Splenic atrophy, considered by some to be a component of this clinical syndrome, may be an associated findings.⁶¹

Atypical infections

There are various atypical infections that can mimic peritoneal carcinomatosis at CT, some of which have characteristic imaging features (Table 3). Broad categories include bacterial, fungal, and parasitic agents, most of which more commonly cause disease in immunocompromised patients.

Table 3.

Clinical and imaging clues to the diagnosis of atypical infections mimicking peritoneal carcinomatosis.

Pathology	Clinical clues	Imaging clues
Bacterial infections:
Tuberculosis	Associated with pulmonary tuberculosis in 15% of cases.	Associated ileal involvement, coned cecum, or low attenuation lymphadenopathy.
Actinomycosis	Long-term use of contraceptive IUD.	Transgression of anatomic tissue planes.
Fungal infections	Immunocompromised states such as advanced HIV/AIDS.
Parasitic infections:
Echinococcosis	Associated with hepatic or splenic hydatid disease.	Well-defined cystic lesions with mural calcifications.
Paragonimiasis		Concurrent pulmonary involvement.
Fascioliasis		Concurrent hepatobiliary involvement.
Sparganosis		Concurrent involvement of the muscle and subcutaneous tissues.

IUD, intrauterine device.

Bacterial causes

Peritoneal tuberculosis is an infrequent manifestation of this common worldwide infection, accounting for less than 1% of all cases.⁶² Perhaps surprising is that concomitant pulmonary disease is present in less than 15% of patients with tuberculous peritonitis.⁶³ It has been classified into three subtypesat CT, with significant overlap: (1) the wet ascitic type (most common), associated with abundant free or loculatedhigh-attenuation ascites; (2) the fixed-fibrotic type (less common), associated with omental and mesenteric involvement, as well as bowel adhesions; and (3) the dry-plastic type (least common), characterized by dense fibrotic peritoneal thickening, associated with adhesions and caseous nodules (Figure 7).⁶⁴ Affected lymph nodes often demonstrate central low-attenuation from caseous necrosis, with occasional calcifications.⁶⁵ Peritoneal thickening is associated with all three subtypes of peritoneal tuberculosis, thus simulating malignancy, although it tends to be more smooth and uniform than most cases of carcinomatosis. Associated ileal involvement, coned cecum, or low-attenuation lymphadenopathy may help suggest the diagnosis (Figure 7).

Figure 7.

Atypical bacterial infections that can mimic peritoneal carcinomatosis. A–D Peritoneal TB. Contrast-enhanced CT scans in four different patients with peritoneal TB show various wet (B) and dry (C) variations, as well as ileal thickening and adjacent coned cecum (A) and low-attenuation lymphadenopathy (D). (E) Actinomycosis. Coronal image from contrast-enhanced CT in 48-year-old female shows diffuse peritoneal thickening and enhancement, as well as ascites and an IUD, which had been in place for 23 years. (C and D from Pickhardt Bhalla⁴and, with permission). IUD, intrauterine device; TB, tuberculosis.

Abdominal actinomycosis is an uncommon subacute or chronic infection.⁶⁶ Despite the well-known association with long-term use of contraceptive intrauterine devices (IUDs) in females, abdominal actinomycosis is actually three times more common in males.⁶⁷ Actinomycosis demonstrates a variable appearance on imaging, often reflecting an aggressive inflammatory response with a predilection for tissue invasion (Figure 7).⁶⁸ Uni- or multifocal areas of enhancing soft tissue infiltration, a complex solid and cystic appearance, and associated bowel wall thickening are typical CT findings.^67,68 Transgression of anatomic tissue planes that often limit other etiologies may suggest actinomycosis. However, in the absence of a longstanding IUD (Figure 7), a specific imaging diagnosis can be challenging.

Fungal causes

Peritoneal dissemination is an uncommon manifestation of fungal infection, and generally affects immunocompromised patients. Examples include coccidioidomycosis, cryptococcosis, histoplasmosis, and candidiasis; the CT features are non-specific and mimic carcinomatosis.⁶⁹ Coccidioidomycosis is endemic to the southwestern United States, and usually presents as a respiratory infection.⁷⁰ Peritoneal dissemination is rare but can closely mimic carcinomoatosis at CT (Figure 8).^71,72 In the absence of a concomitant pulmonary infection, the diagnosis is usually retrospective after tissue culture and histology. Similarly, cryptococcal peritonitis is a rare manifestation of a disease that is generally limited to immunocompromised patients, particularly those with advanced HIV/AIDS. The CT findings are again non-specific (Figure 8). Disseminated histoplasmosis is also associated with advanced HIV/AIDS, although it too can affect other severely immunocompromised patients. Retroperitoneal lymphadenopathy is more common than mesenteric involvement, and may be associated with hepatosplenomegaly.⁷³

Figure 8.

Fungal and parasitic infections that can mimic peritoneal carcinomatosis. (A) Coccidiodomycosis. Contrast-enhanced CT in 40-year-old male with Type 1 diabetes mellitus shows non-specific peritoneal findings that mimic carcinomatosis. Peritoneal coccidiodomycosiswas proven at surgery. (B) Cryptococcosis. Contrast-enhanced CT in immunocompromised male shows ascites and diffuse peritoneal thickening. (C) Candidiasis. Contrast-enhanced CT image in 30-year-old female with right lower quadrant pancreas transplant (arrow) shows diffuse peritoneal thickening surrounding the transplant that proved to be disseminated Candida infection at biopsy. (D) Echinococcosis. Coronal CT image in 33-year-old Mexican immigrant with recently diagnosed colon cancer shows concurrent evidence of peritoneal hydatid disease from E. granulosus, with innumerable serosal hepatic implants, many of which show rim calcification. (D from Orlowski et al⁶⁹, with permission)

Parasitic causes

Hydatid disease of the peritoneum, or peritoneal echinococcosis, generally results from rupture of hepatic or splenic involvement.⁷⁴ Cross-sectional imaging allows for the peritoneal delineation of single or multiple well-defined thick-walled cystic lesions.⁷⁵ Mural calcification is a distinguishing feature of hydatid disease and should suggest the diagnosis, particularly in the presence of hepatic disease or demographic clues (Figure 8).⁷⁶

Some parasitic infestations penetrate through the gastrointestinal tract and into the peritoneumen route to their final destination.⁷⁷ In the case of paragonimiasis, the end organ is the lung, whereas for fascioliasis and sparganosis, the ultimate stops are the hepatobiliary system and muscle/subcutaneous tissues, respectively. During peritoneal migration, these parasites can incite an inflammatory reaction that may manifest as a nonspecific finding at abdominal CT.

Fat-based conditions

A spectrum of benign vascular and inflammatory conditions are associated with intraperitoneal fat, which allows for a confidents diagnosis at abdominal CT (Table 4). These include entities characterized by infarction and necrosis, such as epiploic appendagitis and omental infarction, and the common inflammatory appearance of mesenteric panniculitis. Clear recognition of these benign and generally self-limited conditions is important to avoid confusion with more clinically significant disease, such as liposarcoma. Two other characteristic benign fat-based conditions, ruptured teratoma (dermoid) and the growing teratoma syndrome, will not be discussed further since they derive from a neoplastic entity.

Table 4.

Clinical and imaging clues to the diagnosis of fat-based conditions mimicking peritoneal carcinomatosis.

Pathology	Clinical clues	Imaging clues
Epiploic appendagitis		Well-defined ovoid fat-attenuation lesion with visible central thrombosed vessel. Eccentric colonic wall thickening.
Omental infarction		Encapsulated heterogeneous mass containing fat, fluid and/or soft tissue.
Mesenteric fat necrosis	History of acute pancreatitis, trauma, or abdominal surgery.	Evolution of CT findings over time.
Mesenteric panniculitis		Involvement of the jejunal mesenteric fat. Sparing of the perivascular and peri-nodal fat.

Epiploic appendagitis

The epiploic appendages are normal elongated pedunculated outpouchings of fat surrounded by visceral peritoneum that hang off the antimesenteric serosal surface of the colon.⁷⁸ They are typically not distinguishable from adjacent peritoneal fat unless infarcted or surrounded by free peritoneal fluid. Ischemic or hemorrhagic infarction of these fatty appendages, from either torsion along their pedicles or venous thrombosis, results in symptomatic epiploic appendagitis. On CT, epiploic appendagitis appears as a well-circumscribed ovoid fat-attenuation lesion encased by a soft tissue rim of thickened peritoneum, often with a visible thrombosed vessel centrally (Figure 9).⁷⁹ Adjacent fascial thickening and hazy stranding are common associated findings; adjacent eccentric colonic wall thickening can also be seen. Although epiploic appendagitis often results in acute pain, it is a self-limited condition that typically requires no treatment.⁸⁰ It is important to distinguish this entity from acute diverticulitis, which is managed differently. Not infrequently, chronically infarcted epiploic appendages may transform into fibrotic or calcified nodules on imaging. These torsed appendages may become amputated and free within the peritoneal cavity (“pearls”), which are generally obvious at CT BUT can mimic lymphadenopathy at MR (Figure 9). Amputated appendages may also give rise to so-called pseudolipomas of Glisson’s capsule over the liver.

Figure 9.

Epiploic appendagitis. (A) Contrast-enhanced CT shows typical appearance of acute epiploic appendagitis, with the ovoid fatty appendage (arrow) surrounded by localized peritoneal thickening, hazy stranding, and mild wall thickening of the adjacent sigmoid colon. (B, C) Coronal CT image (B) in another case, this time involving an infarcted appendage (arrow) off the descending colon. Due to lingering symptoms, the patient underwent surgical resection (C), which is rarely necessary for this typically self-limited condition. (D–F) Coronal CT (D) and transverse T₂ weighted MR (E) images in a patient with rectal cancer (asterisks) show a rim-calcified “pearl” (arrow) at the inferior peritoneal reflection, which is easily recognized at CT, but mimics a perirectal lymph node at MR staging. The loose amputated epiploic appendage was confirmed at laparoscopic low anterior resection (F).

Omental infarction

Omental infarction is an uncommon event that predominantly involves the right side (or segment) of the omentum, presumably due to its more delicate blood supply.⁸¹ The clinical presentation of right-sided omental segmental infarction often mimics acute appendicitis. Left-sided omental infarctions are less frequent but do occur, especially in the setting of surgical trauma or post-operative changes. Although the severity of findings varies considerably on CT, the typical appearance is that of a relatively large, encapsulated heterogeneous omental mass containing an admixture of fat, fluid, and soft tissue (Figure 10).⁸² As with epiploic appendagitis, reactive thickening of the adjacent bowel is sometimes present. As with other benign fat-based conditions, omental infarction should not be confused for liposarcoma, and is self-limited without the need for intervention.

Figure 10.

Omental and mesenteric fat infarction. (A–C) Omental infarction. CT images from three different patients show the typical right-sided segmental omental infarction in two patients (A and B), and an example of the less common left-sided infarction (C), with associated fat-fluid level. (D–F) Post-traumatic mesenteric fat necrosis. Initial CT (D) for blunt trauma in 52-year-old male shows findings of bowel (arrows) and mesenteric injury. Subsequent CT (E) 1 month after exploratory laparotomy with extensive bowel repair and resection shows an area of evolving mesenteric fat necrosis (arrow). Extensive saponification of intra-abdominal fat was also noted at re-exploration. CT year later (F) shows more mature fat necrosis, now with fat-fluid levels.

Mesenteric fat necrosis

Beyond the characteristic named fat necrosis entities associated with epiploic appendagitis and omental infarction, other foci of intra-abdominal fat may become infarcted. Acute pancreatitis, trauma, and abdominal surgery are common causes. On CT, this benign condition may mimic malignancy early on, as initial saponification can result in a complex appearance (Figure 10). Over time, however, the CT appearance becomes more characteristic and reassuring (Figure 10).⁸¹ Correlation with clinical history can be important to avoid misdiagnosis in these cases.

Mesenteric panniculitis

Mesenteric panniculitis is a common incidental finding at abdominal CT. This poorly understood self-limited fibroinflammatory condition typically involves the jejunal mesentery is almost always subclinical in nature.⁸³ Some consider this to be the acute phase of sclerosing mesenteritis, but progression to retractile mesenteritis at the other end of the fibroinflammatory spectrumis very rare.⁴ Some have even posited an association between mesenteric panniculitis and malignancy.⁸⁴ Characteristic CT findings, which vary in degree, include hazy infiltration and fullness of the jejunal mesenteric fat, with surrounding peritoneal thickening that creates a pseudocapsule appearance (Figure 11).⁸⁵ A halo that partially spares the perivascular and perinodal fatis also a characteristic feature (Figure 11). At CT evaluation, this benign condition should be differentiated from other causes of the so-called “misty mesentery”.⁸⁶

Figure 11.

Mesenteric panniculitis. (A, B) Contrast-enhanced CT in two different patients show hazy infiltration of the typical jejunal mesenteric leaf, with associated mass effect and pseudocapsule. (C, D) Screening CTC in asymptomatic 53-year-old female shows findings of incidental mesenteric panniculitis, which resolved by the time of follow-up CTC 8 years later (D). CTC, CT colonography.

Conclusion

A broad and fascinating spectrum of uncommon non-neoplastic entities can involve the peritoneum, often manifesting with a relatively non-specific appearance at CT. As such, many of these conditions can be initially misinterpreted to represent peritoneal carcinomatosis or primary malignancy. Fortunately, for many of these entities there are specific clues from the patient history or additional CT imaging findings that may allow for a specific benign diagnosis. Awareness of these details can avoid common pitfalls, may avoid the need for biopsy for tissue diagnosis, and will generally lead to better patient care.