Benign Sclerosing and Fibrosing Conditions of the Abdomen and Their Potential Mimics

PSC is a chronic, idiopathic, fibrosing, and stricturing disease of the biliary tree. There is a strong association with other autoimmune-mediated conditions such as inflammatory bowel disease, particularly ulcerative colitis, RPF, mediastinal fibrosis, and Sjogren's syndrome.[ 24 ] The mean age at presentation is 40 and is twice as common in men.[ 24 ] US will often show intra- and extra-hepatic duct dilatation with a varied duct caliber [ Figure 14 ]. In advanced disease, the liver will appear cirrhotic often with caudate lobe hypertrophy. CT will also show intrahepatic duct dilation and evidence of cirrhosis. Selected ultrasound image and thick slab maximum intensity projection magnetic resonance cholangiopancreatography image showing the dilated left-sided intra-hepatic ducts with evidence of beading in patient with primary sclerosing cholangitis. Cholangiography is the gold standard for diagnosis of PSC.[ 24 ] Both endoscopic retrograde cholangiopancreatography (ERCP) and MRCP will show multifocal intrahepatic bile duct strictures interspersed with normal caliber or slightly dilated bile ducts resulting in a “beaded” appearance[ 24 ] [ Figure 14 ]. Other reported findings include biliary webs and diverticula. Later, in the disease process, bile duct diseases become obliterated in the periphery resulting in a “pruned-tree” appearance.[ 24 ] ERCP has the advantage of interventional capabilities including biopsy, stent placement, and stricture dilatation but also carries the significant risks of cholangitis, pancreatitis, and perforation.[ 24 ] MRCP, on the other hand, is noninvasive and allows the assessment of other organs such as the pancreas and thus is the first-line cholangiographic technique. Especially, in the early disease, PSC can resemble cholangiocarcinoma.[ 25 ] Cholangiocarcinoma also occurs in 10%–15% of PSC patients.[ 25 ] It should be considered clinically if there is a rapid decline in LFTs and/or weight loss. On MRI, periductal high signal on T2-weighted imaging (periportal edema) and periductal soft tissue (>1 cm) with delayed and persistent enhancement suggest cholangiocarcinoma.[ 24 ] Rapid progression of focal strictures on follow-up imaging in PSC should also raise the suspicion of transformation to cholangiocarcinoma. Other imaging features more suggestive of cholangiocarcinoma include shouldered, high-grade strictures, liver capsule retraction, and marked bile duct dilatation.[ 25 ] IgG4-related sclerosing disease can result in multiple extrahepatic strictures mimicking PSC and cholangiocarcinoma,[ 3 5 ] thus careful histological analysis is recommended. There are a number of other causes of secondary sclerosing cholangitis, which share many imaging features and should be excluded before a diagnosis of PSC is made [ Table 5 ]. Causes of secondary sclerosing cholangitis Mesenteric fibromatoses are a group of fibroproliferative processes that are benign but can be locally aggressive. They are also known as desmoid tumors due to their tendon-like appearance histologically. In this review, we will focus on mesenteric fibromatosis, which can be superficial or deep. Intra-abdominal fibromatosis can infiltrate local organs and reoccur after resection, but it does not metastasize.[ 15 26 ] The majority of cases occur sporadically and have no gender predilection with a wide age range at presentation. Thirteen percent of patients have the Gardner syndrome variant of familial adenomatous polyposis.[ 15 ] An additional risk factor in these patients is previous surgery usually developing within 4 years of the first surgical procedure.[ 15 ] Abdominal fibromas contain a mix of collagenous tissue and myxoid stroma, thus may have varied image characteristics depending on the predominant tissue type.[ 15 ] US performed for nonspecific abdominal pain may reveal homogeneously anechoic or hypoechoic masses. Cross-sectional imaging is preferred for the preoperative assessment of mesenteric masses, but imaging characteristics are variable due to varied histological components [ Figure 15 ]. The mass is often homogenous and isoattenuating to muscle. If mixed then a striated or “whorled” texture is seen. In postintravenous contrast administration, there is mild-to-moderate enhancement depending on vascularity. Lesions with a predominant myxoid histology are classically hypoattenuating and do not enhance postcontrast.[ 15 ] Axial and coronal contrast enhanced computed tomography showing homogenous mass in the root of mesentery which is isoattenuating to muscle and shows minimal contrast enhancement (yellow arrow). Surgical biopsy confirmed desmoid tumor. At MRI, most lesions are low or intermediate signal intensity on T1. T2 characteristics are more variable with heterogeneous intermediate/high signal with bands of low signal. If there is predominantly high T2 signal, the lesion is likely to be predominantly made of myxoid stroma. On postcontrast MRI, most lesions will show variable enhancement much like CT.[ 15 ] In general, malignant disease of the mesentery can mimic mesenteric fibromatosis. More specifically, lymphoma can present with a well-marginated lesion of homogenous attenuation involving the small bowel. Gastrointestinal stromal tumors of the small bowel can have extensive mesenteric involvement and occur as a primary mesenteric mass mimicking benign fibromatosis both radiologically and histopathologically.[ 15 27 ] Benign mimics include mesenteric hemangioma, which may have a similar appearance to myxoid mesenteric fibromatoses. They are predominantly hypoattenuating to muscle. Hemangiomas often have prominent peripheral vessels that help in differentiating from other benign fibromatoses [ Figure 16 ]. Coronal contrast-enhanced computed tomography image showing a low-attenuation lesion with multiple prominent peripheral veins in keeping with a mesenteric hemangioma. Other differentials for mesenteric deposits include endometriosis, soft-tissue sarcomas, RPF, and ITPs.

Sclerosing peritonitis (SP) is a chronic fibrotic thickening of the peritoneum. When severe, it can progress to envelop and encapsulate small-bowel loops in what has been called an “abdominal cocoon” or sclerosing encapsulating peritonitis (SEP).[ 28 29 ] There is a strong association with patients receiving continuous ambulatory peritoneal dialysis (CAPD). Although rare among the entire population of patients on CAPD with an incidence of 0.7%, this rises to 19.4% in patients on CAPD for greater than 8 years.[ 30 ] Etiology, although unclear, is likely to be a result of chronic inflammation or infection. Other factors linked with SP include ventriculoperitoneal shunts, granulomatous disease, malignancy, familial Mediterranean fever, liver transplantation, abdominal surgery, protein S deficiency, luteinizing ovarian thecoma, and the beta-blocker practolol.[ 30 31 ] Abdominal radiography may show focal or linear calcifications. In advanced stages, there may be pockets of small-bowel dilatation [ Figure 17a ]. Sclerosing peritonitis. (a) Abdominal radiograph in patient with known sclerosing peritonitis showing calcification in the right lower quadrant (red circle) with a paucity of bowel gas and gas filled bowel loops in the left upper quadrant. Note the JJ stent in the transplant ureter (yellow arrows). In advanced stages there may be pockets of small bowel dilatation. (b) Selected ultrasound image showing a complex loculated intra-abdominal collection in the subhepatic region with thick septations (yellow arrows) in a patient with encapsulated sclerosing peritonitis. (c) Coronal contrast-enhanced computed tomography selected image of the same patient in picture B, showing smooth, calcified thickening of the peritoneum. (d) Axial contrast-enhanced computed tomography images from a patient with a history of peritoneal dialysis showing smooth, calcified thickening of the peritoneum with encapsulation of the small bowel loops with multiple fluid collections. Note the transplant kidney in the right iliac fossa. US scan findings are not specific but will show loculated fluid collections. It may also show trilaminar small-bowel wall thickening and foci of calcification[ 28 ] [ Figure 17b ]. CT shows smooth or nodular thickening of the peritoneum that can enhance postcontrast.[ 31 ] It eventually progresses to envelop the small-bowel loops with focal areas of upstream intestinal dilatation [Figures 17c and d ]. The SEP form of the disease is classically widely and densely calcified. Patients on CAPD often have free intra-abdominal fluid, so the presence of ascites is nonpecific. However, loculated fluid collections trapped within the fibrotic, calcified peritoneum are often described in SEP [Figure 17c and d ], and are seen to persist in patients who are no longer on CAPD. Peritoneal calcification and loculated fluid collections can be seen in tuberculosis and peritoneal carcinomatosis, specifically pseudomyxoma. Spontaneous bacterial peritonitis can show smooth peritoneal thickening and bowel encapsulation mimicking SEP.[ 31 ]

There is uncertainty and disparity in the literature regarding the understanding and nomenclature of mesenteric lipodystrophy, mesenteric panniculitis (MP), and sclerosing mesenteritis (SM). In the past, the authors have described SM as the end-point of a single disease process progressing from mesenteric lipodystrophy to panniculitis and then mesenteric sclerosis. There is little or no evidence to support this theory, and there is a growing consensus that these processes are separate entities that share some histopathological and imaging features. However, there are imaging features specific to MP and SM enabling confident distinction on imaging.[ 17 ] This is an important distinction to make as MP, and “misty mesentery” is a common finding that requires limited or no further imaging or follow-up.[ 17 18 19 ] The US findings are nonspecific, but echogenic mesenteric fat or lymphadenopathy with the root of the mesentery and with or without mass effect can be seen. Mesenteric panniculitis has been suggested to be present in up to 3% of the abdominal CTs by some authors[ 19 ] and is, therefore, often discovered as an incidental finding. Appearances on CT are predominantly inflammatory and vary in severity from ill-defined hyperattenuating mesenteric fat? (-60 to -40 Hounsfield Units) similar to ground-glass opacification seen in the chest imaging, also known as “misty mesentery” to a well-demarcated lesion causing local mass effect[ 20 21 ] [Figures 10 and 11 ]. The soft tissue or mixed fat and soft-tissue mesenteric root mass may or may not have surrounding fluid density secondary to lymphatic or venous obstruction. The presence of a capsule-like soft-tissue attenuation surrounding the lesion called a “pseudocapsule” has been described in 60% of cases.[ 19 22 ] Surrounding fluid, a soft-tissue rim, or apparent mass increase the likelihood of need for biopsy. Axial and coronal contrast-enhanced computed tomography image of early mesenteric panniculitis. There is an increased attenuation (fat stranding) within the bowel mesentery with sparing of the fat surrounding mesenteric vessels and small lymph nodes (yellow arrows). The fat stranding is relatively well circumscribed and limited to the mesentery. Axial contrast-enhanced computed tomography image of mesenteric panniculitis of indeterminate cause. The features resolved on follow-up computed tomography. The process is enveloping mesenteric vessels and local lymph nodes with preservation of a ring of fat around the vessels and nodes called the “fat halo” or “fat ring” sign which is fairly specific for mesenteric panniculitis and can help differentiate from sclerosing mesenteritis and other mimics. The process envelops mesenteric vessels and local lymph nodes with preservation of a ring of fat around the vessels and nodes called the “fat halo” or “fat ring” sign which is fairly specific for MP and can help differentiate from SM and other mimics[ 15 19 20 21 ] [ Figure 11 ]. There are often minimally enlarged lymph nodes related to inflammatory MP, which have been described as an additional supportive finding in MP.[ 17 ] These nodes are homogeneous in appearance and connected along a chain. There is sparse literature of the MR findings of in MP. Many of the findings on CT such as the “fat halo” sign translate to MR. Other findings such as diffuse-increased T2-signal intensity of the affected mesenteric fat and postcontrast enhancement have been recently described.[ 17 ] Misty mesentery is nonspecific and is also seen in mesenteric hemorrhage, edema, lymphoma, and metastatic disease.[ 14 15 ] A mesenteric root soft-tissue mass commonly seen in MP can be mimicked by a number of malignant conditions including lymphoma and RPF. However, if the fairly specific imaging features of ground-glass changes in the mesenteric fat, with preservation of the perivascular/perilymphatic soft tissue and connected homogenous minimally enlarged lymph nodes, are present, a confident diagnosis can be made on imaging alone. SM is an idiopathic disease process characterized by chronic inflammation, fat necrosis and fibrotic thickening of the small bowel mesentery and mesocolon, shortening and kinking the mesentery.[ 15 ] It is often associated with other idiopathic inflammatory disorders including RPF, Riedel's thyroiditis, and pseudotumor and have been linked with prior abdominal surgery. The disorder occurs more frequently in men with an average age of presentation of 60 years.[ 15 20 ] The condition is now considered separate to MP by a number of authors and has more severe inflammation, imaging findings, and clinical symptoms.[ 17 ] CT is the investigation of choice in the assessment of SM and shows ill-defined soft tissue extending within and into the mesentery [ Figure 12 ]. Infiltration of the porta hepatis and retroperitoneal extension has also been described. Axial contrast-enhanced computed tomography image showing a mesenteric root, ill-defined soft-tissue mass extending within and into the mesentery. Unlike MP, the soft tissue will encircle and encase the mesenteric vessels without preservation of fat. It may also involve bowel loops [ Figure 12 ]. Severe cases of retractile SM have been shown to cause bowel obstruction and ischemia.[ 21 ] Calcification of the mass lesion is a rare but recognized finding affecting areas of fat necrosis[ 20 ] [ Figure 12 ]. MRI features of SM predominantly fit the pattern expected for fibrosis low T1- and T2-signal intensity mesenteric root soft tissue, but high T2-signal intensity has been described.[ 15 ] Histopathological analysis is necessary for definitive diagnosis of SM due to the number of important mimics. Neuroendocrine tumors, specifically carcinoid of the ileum and jejunum, often have mesenteric lymph node involvement. The desmoplastic reaction is an important mimic of the retractile type of SM. Infiltration rather than encasement of local vessels is more consistent with carcinoid [ Figure 13 ]. Differentiation of these conditions is often impossible on CT alone.[ 20 ] Tumor markers and a positive somatostatin receptor scintigraphy scan (indium-111 pentetreotide) are specific for carcinoid tumor and a helpful imaging tool to differentiate the two,[ 2 ] but biopsy is often still required. Coronal contrast-enhanced computed tomography image showing a heterogeneously enhancing mass within the root of the mesentery with surrounding satellite nodules, in-drawing bowel loops, and obliterating the superior mesenteric vein (red circle). Histopathology confirmed carcinoid. Other mimics of SM include treated lymphoma, mesenteric carcinomatosis, primary mesenteric mesothelioma, and lipogenic liposarcoma.[ 20 ] The presence of large discrete nodes will point to a malignant cause[ 20 ] and calcification suggests carcinoid tumor.[ 23 ] Although this is not always the case as both SM and carcinoid metastases can show calcification [ Figure 12 ].

The process of abnormal reparative or reactive processes can lead to sclerosis and fibrous deposition producing characteristic and shared imaging features [ Table 1 ]. Benign sclerosing and fibrosing diseases of the abdomen are relatively rare and can be broadly categorized into two main groups idiopathic/immune-mediated and iatrogenic conditions. The group of immune-mediated fibrosing conditions can be further subdivided into those conditions that have been linked to immunoglobulin (Ig) G4 and those that do not have any current link to IgG4. Multimodality shared imaging features of fibrosing conditions The relatively recent discovery of an IgG4 subgroup of immune-mediated sclerosing disease[ 1 2 ] has thrown some light on the pathophysiology of these conditions. Despite this, our understanding of this heterogeneous group of diseases is constantly evolving. There remains uncertainty and disparity in the imaging features and nomenclature used to describe and categorize these benign sclerosing and fibrosing abdominal conditions. As such, there is an understandable reliance on histopathology for a definitive diagnosis[ 3 ] as a number of malignant and infectious/inflammatory intra-abdominal processes can mimic the imaging features of benign sclerosing and fibrotic diseases within the abdomen [ Table 2 ]. Common conditions that can mimic fibrosing conditions of the abdomen We present a pictorial review that aims to describe and clarify imaging findings to enhance the general radiologist's recognition and interpretation of this varied group of benign sclerotic and fibrotic abdominal processes. Some of these findings are shared with inflammatory and malignant processes and require further investigation. Other imaging features are more specific and recognition may help differentiate benign from malignant or infective processes [ Table 3 ]. Cross-modality imaging features to aide differentiation of benign and malignant retroperitoneal fibrosis

Benign fibrosing diseases of the abdomen are a relatively rare, varied, and complex group of conditions that have both overlapping and more specific imaging features. It is important to recognize that there are a number of malignant and infective/inflammatory processes that can also mimic these benign conditions. To ensure a correct diagnosis, thorough clinical and histopathological assessment is required to support the imaging findings presented in this review. Nil. There are no conflicts of interest.