Inflammatory pseudotumor of the omentum in contrast-enhanced ultrasound

The term “inflammatory pseudotumor” (IPT) includes a variety of rare neoplastic and reactive lesions that share a similar histological appearance. IPT embraces inflammatory myofibroblastic tumor (IMT), pseudosarcomatous myofibroblastic proliferations of the bladder (PMP), postinfectious lesions, and IPT of the lymphatic tissue of the orbit. Histologically, it is characterized by myofibroblastic cell proliferation and mixed inflammation cells⁽¹⁾. Due to its rarity the knowledge of diagnostic and therapeutic procedures in IPT is limited. Radiographic descriptions, based on case reports and small sample studies, are both nonspecific and heterogeneous⁽²⁾. The therapeutic concepts vary from conservative approaches to immunosuppressive therapy and surgery⁽³⁾. Despite the above considerations, IPT represents an important benign differential diagnostic option when considering a malignant disorder. This case report describes the contrast-enhanced ultrasound (CEUS) pattern in IPT of the omentum.

A 71-year-old male patient was admitted to the hospital with right upper quadrant abdominal pain for three days. There were no other symptoms including nausea, vomiting, change in bowel habits, night sweats, fever or weight loss.

The patient had a history of coronary heart disease, type 2 diabetes mellitus with diabetic foot syndrome, obesity (body mass index 30.4 kg/m², 104 kg) and nicotine consumption (40 pack-years). A cholecystectomy was performed three years previously.

The physical examination revealed normal vital parameters and normal body temperature. No abnormalities were identified apart from right-sided subcostal pain on palpation.

The blood tests were normal except for an elevated CRP level – 88 mg/l (normal <5 mg/l) and slightly elevated bilirubin – 1.44 mg/dl (0.1–1.2 mg/dl). Blood cultures remained negative. A chest X-ray and urine sample showed no signs indicative of an inflammatory focus.

B-mode ultrasound (US) examination revealed a subhepatic retroperitoneal, hypoechoic, inhomogeneous lesion, measuring 3.5 × 4.0 cm, with imbibition of the surrounding omental tissue, and without contact with the liver, right kidney or ascending colon (Fig. 1A). On CEUS, the lesion was not vascularized and hence consistent with an abscess (Fig. 1B). The lesion was biopsied under sonographic guidance, and 50 ml of pus was removed. The microbiological results revealed Enterobacter cloacae and Klebsiella pneumonia. In the follow-up, repetitive abscess punctures as well as intravenous antibiotic treatment were necessary for healing. In the context of treatment, the pain resolved and the CRP level normalized.

Fig. 1

Furthermore, a 1 cm small hypoechoic lesion was seen in the thickened and hyperechoic lesser omentum by sonography (Fig. 2). The lesion showed homogeneous and isoechoic enhancement in the arterial phase (Fig. 3A) and a washout phenomenon in the parenchymal phase (Fig. 3B) in CEUS. Based on the findings, peritoneal carcinomatosis was suspected. An ultrasound-guided core needle biopsy was recommended.

Fig. 2

Fig. 3

For further clarification of the omental lesion and the cause of the abscess, a CT of the abdomen (Fig. 4) and a colonoscopy were carried out, without revealing any other abdominal pathology. The histology of the omental lesion demonstrated connective and fatty tissue of the omentum with a dense infiltrate of plasma cells and macrophages, sharply demarcated by a fibrotic rim. Single eosinophils were intermingled, and residues of hemorrhage were evident. The morphological pattern was consistent with the diagnosis of inflammatory pseudotumor (IPT). An IgG4-associated pathogenesis was excluded immunohistochemically, and a myofibroblastic component was not evident.

Fig. 4

IPT is an unusual entity, and thus difficult to identify in the differential diagnosis. In the past, a broad spectrum of synonyms was used to describe this benign tumor entity, mimicking malignancies. Two main types can be distinguished: inflammatory myofibroblastic tumors (IMT), occurring predominantly in children and younger adults, and distinguishable from other not precisely defined inflammatory pseudotumors, which are more often seen in middle-aged or older adults⁽⁴⁾. The etiology of IPT is unknown. One theory suggests an inflammatory reaction to an infection, trauma or autoimmune disease. Consequently, different agents have been linked to the development of IPT. The possibility of low-grade sarcoma has been discussed by some authors as well⁽¹⁾. In the patient reported here, Enterobacter cloacae and Klebsiella pneumonia were isolated from the abscess, which could have been the trigger for IPT formation⁽⁵⁾. The etiology of the abscess remained unclear after diagnosis. Local infection of the kidney or intestine could not be demonstrated. Metabolic syndrome is known to contribute to an immunosuppressive and hypoxic environment on the cellular level, and thus promotes abscess formation. The hematogenous origin could not be ruled out despite a negative blood culture.

In B-mode-ultrasound and CEUS, IPT of the liver has been the main subject of study^(6,7).

In the present case, it was particularly important to exclude another abscess within the omental tissue. CEUS showed marked homogeneous enhancement in the arterial phase and a washout phenomenon in the parenchymal phase, indicating a vital perfused tumor lesion and excluding an abscess. Hence, peritoneal carcinomatosis is an important differential diagnostic option, as it is characterized by a similar perfusion pattern⁽⁸⁾. In this study, CEUS showed marked enhancement (arterial phase) with consecutive hypoechoic enhancement (parenchymatous phase). This is the pattern seen in approximately 85% of malignant omental lesions. In this situation, malignancy was excluded by ultrasound-guided needle biopsy. If there had been no enhancement of the omental lesion, the differential diagnosis of acute epiploic appendagitis should have been considered⁽⁹⁾.

Therefore, IPT is a rare differential diagnosis of peritoneal carcinomatosis in the work-up of omental lesions. CEUS is helpful in identifying vital tissue for a targeted biopsy for subsequent histological examination that is essential for the exclusion of malignancy.