Inflammatory pseudotumor, also called inflammatory myofibroblastic tumor or plasma cell granuloma, is a rare lesion that affects both children and adults [91,97]. Histologically, it is characterized by a proliferation of spindle-shaped cells, myofibrob-lasts mixed with inflammatory plasma cells, lymphocytes, and, occasionally,histiocytes (see Chapt. 5, "Hepatic Pseudolesions", section 5.2.3, "Inflammatory Pseudotumors"). The lesion arises in a variety of tissues and organs including the lungs, mesentery of the intestines, omentum, stomach, and liver [36,87]. The lesion is generally considered to be benign, but some inflammatory pseudotumors may recur or metastasize, and some patients die of the disease. On the other hand, it is known that some inflammatory pseudotumors regress and completely resolve without treatment .
An inflammatory reaction is believed to cause inflammatory pseudotumor. Affected patients have varying degrees of non-specific symptoms and inflammatory responses, such as fever, impaired growth, leukocytosis, anemia, thrombocytosis, hy-pergammaglobulinemia, and an increase in the ery-throcyte sedimentation rate or C-reactive protein (CRP). In some patients, inflammatory pseudotumor arises after trauma, surgery, or infection.
The radiologic findings for inflammatory pseudotumor are non-specific on all imaging modalities. On US, lesions typically present het-erogeneously hypoechoic or mosaic patterns that are similar to those observed for other focal liver neoplasms .
The lesion is usually hypodense on unen-hanced CT but presents an early intense and peripheral enhancement immediately after contrast medium administration, followed by homogeneous, complete and persistent enhancement. Thereafter peripheral enhancement and a hypo-dense core can often be observed. These features are due to the presence of fibroblastic cells and chronic inflammatory cells, respectively [30,36].
The signal characteristics on MRI are similarly non-specific . On unenhanced T1-weighted MR images inflammatory pseudotumor is often hypointense in the central portion, while on T2-weighted images the lesion frequently demonstrates isointensity or slight hyperintensity (Fig. 7).
Fig. 6a-e. Cystic dilatation of the bile ducts in an asymptomatic 3 year old. The T2-weighted HASTE images in sagittal (a, b) and coronal (c) orientation demonstrate cystic dilatation of the bile ducts (arrows) corresponding to type IVa of the Todani Classification (for details see Chapter 7). Massive dilatation (asterisk) of the chole-dochal duct surrounded by a small rim of pancreatic tissue (arrowheads) is apparent on the unenhanced axial T1-weighted fat-suppressed image (d) at the level of the pancreatic duct. The post-contrast T1-weighted fat-suppressed image (e) reveals normal enhancement of the pancreatic tissue. Increased enhancement of the wall of the choledochal duct wall, which would indicate inflammation, is not seen c e
Fig. 7a-i. Inflammatory pseudotumor of the liver. On the T2-weighted respiratory-gated TSE image (a) and on the single-shot T2-weighted HASTE image (b) the lesion demonstrates slight hyperintensity with a hyperintense rim (arrowheads in a). The corresponding single-shot HASTE image in sagittal orientation (c) shows pleural reaction (arrow/) neighbouring the lesion. On the unenhanced T1-weight-ed image (d), the inflammatory pseudotumor is hypointense. Peripheral enhancement is seen on the Tl-weighted dynamic images (e-h) after the bolus injection of contrast agent; this reflects the cellular components and inflammatory changes within the lesion. On the Tl-weighted fat-suppressed image in the equilibrium phase (i), a hyperintense rim surrounding the lesion is seen together with enhancement of the central portions of the lesion. The hyperintense rim is due to edema of the surrounding liver tissue
However, the appearance on T2-weighted images may vary in relation to the histologic components: a strong fibrotic predominance may result in slight hypointensity compared to the normal liver parenchyma while a greater predominance of inflammatory cells may produce a stronger hyperintense appearance. Early peripheral enhancement is typically seen on T1-weighted dynamic imaging after the bolus injection of Gd contrast agent. This reflects the cellular components and inflammatory changes within the lesion. In the equilibrium phase a hyperintense rim may be seen due to edema of the surrounding liver tissue. During this phase the central portions of the lesion are typically hyper-intense .
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