Understanding of the retroperitoneal lesions of IgG4-RD is facilitated by classifying them into three principal types, namely, those developing in soft tissues around ureteropelvic lesions; periarterial lesions; and lesions occurring unrelated to any existing organs. Of these, periarterial lesions show the highest frequency. Periarterial lesions are recognized on diagnostic imaging as a thickening of the arterial wall in the absence of stenosis or occlusion of the affected artery. Distinctive features such as penetration of the lesion by small vessels are present. In addition, this manifestation of IgG4-RD sometimes presents as inflammatory aortic aneurysms associated with luminal dilatation. The differentiation of periarterial lesions in IgG4-RD from other types of periarterial pathology, including lymphoma, must proceed comprehensively by considering the presence or absence of lesions in other organs, the serum IgG4 concentration, and (in some cases) a histological diagnosis.
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