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Histiocyte-rich xanthomatous pseudotumor mimicking relapse on positron emission tomography imaging in an adolescent with primary mediastinal diffuse large B-cell lymphoma.

Inflammatory pseudotumors (IPTs) are rare, enigmatic lesions that may develop as a late manifestation of a reparative process. We describe the case of a teenager with primary mediastinal subtype of diffuse large B-cell lymphoma who developed an IPT at the site of the original lymphoma, mimicking relapse of disease on positron emission tomography/computed tomography imaging. This is the first report of IPT in a teenager with mediastinal lymphoma. This case is an important reminder of the limitations of positron emission tomography/computed tomography imaging in patients with lymphoma and stresses the importance of histologic confirmation of suspected treatment failure or relapse.