http://repub.eur.nl/res/aut/53408/ List of Publications en http://repub.eur.nl/static-eur/img/logo.png http://repub.eur.nl http://repub.eur.nl/res/pub/34287/ 2011-10-06T00:00:00Z We present a case of atypical idiopathic retroperitoneal fibrosis (iRPF) presenting as a large pelvic tumor, for which it proved difficult to exclude T-cell malignant lymphoma. Histopathological examination of biopsy material showed collagenous tissue and fat with an exuberant and predominant T-cell infiltrate, largely consisting of CD4+cells expressing the IL-2 receptor-α chain (CD25). Focal plasma cells were negative for the immunoglobulin G4 (IgG4) isotype. T-cell receptor gene rearrangement (TRGR) pattern showed a Gaussian distribution, in keeping with a polyclonal T-cell population. Awareness of the sometimes exuberant and predominant T-cell infiltrate in iRPF should lead to earlier consideration of this disorder. This is particularly the case where there is an atypically localized and/or extensive mass, for which early exclusion of monoclonality with TRGR may provide helpful. Immunohistochemical findings suggest that CD4+CD25+cells, which are part of a naturally occurring population of regulatory T-cells, may be involved in the pathogenesis of iRPF. © 2011 The Authors. Pathology International