Inflammatory fibroid polyp of the renal pelvis: first report at an extra-gastrointestinal site with molecular confirmation

Inflammatory fibroid polyps (IFP) are known as rare benign mesenchymal tumours of the gastrointestinal tract. This entity was first described in 1949 as a gastric submucosal granuloma with eosinophilic infiltration by Vanek [1]. He reported on various ill-defined submucosal masses located in the stomach, histologically consisting of fibroblasts, collagenous fibres, lymphocytes with occasional rudimentary lymph follicle formation, and eosinophilic granulocytes, with multiple blood and lymph vessels. He already discussed the possibility of a neoplastic process behind these lesions, although he personally considered them rather an inflammatory reaction, and based on Kaijser’s article from 1937, he even suggested a possible allergic cause behind it. In 1953, Helwig et al. [2] suggested the term “inflammatory fibroid polyp” for this entity.

Histologically, inflammatory fibroid polyps are polypoid submucosal lesions which may infiltrate the mucosa. They consist of spindle cells without atypia, and often a so called “onion skin” pattern can be found around blood vessels. The mixed inflammatory infiltrate in the background is commonly rich in eosinophilic granulocytes. On immunohistochemistry, the spindle cells of these tumours typically show a CD34 positivity.

Mainly because of the bland cytology of fibrocytes and fibroblasts and the considerable inflammatory reaction, these lesions were long considered as reactive processes, just as Vanek had originally suggested. In 2008, Huss and Wardelmann et al. described that IFPs harbour mutations in the platelet-derived growth factor receptor alpha (PDGFRA) gene [3] and therefore represent true neoplasms of the gastrointestinal tract.

PDGFRA is a proto-oncogene and belongs to the receptor tyrosine kinases (RTKs). Besides playing an important role in embryonic development, its role has been described in the pathogenesis of various haematopoietic malignancies [4], glioblastoma [5], and tumours of the gastrointestinal tract, including gastrointestinal stromal tumours [6] and aforementioned inflammatory fibroid polyps [3]. In 2012, Huss et al. [7] reported two cases with PDGFRA-exon 14 mutations besides the previously described exon 12 and exon 18 mutations. They suggested two different phenotypes, the “gastric” one which is associated with exon 18 mutations and the “small bowel” type which harbours exon 12 alterations.

Although inflammatory fibroid polyps have been described on various sites of the gastrointestinal tract, even in recent literature reviews [8, 9], the possibility of this entity occurring outside of the alimentary canal has not been considered or examined. To the best of our knowledge, we report here the first case of an IFP in the urogenital tract.

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