Retroperitoneal fibrosis: A rare mimicker of a perirenal hematoma

Retroperitoneal fibrosis (RPF) is a relatively rare entity, characterized by chronic inflammation and fibrosis in the retroperitoneal periaortic tissues. Due to this rarity, the diagnosis might be delayed or challenging, especially with atypical presentations of RPF. We report the imaging findings of an incidental finding of active RPF in an asymptomatic 84-year-old male patient. This patient was initially diagnosed with a perirenal hematoma due to an atypical imaging presentation on a routine staging computed tomography scan of the chest and the abdomen to stage a recently discovered prostate cancer. However, due to the persistence of the perirenal lesion, subsequent magnetic resonance imaging of this lesion was conducted, which resulted in the final diagnosis of active RPF. With this case report, we want to address attention to the potential atypical presentation of RPF, which could mimic a perirenal mass and demonstrates the importance of a broad differential diagnosis. Secondly, we want to point out the importance of magnetic resonance imaging in the evaluation of RPF, since it can differentiate between inactive or chronic idiopathic RPF and active idiopathic RPF or malignant RPF. Finally, this case shows the importance of comparing current imaging findings with previous imaging results, since a perirenal hematoma would reduce in size on follow-up imaging, which was not the case in our patient.