 So here we have a 72-year-old male with pain. And so they did a pan scan to evaluate the etiology of the pain. And I'm gonna focus on the abdomen. And there's a whole bunch of things going on, but let's just focus on what's happening around the kidneys. And let's just start focusing on what's happening around the right kidney. Kidneys themselves look on the smaller side, I would say, but let's look at the fat around the kidney and notice that there's rinds of soft tissue are in that location. And this is more than just sort of peripheral stranding. This really does look like rinds of soft tissue, quite thick surrounding the kidney and perhaps some of it involves the renal sinus and maybe you surround some portions of the ureter as well. So that is sort of an unusual look, but it's there. Look at the left kidney and you notice that it's present in the left kidney as well. Now some of this may be more difficult to appreciate in the left kidney because you got this large cystic mass, but if we were to take that away and focus on everything around it, look at the bottom of this kidney, you have this soft tissue that's surrounding it over here. And so there's a couple of entities that can manifest in this way. All of them are very, very rare. Things to think about retroperitone fibrosis. Well, retroperitone fibrosis is a possibility, but typically that surrounds the inferrenal abdominal aorta, the epicenter is around the infromesenteric artery, it goes downwards to the iliacs. We have seen a few rare cases of retroperitone fibrosis isolated to the perinephric fat, but again, it's a rare example of a rare entity. So, that's certainly something you could suggest in your differential diagnosis. Something that we've covered in one of the cases is lymphoma, which can certainly manifest with perinephric soft tissue. I think that's not an unreasonable thought, but for it to manifest as only isolated perinephric involvement is very unusual. I'd like to see some more involvement systemically, whether it's adenopathy, whether it's slenomegaly, whether it's other lesions, isolated perinephric involvement, bilateral symmetric like this would again be unusual for lymphoma. Now, another entity to think about, which is really, really rare, but which actually, when it does occur, likes to do this in the retroperitone aim, and this is what this turned out to be, is Erdheim Chester disease. Erdheim Chester disease. So that's a non-long-your-hand cell histiocytosis. Clinical presentation is variable. Patients can just have this disease and for the most part, asymptomatic. They can have constitutional symptoms including weight loss, favor, malaise. It is often a disease that affects the bones, and so patients may manifest with bone pain. And so in terms of the bony lesions, there are gonna be these bilateral symmetric involvement, the metathesial and diathesial regions of bones where you see areas of sclerosis. And so that's what you may see on sort of the osseous plane films that you get on these patients. In about 50% of the cases, you may have extra skeletal involvement. And of those in the retroperitone and the most common involvement, you see this rind of soft tissue, bilateral symmetric surrounding the kidneys, potentially surrounding portions of the ureters, and someone in the literature called it a hairy kidney, which I certainly don't use in my reports, but I think is maybe a way to remember it when you look at it on imaging. Now this turned out to be an exceedingly rare case of Erdheim-Chester disease where patients also had a cardiac involvement where you can see involvement here surrounding the right atrium and surrounding some of the valves in that location. And so this was just sort of an unfortunate case of Erdheim-Chester disease with quite pronounced systemic involvement. And of course, you can't make this diagnosis prospectively. You can certainly suggest in your differential one would need to biopsy it to make sure that this is what you're dealing with. And this indeed was a case of Erdheim-Chester disease.