Arrived at the ER due to bilateral lower limb edema.
All vital signs were normal. CRP 56. 
Doppler ultrasonography of the lower limb veins, both deep and superficial, and arteries was essentially normal.
CT abdomen was performed to investigate for a pelvic mass, which showed huge bilateral renal masses and periaortic soft tissue. The putative diagnosis was of Erdheim-Chester disease, the differential diagnosis for the perirenal masses consisting of perirenal fibromatosis, lymphoma, IgG4-related disease, and desmoid tumor.

An ultrasound-guided biopsy was obtained from one of the perirenal masses.

Interestingly, there was virtually no radiologic sign of bone involvement, though when asked by the radiologist whether he had been suffering from bone pain, he nodded vehemently.

Pathology:
Perirenal mass, biopsy:
Histiocytic lesion (SEE COMMENT).
COMMENT:
The specimen consists of fibrous tissue in which cells with abundant foamy cytoplasm are seen. Giant cells are not seen. The cells are positive for CD68 and vimentin and are negative for CD1a, CD34, CD117, DOG1, MUC4, EMA, MDM2, S-100, SOX 10, desmin, beta-catenin, actin, caldesmon, IgG4, Langerin and BRAF. The morphology and immunohystochemical results are those of a histiocytic process and essentially rule out fibromatosis, lymphoma, IgG4 disease, desmoid tumor and liposarcoma. "The diagnosis of Erdheim-Chester disease is based on clinical features, imaging and histology" (WHO fascicle). In the appropriate clinical setting, these findings would be consistent with a diagnosis of Erdheim-Chester disease.

A renal MRI was performed, which, in addition to the CT, showed two small arterially-enhancing hepatic foci and showed the circular thickening at the distalmost aorta to be in fact periaortic soft tissue, not soft atheromatous plaque. 

A brain-orbits MRI and a cardiac MRI were performed (not shown), both of which were normal.

An additional biopsy specimen was sent to a world-renowned lab for both pathological and molecular studies, which corroborated the diagnosis of Erdheim-Chester disease and found the C121S mutation in the MAP2K1 gene.

Same-day baseline PET-CT and PET-MRI were done before initiating treatment with the MEK inhibitor cobimetinib.

3 months after starting treatment, a follow-up PET-CT (not shown) revealed a modest size regression and decreased radiotracer uptake in the perirenal and periaortic masses.

Several weeks later, renal ultrasonography revealed yet further size regression of the perirenal masses.