The position of total hip replacements can be reliably assessed with use of simple electronic tools and standard radiology workstations.

Background A 56 year old female was hospitalised in July 2019 with abdominal pain and significant weight loss, but little in the way of bone pain. Examination showed no evidence of xanthelasma. A large pericardial effusion was detected, requiring pericardiocentesis. The pericardial fluid contained numerous macrophages staining with CD68. Methods CT scans were reported as consistent with metastatic carcinoma. There were multiple sclerotic bone lesions in the manubrium, T7, T11, L2 and L4. She also had a periaortitis with soft tissue infiltrate around the ascending aorta and aortic arch, in the mediastinum, posterior paravertebral region, and in the retroperitoneum, obscuring both adrenal glands and surrounding both kidneys. Bone marrow aspirate and trephine demonstrated reactive appearances only. Results She required re-admission with breathlessness due to recurrence of the pericardial effusion. Biopsy from the left perinephric region was performed. Histopathology revealed a fibroinflammatory disorder. Some histiocytes were seen. There was no evidence of the typical features of IgG4-RD such as storiform fibrosis or obliterative phlebitis, which had been the working diagnosis thus far. Serum IgG4 level was normal. The classical histological features of Erdheim-Chester disease (ECD, an ultra-rare non-Langerhans cell Histiocytosis) in terms of foamy macrophages and Touton cells, were not obvious, but in the literature, it is not uncommon for the typical histological appearances of ECD to be absent. Further investigation demonstrated the classical radiographic findings of ECD with symmetrical sclerotic lesions in the long bones of the lower limbs. Radionuclide bone scan showed multifocal symmetrical increase in isotope uptake, predominately in the distal femora, proximal and distal tibiae, mandible and maxillae with multiple lesions in the thoracic and lumbar spine. Endocrine failure is frequently seen in ECD. Fortunately, our patient had neither pituitary disease nor hypoadrenalism. Furthermore, CNS involvement, ataxia and retro-orbital disease have all been reported in ECD. Our patient has experienced daily episodes of right retro-orbital pain, dizziness on upward gaze without diplopia, and occasional staggering. An MRI of brain, orbits and whole spine is scheduled, as are a whole body FDG-PET scan and cardiac MRI (to exclude myocardial infiltration). BRAF V600E mutation analysis is in progress since around 50% of ECD patients with this mutation may respond to vemurafenib treatment. Conclusion In this illustrative case, the combination of a fibroinflammatory disorder surrounding both kidneys, along with recurrent pericardial effusion and sclerotic bone lesions, was clinically and radiologically diagnostic of Erdheim-Chester disease. ECD is a recognised mimic of IgG4-RD, which itself is a mimic of multiple other conditions, including metastatic carcinoma. We present this case to highlight this little-known condition. Rheumatologists and Physicians should consider ECD in the differential diagnosis of IgG4-RD, periaortitis, pericardial effusion, symmetrical sclerotic bone lesions, endocrine failure and neurological features. Disclosures J.S. McLaren None. V. Campbell None. M. Rahilly None. J.M. Rehman None. R. Cargill None.