18F-FDG PET/CT findings of Erdheim Chester disease, a rare inflammatory condition – a pictorial presentation

Introduction: Erdheim Chester disease (ECD) is a rare multisystem inflammatory disorder characterised by infiltration of affected tissues with histiocytes and mononuclear cells. It is described in the spectrum of non-Langerhans cell histiocytosis. The mean age of onset is between 40 to 70 years of age with a male predominance. There are approximately more than 1500 cases of Erdheim Chester disease across the globe by July 2019 with a growing number every year. Hence there is increased recognition and interest in the early detection of ECD in which imaging plays an important role.

Positron emission tomography (PET/CT) examination is commonly used in the final imaging pathway in ECD. 18F-FDG (Fluorodeoxyglucose) PET-CT plays a pivotal role not only in the assessment of active disease but also in treatment response and incidental detection of unknown sites of disease. We here present a range of PET/CT findings of ECD affecting multiple organ systems.

Methods: Our centre is one of the largest regional referral centre for non-oncological 18F-FDG PET/CT service in the United Kingdom. We perform approximately 1000 examinations every year for non-oncological indications particularly in the assessment of various inflammatory conditions. We here share a constellation of 18F-FDG PET/CT findings in histologically proven ECD patients who have been imaged in our department from 2011 till December 2021.

Results: ECD is an indolent condition with progressive soft tissue infiltration affecting almost all organs of the body. Central nervous system (CNS), cardiac, pulmonary, orbital, skin, renal and skeletal systems manifestations are clinically evident. Hydronephrosis due to retroperitoneal fibrosis, bone pain, cutaneous lesions (skin plaques), exophthalmos, optic neuropathy, diabetes insipidus, focal neurological deficit are some of the common manifestations that we come across in our clinical practice.

Various PET/CT findings are as follows:

- High FDG avid soft tissue infiltration in retroperitoneum, prevertebral regions, perirenal, mesenteric and along pelvic sidewalls are evident. Histological confirmation is often required to exclude lymphoma in these circumstances.

- Vascular findings are usually incidental which manifests as avid periaortic soft tissue cuffing, diffuse mural uptake of coronary arteries.

- Central nervous system findings on PET/CT include focal pituitary uptake and orbital soft tissue infiltration.

- Isolated skeletal disease is demonstrated as sclerotic FDG avid lesion usually in long bones with monostotic or polyostotic in distribution. Bone metastasis or primary bone malignancies are to be excluded in isolated bone disease.

- Slow growing thick cutaneous plaques of ECD are high FDG avid with underlying subcutaneous involvement.

- Small organ involvement in ECD is rare and is demonstrated as a metabolically active disease in soft tissue thickening along the spermatic cord, avid focus in testis and diffuse uptake of salivary glands are also seen.

Conclusions: There are no specific clinical manifestations or serologic biomarkers for ECD which can result in delayed diagnosis making ECD a challenging and complex diagnosis. Malignancies, systemic sarcoidosis, Ig4 related disease and idiopathic retroperitoneal fibrosis are important imaging differentials for ECD. In the active phase of the disease, areas of infiltrative process are highly avid on 18F-FDG PET/CT. Semiquantitative analysis of disease severity in 18F-FDG ET/CT by measuring uptake values (SUVmax) of the lesions before and following treatment has been crucial for clinicians to guide on medical management.

This poster highlights the FDG PET/CT findings of ECD and emphasises the importance of considering ECD as an alternate diagnosis in unusual presentations of inflammatory conditions. Early diagnosis and targeted management can prevent life threatening neurological and cardiovascular complications of ECD.