Potential Role of ¹⁸F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography in Response Assessment of Rare Multisystemic Disease Like Erdheim–Chester Disease

Erdheim–Chester disease (ECD) is a form of rare systemic histiocytosis with deposition of lipid-laden macrophages in multiple organs, first described by William Chester and Jakob Erdheim in 1930.^[1] ECD histiocytes particularly stain positive for CD68 while IHC markers such as CD1a, langerin, and S100^[2] usually appear negative. The most common symptoms of ECD include dyspnea, bone pain, ataxia, and visual disturbance.^[2] In 2012, it was reported that ECD harbors BRAF mutation,^[3] with most of the studies reporting nearly 50% involvement.^[4,5] The most common sites of involvement include skeletal system (osteosclerotic lesions), kidneys, lungs, and central nervous system, along with other less involved areas such as orbit, blood vessels, and paranasal sinuses^[6] [Fig 1].

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Fig 1:

(a-g) We present the case of a 55-year-old male patient with documented active ECD who underwent FDG PET/CT for baseline evaluation to assess the extent of the disease. (b and e) Axial sections of PET/CT and CT show osteosclerotic lesions (white arrows in b) in bilateral maxillary sinuses (causing complete obliteration), (c and f) interlobular septal thickenings in the left lung, and (d and g; blue arrows in g) large bilateral heterogeneous retroperitoneal masses (right side: 17 cm × 10 cm × 16 cm [AP × TR × CC], left side: 15.8 cm × 9 cm × 16 cm) involving and encasing bilateral renal parenchyma leading to poor corticomedullary differentiation (atypical presentation in contrast to typical findings). The patient was treated with high-dose subcutaneous interferon-alpha 2b (1,000,000 units 3 times a week), but treatment was interrupted due to COVID-19. (FDG: Fluorodeoxyglucose, PET/CT: Positron emission tomography–computed tomography, ECD: Erdheim–Chester disease)

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Imaging plays a critical role in the diagnosis of ECD to evaluate the organs involved and to assess its extent. Many studies have highlighted the role of ¹⁸F-fluorodeoxyglucose positron emission tomography–computed tomography (¹⁸F-FDG PET/CT) in diagnosis and assessing the disease extent;^[6-10] however, there is only one case report^[11] that emphasizes its role in the evaluation of treatment response [Fig 2].

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Fig 2:

(a-g) The patient came for follow-up PET/CT after 2 years to assess disease status and response to therapy. (b and e; white arrows in b) The fused transaxial PET/CT and CT sections demonstrate lesions in bilateral maxillary sinuses, (c and f), lungs, (d and g; blue arrows in g) retroperitoneum, and bilateral kidneys as described above with no significant interval change, thereby suggestive of stable disease in the posttherapy FDG PET/CT scan acquired for the purpose of response assessment. (FDG: Fluorodeoxyglucose, PET/CT: Positron emission tomography–computed tomography)

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As per the literature, retroperitoneal and renal involvement accounts for only 30% of ECD cases.^[12] This particular case pictorially depicts the atypical involvement of kidneys compared to the usual findings such as hairy kidneys (perirenal fat stranding) and goose kidneys (rim of cortical calcification).^[6,13] It also brings into light the clinical utility of ¹⁸F-FDG PET/CT for response assessment in ECD patients. The information provided by ¹⁸F-FDG PET/CT can impact clinical decisions based on a future treatment plan (for example, in this case, it demands a change of management), and therefore, ¹⁸F-FDG PET/CT provides an insight into the management of such rare disease.