Erdheim-Chester disease (ECD) is a rare inflammatory neoplasm in the non-Langerhans cell histiocytosis group. BRAF V600E mutation is detected in 50-60% of patients and offers targeted therapy opportunity. In retroperitoneal involvement, 'coated aorta sign' (periaortic fibrous sheath formation — concentric soft tissue thickening around the aortic wall) and 'hairy kidney' (infiltrative thickening of perirenal fat creating a hairy/shaggy appearance around kidneys) are pathognomonic findings. Bilateral symmetric osteosclerotic lesions of long bones support the diagnosis, characteristically showing meta-diaphyseal location. Multisystemic involvement (bone, retroperitoneum, orbit, brain, lung, heart) is typical and the extent of disease determines prognosis. Mean age at diagnosis is 53 years with slight male predominance.
Age Range
40-70
Peak Age
55
Gender
Male predominant
Prevalence
Rare
In ECD, lipid-laden (foamy) histiocytes (CD68+/CD1a-) accumulate in various tissues creating fibrosis and inflammatory infiltration. BRAF V600E mutation constitutively activates the RAS-RAF-MEK-ERK signaling pathway, triggering histiocyte proliferation and accumulation. In retroperitoneal involvement, histiocytes infiltrate periaortic and perirenal areas leading to coated aorta sign (concentric fibrous sheath around the aortic wall) and hairy kidney (homogeneous infiltrative thickening of perirenal fat). Coated aorta sign appears as periaortic soft tissue thickening on CT, creating a distinct contrast difference between the aortic wall and fat that produces the characteristic 'coating' appearance. In hairy kidney, infiltrated perirenal tissue causes renal fascial thickening and an appearance mimicking perirenal fluid accumulation; renal parenchyma is preserved but surrounded by hairy/shaggy infiltration. In bone involvement, histiocytic infiltration stimulates osteoblastic activity leading to trabecular thickening and cortical sclerosis. Cardiac involvement with pericardial and myocardial infiltration can be life-threatening; pericardial effusion and right atrial pseudotumor have been reported.
Concentric fibrous/histiocytic sheath around the aortic wall — pathognomonic for ECD. Diagnosis is confirmed when coexisting with hairy kidney. Unlike retroperitoneal fibrosis (RPF), ECD is accompanied by bone lesions and multisystemic involvement. RPF typically involves the anterolateral aorta sparing the posterior wall, while ECD shows concentric involvement.
On contrast-enhanced CT, concentric, smoothly marginated, soft tissue density thickening is seen around the aortic wall — 'coated aorta sign'. This thickening may involve thoracic and abdominal aorta, most commonly affecting the infrarenal aorta. May show mild enhancement but is not hypervascular. Aortic lumen remains patent and the thickening is peripheral, separate from the aortic wall. Thickness is typically 5-15 mm.
Report Sentence
Concentric soft tissue thickening around the aortic wall is seen; coated aorta sign consistent with Erdheim-Chester disease.
Bilateral homogeneous infiltrative thickening of perirenal fat — 'hairy kidney' or 'coated kidney'. Perirenal fascia appears thickened and irregular. Does not invade renal parenchyma but encases it. Bilateral and symmetric involvement is typical. Infiltration is homogeneous and necrosis or cystic change is not expected.
Report Sentence
Infiltrative thickening of bilateral perirenal fat is seen with hairy kidney appearance consistent with Erdheim-Chester disease.
Bilateral symmetric osteosclerotic lesions are seen in meta-diaphyses of long bones — femur and tibia most commonly involved. This osteosclerosis strongly supports ECD diagnosis and forms a diagnostic triad with retroperitoneal findings. Epiphyses are usually spared — this differs from neurogenic bone tumors. Cortical thickening and medullary sclerosis may coexist.
Report Sentence
Bilateral symmetric osteosclerotic lesions in meta-diaphyses of long bones are identified, consistent with Erdheim-Chester disease.
On T2, periaortic and perirenal infiltrative tissue generally shows low-to-intermediate signal — low T2 when fibrous component is dominant, intermediate T2 when active inflammatory component is dominant. These T2 characteristics reflect the collagen content of inflammatory fibrous tissue. On gadolinium-enhanced series, mild-to-moderate enhancement is seen and active inflammatory areas show more prominent enhancement.
Report Sentence
Low-to-intermediate T2 signal infiltrative tissue is seen in the periaortic and perirenal area consistent with fibroinflammatory process.
On FDG PET-CT, ECD shows moderate-to-high FDG uptake in active inflammatory areas. Retroperitoneal (periaortic, perirenal), orbital, bone, and other involvement sites are evaluated simultaneously, mapping the multisystemic extent of disease in a single session. SUVmax decrease in follow-up indicates therapeutic response. FDG uptake rapidly decreases with BRAF inhibitor (vemurafenib) treatment — typically significant metabolic response is observed within 2-3 months.
Report Sentence
FDG uptake in periaortic, perirenal, and skeletal areas is identified, consistent with multisystemic Erdheim-Chester disease.
On MRI, bilateral infiltrative thickening of retrobulbar fat is seen — orbital involvement occurs in 25-30% of ECD cases. Infiltrative tissue partially or completely fills retrobulbar fat causing exophthalmos. Lacrimal gland involvement may accompany. Mild-to-moderate enhancement on post-contrast series. In differential diagnosis with IgG4-related disease, coexisting bone lesions and perirenal involvement support ECD diagnosis.
Report Sentence
Infiltrative thickening of bilateral retrobulbar fat is seen consistent with orbital ECD involvement.
In pulmonary involvement, bilateral interlobular septal thickening, centrilobular nodular densities, and subpleural ground-glass opacities may be seen on CT. Pleural effusion and pleural thickening may accompany. This pattern may be confused with lymphangitic carcinomatosis, but coexisting bone and retroperitoneal findings suggest ECD.
Report Sentence
Bilateral interlobular septal thickening and subpleural ground-glass opacities are seen; pulmonary ECD involvement should be considered.
Criteria
BRAF V600E mutation positive (50-60%), confirmed by immunohistochemistry and/or PCR
Distinct Features
Dramatic treatment response with BRAF inhibitor (vemurafenib) with >80% response rate, targeted therapy opportunity, rapid metabolic response on PET-CT
Criteria
BRAF V600E mutation negative (40-50%), alternative MAPK pathway mutations should be sought (MAP2K1, NRAS, KRAS, PIK3CA)
Distinct Features
May have MEK or RAS mutations, treatment with MEK inhibitors (cobimetinib, trametinib), interferon-alfa as alternative
Criteria
Mixed form showing features of both ECD and Langerhans cell histiocytosis (LCH), CD1a+ and CD1a- histiocytes coexisting in the same biopsy
Distinct Features
CD1a+ and CD1a- cells coexist, high BRAF frequency (>80%), skin and bone involvement dominant, may have aggressive course
Distinguishing Feature
Lymphoma shows nodal sandwich sign encasing vessels without occlusion; ECD shows periaortic fibrous coating (coated aorta) and bone osteosclerosis — when bone involvement is present in lymphoma, lytic pattern predominates
Distinguishing Feature
RPF typically involves anterolateral aorta sparing posterior wall, ureteral obstruction is typical; ECD shows concentric periaortic involvement, bone lesions and multisystemic findings
Distinguishing Feature
Abscess shows rim enhancement, gas and clinical infection signs; ECD shows fibrous coating, systemic involvement and chronic course — in abscess fever and leukocytosis dominate while in ECD bone pain and exophthalmos are prominent
Urgency
urgentManagement
medicalBiopsy
NeededFollow-up
3-monthECD diagnosis is confirmed by biopsy (CD68+/CD1a- foamy histiocytes). BRAF V600E testing is mandatory — if positive, vemurafenib (BRAF inhibitor) is primary treatment with >80% response rate. If BRAF negative, MEK inhibitor (cobimetinib, trametinib), interferon-alfa, or anakinra (IL-1 receptor antagonist) is used. Cardiac and CNS involvement can be life-threatening and require urgent treatment — pericardial effusion carries tamponade risk, cerebellar infiltration may cause ataxia. Median survival without treatment is 3-5 years; prognosis has dramatically improved with BRAF inhibitors. Treatment response is monitored with PET-CT every 3-6 months. Long-term treatment is required because relapse risk is high when medication is discontinued.
ECD is rare but can affect vital organs. Vemurafenib (BRAF inhibitor) has revolutionized treatment in BRAF V600E positive patients. Bone, retroperitoneum, orbit, CNS, and cardiovascular involvement may occur. Early diagnosis and targeted therapy improve prognosis.