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E1984. Imaging Review of Langerhans Cell Histiocytosis
Authors
  1. Desi Schiess; University of Texas Southwestern Medical Center
  2. Cory Pfeifer; University of Texas Southwestern Medical Center
Background
Langerhans Cell Histiocytosis (LCH) is a multi-system entity with variable involvement and prognosis. Disseminated and osseous forms are most common in the pediatric population, while isolated pulmonary LCH is seen in adult smokers. The underlying histopathology is unchecked monoclonal proliferation of Langerhans cells, which is accompanied by inflammation and granuloma formation. It can affect multiple organ systems including skeletal, central nervous system (CNS), pulmonary, hepatobiliary, and reticuloendothelial/lymphatic manifestations. Prognosis is highly variable, with some patients demonstrating spontaneous regression while others have a rapidly progressive course. Survival largely depends on extent of disease and ranges from 50-95%. It is important for radiologists to be familiar with the characteristic features and suggest the diagnosis of LCH when appropriate.

Educational Goals / Teaching Points
Review the multi-system effects of LCH in pediatric patients including the classic osseous, neurologic, and pulmonary manifestations. Review imaging findings of LCH using multiple imaging modalities including radiography, computed tomography (CT), and magnetic resonance imaging (MRI).

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The skeletal system is most frequently involved, with one or more lytic lesions. Although any bone can be involved, LCH has a predilection for the flat bones (skull, mandible, ribs, pelvis, and spine). Skull lesions have a characteristic beveled edge appearance due to asymmetric destruction of the inner and outer tables. Calvarial lesions can coalesce to form a “geographic skull” appearance. Mandibular lesions can destroy the alveolar ridge, causing the “floating tooth” sign. LCH is the most common cause of vertebra plana in children. By radiography and CT, osseous lesions are lytic without a sclerotic margin. On MRI, they are T2 hyperintense and T1 hypo- to isointense with diffuse contrast enhancement. On nuclear medicine bone scans, they typically demonstrate increased radiotracer uptake, though this depends on chronicity. MRI is the most useful modality to evaluate CNS involvement, which can include absence of the normal T1 hyperintense neurohypophysis and thickening of the pituitary stalk (associated with diabetes insipidus), bilateral symmetric neurodegenerative lesions of the cerebellum and basal ganglia, or other mass lesions with or without obstructive hydrocephalus. Pulmonary involvement typically manifests as nodules and cysts, which are best seen on CT. Imaging features are often not pathognomonic, and tissue is usually required for definitive diagnosis.

Conclusion
LCH is a multi-system disorder with variable involvement and presentation. It can even be asymptomatic and found incidentally in some patients; therefore, the radiologist may be the first provider to suggest the diagnosis.