1.  Langerhans cell Histiocytosis

LCH can affect most organ systems and imaging features are often not pathognomonic. Definitive diagnosis usually requires clinical features, histopathology and immunohistochemistry in addition to radiologic findings. A review of typical radiologic findings in LCH is shown in Fig. 3 (4).

- Skeleton:

The skeleton involvement is the most common radiographic manifestation and flat bones are the most common location for single-lesion, especially in adults. The skull is the most common flat bone involved. Lesion appears as well-defined “punched out” lytic lesion that typically lack periosteal reaction. The double contour or “beveled edge” appearance of the skull lesion is caused by asymmetric destruction of the inner and outer cortices ( Fig. 4 ).

Whereas isolated flat bone involvement is more frequent in adults, long bone lesions are more common in children. Unlike calvarium disease, long bone lesions have been associated with periosteal reaction.

Another location of skeletal involvement of LCH is the vertebral body, which typical “vertebra plana” appearance, especially in children. This feature is due to the symmetric uniform collapse of the vertebral body with the preservation of the intervertebral disk spaces.

At MRI imaging, lesions signal characteristics include typically hypointense to isointense on T1-weighted images, hyperintense on T2 and STIR-weighted images, with enhancement after gadolinium-based contrast agent administration.

Differential diagnosis of bone involvement in LCH includes a wide range of benign and malignant bone lesions ( Fig. 5 ).

- Lung:

At CT imaging, the earliest manifestation of LCH is characterized by centrilobular micronodules with a predominantly bilateral symmetric upper-to mid-lung distribution, with costophrenic angles usually spared. Later, cysts develop and can become the dominant imaging finding ( Fig. 6 ; Fig. 7 ). Distribution is the key to differentiating LCH from other cystic lung diseases like lymphangioleiomyomatosis, lymphocytic interstitial pneumonia and bullies emphysema ( Fig. 8 ). Typically in LCH, there is a preservation of lung volumes or even hyperinflation.

- Liver, Spleen and Lymph nodes:

Cervical lymph nodes are the most common location for LCH but lymph nodes involvement can occur anywhere, manifesting as groups of nodes with lymphedema. Liver and spleen involvement is less frequent but they are considered “risk organs” that indicate a worse prognosis. ( Fig. 9 ).

- CNS:

Patients with LCH CNS disease clinically present diabetes insipidus secondary to infiltration of the posterior pituitary gland, which results in a lack of T1-weighted high intensity of the posterior pituitary with associated enhancement and thickening of the pituitary stalk.

2.  Erdheim-Chester disease

Musculoskeletal involvement is most common, with characteristic radiographic findings which appears as patchy medullar osteosclerosis. ( Fig. 10

Kidneys and retroperitoneum are often involved and are usually asymptomatic. Imaging findings include a characteristic “hairy kidney sign” that is a perirenal rind of soft tissue as a result of an irregular symmetric infiltration of the bilateral perirenal and posterior pararenal spaces.

Aortic involvement is also reported as a periaortic soft tissue which results in the “coated aorta sign”. ( Fig. 11;  Fig. 12 ; Fig. 13 ).

Differential diagnosis includes lymphoma and retroperitoneal fibrosis. The inferior vena cava and pelvic ureters are typically spared, which are useful cross-sectional imaging findings for differentiation of retroperitoneal ECD from retroperitoneal fibrosis (Fig. 14 ; Fig. 15 ).

The most frequent CNS manifestations are diabetes insipidus, cerebellar syndromes, orbital lesions, and extra-axial masses involving the dura, that can be confused with meningiomas at imaging. In the case of our hospital, retroperitoneal lymph node involvement of ECD was diagnosed in a 57-year-old man, after histologycal analysis of a brain lesion, that was initially identified as glioblastoma. ( Fig. 16 ; Fig. 17 ). There are some cases described in the literature, but there is a very low incidence of EDC intra-axial lesions (5,6).

3.  Rosai-Dorfman disease

The most common manifestation of RDD includes nodal involvement, being cervical lymph nodes the most frequent location, and extranodal involvement. ( Fig. 18 ).

Extranodal disease includes extra-axial lesions, polypoid masses into the paranasal sinus, extraconal soft-tissue masses and salivary gland lymphoid hyperplasia. 

Intraabdominal extranodal involvement is uncommon (4%) and tends to affect adults. The kidney is the most common location and it is reported to be a sign of poor prognosis (7). Imaging typical findings are bilateral infiltrative perihiliar masses or subcapsular solid masses that are mildly homogeneously enhancing on CT or MRI images. Infiltrative masses of the kidneys can replace the renal parenchyma and can result in acute renal failure. ( Fig. 19 ).

Differential diagnosis includes lymphoma, transitional cell carcinoma, renal cell carcinoma or hemorrhage.

4.  Juvenile Xanthogranuloma

JXG´s most common manifestations are dermal and the diagnosis is made clinically based on typical dermal findings. In MRI imaging, the lesions appear as masses in the subcutaneous tissue that have an infiltrative appearance, with intermediate T1 signal intensity and no appreciable enhancement. ( Fig. 20 ).

Much less commonly, JXG may involve other organ systems like the liver, spleen, lung, bones, lymph nodes and gastrointestinal tract. Radiologic appearance of systemic JXG is nonspecific and is used when there is clinical concern for systemic involvement.