Eosinophilic granuloma (EG) is the benign form of the 3 clinical variants of Langerhans cell histiocytosis, which include Letterer-Siwe disease, Hand-Schüller-Christian disease, and EG (formerly termed histiocytosis X).
Eosinophilic granuloma is characterized by single or multiple skeletal lesions, and it predominantly affects children, adolescents, and young adults. Solitary lesions are more common than multiple lesions. When multiple lesions occur, the new osseous lesions appear within 1-2 years, and the condition is still classified as eosinophilic granuloma. Any bone can be involved; the more common sites include the skull, mandible, spine, ribs, and long bones (see the images below). [1, 2, 3] Pathologic fractures may ensue. [1, 4, 5]
The diagnosis of eosinophilic granuloma is usually based on radiographic demonstration of a destructive bone lesion arising from the marrow cavity and on characteristic morphologic findings. Radiologists need to be aware that additional eosinophilic granuloma of bone, occurring as long as 4 years after initial diagnosis, should be interpreted as a localized form of Langerhans cell histiocytosis. This differentiation is important because the prognosis is more favorable with focal disease with multifocal disseminated disease, which involves organs other than the skeletal system. Similar lesions may occur within the lungs, skin, and stomach, either as a unifocal lesion or as part of multifocal disease. 
Lung involvement occurs in 20% of patients with eosinophilic granuloma and in an older group (age, 20-40 y), with a strong association with smoking. Diffuse pulmonary infiltrates may be a manifestation of a covert osseous disease. In 50-75% of patients, the disease is monostotic. Skull involvement is seen in 50% of patients.  Rarely, the growing epiphysis is involved with eosinophilic granuloma; in most such cases, transphyseal extension can be demonstrated, both by the radiologic findings and the histopathologic results. 
Eosinophilic granuloma may masquerade as an aggressive periodontitis,  therefore eosinophilic granuloma should be considered when an expanding lytic jaw lesion is encountered.
Plain radiography is the mainstay in the diagnosis of eosinophilic granuloma, although a specific diagnosis cannot always be made without bone biopsy because children and adolescents are not spared skeletal neoplasms. Radionuclide study, computed tomography (CT) scanning, magnetic resonance imaging (MRI), and, occasionally, angiography are complementary examinations. Any or all of these modalities may be used to arrive at a diagnosis. [1, 9, 10, 11, 12, 13]
A wide variety of bone lesions may mimic eosinophilic granuloma; these include infections, traumatic lesions, and neoplasms. A false-negative diagnosis of eosinophilic granuloma is exceptional when plain radiographic findings are used, although difficulty may be encountered with lesions in areas with more complex anatomy, such as the posterior elements of the vertebral bodies. In these cases, conventional tomography or CT scanning may useful. With radionuclide scanning, the false-negative rate is 30%.
Differential diagnosis and other problems to be considered
The differential diagnosis of eosinophilic granuloma includes aneurysmal bone cyst, bone infarct and bone metastases, thoracic eosinophilic granuloma, fibrous dysplasia, as well as acute pyogenic, chronic, and variant osteosarcoma.
When the skull is involved, the following conditions should be considered: venous lake; meningocele, encephalocele, and cranium bifidum; arachnoid granulation; parietal foramen; epidermoid cyst; hemangioma; cholesteatoma; fibrous dysplasia; metastasis; surgical defect; and osteomyelitis.
The presence of vertebra plana should prompt consideration of causes such as fracture, hemangioma, osteomyelitis, metastasis, lymphoma, leukemia, plasmacytoma, chordoma, aneurysmal bone cyst, and Ewing sarcoma.
Other interstitial lung diseases should be taken into consideration with pulmonary involvement.
A solitary eosinophilic granuloma lesion can present a diagnostic and treatment dilemma for the orthopedic surgeon. Pathologists, pediatricians, radiologists, and orthopedic surgeons must consider the wide range of skeletal neoplasms that may occur in children and adolescents.
In descending order of frequency, sites involved with monostotic osseous disease include the calvarium, mandible, ribs, long bones of the upper extremity, pelvis, and vertebrae (see the following images).
When tubular bones are involved in eosinophilic granuloma, diaphyseal and metaphyseal localization is more frequent than epiphyseal localization. Epiphyseal lesions may cross the open physeal plate.
The skull is affected in one half of patients, [3, 6] including the diploic space of the parietal and temporal bones. Skull lesions are lytic, with a beveled edge or sharp and serrated margins and the absence of sclerosis in calvarial lesions. A hole-within-a-hole appearance may occur as a result of uneven erosion of the inner and outer tables of the skull. In additionn, sclerosis may occur around orbital lesions, and marginal sclerosis may occur during the healing phase in up to 50% of patients with a calvarial lesion. A button sequestrum is seen because a central bone opacity within a lytic lesion is an unusual presentation.
A soft-tissue mass may be obvious overlying the skull defect; this mass is often clinically palpable. A soft tissue mass is also occasionally seen with orbital lesions, with or without underlying bone erosion.
Mandibular lesions may be associated with gingival and soft-tissue swelling and floating teeth (see the image below).
The ribs show lytic expansile lesions, which may be associated with pathologic fractures. The scapulae and pelvis show destructive lesions; periosteal elevation is minimal, and some lesions show sclerotic margins, particularly lesions occurring in the supra-acetabular regions.
Long bones below the knees and distal to the elbows are rarely involved. Lesions are lytic, round or oval, and expansile, with ill-defined or sclerotic margins. The medullary cavity may be expanded and may be associated with cortical thinning, intracortical tunneling, and erosion of the cortex, as well as an adjacent soft-tissue mass. Laminated periosteal new bone formation is common around the involved segment of bone, but spread across growth plates is unusual. Tubular long bone lesions may appear rapidly over 3 weeks.
Vertebral destruction may lead to flattening of the vertebral body, which is termed vertebra plana, a finding that is much more common in children than in adults and that is more common in the dorsal spine. [14, 15, 16] An associated paraspinal mass may occasionally occur. Associated kyphosis has not been described, but scoliosis can occur. Eosinophilic granuloma can produce expansile lytic lesions of the vertebral bodies and the posterior vertebral elements. However, involvement of the second cervical vertebra is extremely rare; it may cause atlantoaxial instability. 
Lung involvement is seen in as many as 20% of patients, an older subset (ie, aged 20-40 y), with an incidence of 0.05-0.5 per 100,000 patients annually. Plain radiographic findings may demonstrate an alveolar pattern in an early stage, which may be followed by nodular shadows (3-10 mm) and/or a reticulonodular pattern with a predilection for the apices. Eventually, fibrosis and a honeycomb lung may ensue. Recurrent pneumothoraces occur in 20% of patients. Hilar lymphadenopathy and pleural effusions are rare.
Degree of confidence
Plain radiography remains the mainstay of diagnosis in patients with eosinophilic granuloma, although a specific diagnosis cannot always be made without bone biopsy because children and adolescents are not spared skeletal neoplasms.
A wide variety of bone lesions may mimic eosinophilic granuloma, including infections, traumatic lesions, and neoplasms (see Differential diagnosis and other problems to be considered). A false-negative diagnosis of eosinophilic granuloma made by using plain radiographs is exceptional, although difficulty may be encountered with lesions in areas with more complex anatomy, such as the posterior elements of the vertebral bodies; in these cases, conventional tomography or CT scanning may useful.
CT scans may be particularly useful in osseous lesions in areas with complex anatomy, such as the mastoids, atlantoaxial joints, and posterior elements of the vertebral bodies. Also, soft-tissue components are better depicted with CT scanning than with other imaging modalities. The destruction of the mastoid, petrous ridge, tegmen tympani, and lateral sinus plate and the destruction of the inner and external ear are depicted elegantly on CT scans (see the images below). CT scans may demonstrate an isoattenuating and homogeneously enhancing mass in the hypothalamus/pituitary gland. 
Degree of confidence
CT scanning is considerably better than plain radiography and conventional tomography in depicting an intracranial extension of eosinophilic granuloma.
CT scan appearances of eosinophilic granuloma are nonspecific, and a variety of inflammatory and neoplastic processes may mimic this condition.
Magnetic Resonance Imaging
On spin-echo MRIs, osseous lesions of eosinophilic granuloma reveal decreased signal intensity on T1-weighted images and high signal intensity on T2-weighted sequences. The lesion may enhance after the administration of a gadolinium-based contrast agent (see the following image). 
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). For more information, see the eMedicine topic Nephrogenic Systemic Fibrosis. The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness. For more information, see the FDA Public Health Advisory or Medscape.
Degree of confidence
The value of using MRI for patients with eosinophilic granuloma lies in the sensitivity of this modality; its specificity is low. However, the cost of the procedure and the procedural problems encountered in imaging young children confer no advantages over plain radiography.
The soft-tissue component around the osseous lesion has poor definition and shows signal inhomogeneity; the appearance may mimic that of a malignant tumor, infection, or stress fracture.
Conventional radiography, CT, and radionuclide imaging usually diagnose these lesions. Ultrasound of the skull and a guided core biopsy has previously been reported in 3 cases with excellent results.  Viewing of the frontal bone solitary eosinophilic granuloma with ultrasound has been found instructive and useful. See the images below.
Compare the vascularity of an eosinophilic granuloma in the first image below, where only stretching of the vessels is noted on the angiography. Doppler interrogation in the second image below show internal vascularity not previously shown.
Eosinophilic granuloma shows a variety of activity patterns on radionuclide bone scintigrams obtained by using technetium-99m (99m Tc) diphosphonate. The bone lesions may be hot, cold, or cold with an area of increased surrounding reparative-ring activity. Areas of increased activity vary in intensity. In the lower limbs, eosinophilic granuloma lesions tend to appear more elongated and diffuse than bone metastasis. Recurrences are identified more readily with fewer false-negative findings (see the image below).
Bone lesions in all types of Langerhans cell histiocytosis are not gallium-67 (67 Ga) citrate–avid, but67 Ga imaging may be helpful for detection of nonosseous lesions. Hence, it is useful in the initial assessment and serial follow-up imaging of patients with Langerhans cell histiocytosis. Thallous chloride-201 (201 Tl) uptake detected on single-photon emission CT (SPECT) scans has been reported in a patient with skull eosinophilic granuloma, which was photon deficient on a99m Tc methylene diphosphonate uptake study. 
Degree of confidence
Radiographic examination and radionuclide bone imaging are complementary techniques in detecting bone lesions in bone marrow disorders, including eosinophilic granuloma. Scintigraphy is more useful in cases of unifocal eosinophilic granuloma than in cases of multifocal disease, in which radiography is superior.
Negative radionuclide findings occur in 35% of patients with known eosinophilic granuloma in whom plain radiographic findings are positive.
Eosinophilic granuloma shows no neovascularity, and angiography is usually not performed. Rarely, angiography may be performed in patients in whom staging is required before surgical intervention and to exclude other vascular lesions that can mimic eosinophilic granuloma, such as hemangioma and aneurysmal bone cyst. See the following image
Degree of confidence
In most instances, angiography has little or no role in the investigation of eosinophilic granuloma.