Histiocytosis X includes eosinophilic granuloma, Letterer-Siwe disease and Hand-Schuller-Christian disease, all representing phases of the first mentioned entity.

Eosinophilic granuloma (EG) was first described by Jaffe and Lichtenstein in 1944 and is characterized by single or multiple skeletal lesions occurring predominantly in children and adolescents (mostly between 3 and 10 years of age, rarely after age 15), although occasionally it is seen in older individuals. Statistically it is more common in male Caucasians, and it represents approximately 70% of the total number of cases of histiocytosis X. Since the incidence in the elderly is so low, lymphoma should be the suspected entity when facing similar findings.

Histologically the lesion consists of eosinophils and histiocytes associated with areas of necrosis, with or without lipidization.

Clinically, a solitary painful bone lesion is most common; however, a more disseminated and polyostotic disease may be encountered. The most common sites of involvement are the calvarium, mandible, spine, ribs, and long bones, particularly the femur and the humerus. The prognosis is excellent as the monostotic lesions respond to x-ray therapy and some heal spontaneously.

Since 5% of the patients with unexplained chronic interstitial lung disease prove to have EG, all these patients should be screened for the presence of osteolytic lesions.

The radiographic appearance of EG varies depending on its skeletal location. In general it is a destructive punched-out lesion varying in size from a few millimeters to several centimeters in diameter, occurring in both the membranous and long bones. The lytic borders are well defined and less than half will reveal marginal reactive sclerosis.

In the long bones the lesions appear as relatively well-defined radiolucent areas usually in the medullary region of the epiphysis and the metaphysis. With further growth they encroach on the cortical bone with endosteal erosion and osseous expansion of the cortex. The appearance can resemble that of osteomyelitis or a malignant neoplasm such as Ewing's sarcoma and lymphoma. Periosteal reaction is frequent and it usually suggests a sarcoma. However, EG is distinguished by the solid layer of periosteal new bone rather than the interrupted malignant reactive periosteal change. The periosteal reaction is only a few millimeters in depth, and the solid reaction usually indicates a benign lesion. In the skull and pelvis the lytic areas may be particularly well defined with or without surrounding sclerosis. The lesions tend to erode both tables of the skull. The outer table is more extensively destroyed, at times producing a characteristic double contour, with a radiodense focus within the lytic area termed a "button" sequestrum. Extensions to the dura or the brain substance have been described. Within the calvarium lesions are especially prevalent in the frontal bones. Button sequestrum in the calvaria has also been associated with tuberculous osteitis, radiation necrosis, metastatic carcinoma, fibrous dysplasia, epidermoid cysts and hemangioma. It has also been reported in staphylococcal abscesses, meningioma, and dermoid cysts and in an iatrogenic calvarial burr hole of a 2-year-old child.

Soft tissue masses on the scalp can be palpated and demonstrated radiographically overlaying the lytic processes in the calvaria. The sella turcica is occasionally destroyed; sometimes the mastoid air cells and the petrous portion of the temporal bone are also destroyed. The paranasal sinuses may be affected and show marked opacification due to infiltration. The mandible is often extensively involved giving the "tooth suspended in space" appearance. The changes in the spine have received the greatest attention. Vertebral destruction can lead to a flattened vertebral body, termed vertebra plana, a finding seen in children more frequently than in adults. Occasionally the lesions are lytic and slightly expansile affecting the posterior aspects of the vertebral body and the posterior elements. A paraspinal mass may be evident, simulating a soft tissue abscess related to vertebral osteomyelitis. Rarely neurologic manifestations ensue.