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Findings: AP and lateral view of the skull were obtained and reveal an approximate 3 cm. circular lytic lesion overlying the left frontoparietal region, directly over the coronal suture. Again the margins are well delineated. There was no associated soft tissue swelling over this region. No other similar such lesions are identified.

Diagnosis:Histiocytosis X (Langerhans Cell Histiocytosis)

Differential Diagnosis:The differential for a solitary lytic lesion in the skull includes metastatic lesion, myeloma, fibrous dysplasia, tuberculosis, trauma, histiocytosis X, osteomyelitis, leptomeningeal cyst, and epidermoid/dermoid.

There is no history of previous trauma, nor any previous history of infection, therefore, leptomeningeal cyst and osteomyelitis are remote possibilities. In addition, there is no associated soft tissue swelling so this lesion is not attributable to any recent trauma. Myeloma would also be excluded in this age group. As there is no evidence of sclerotic margin and in addition there is a suggestion of a beveled edge, the most likely diagnosis in this age group is histiocytosis X.

Discussion: The diagnosis histiocytosis X also known as Langerhans cell histiocytosis has a number of variants. In this case it is known as eosinophilic granuloma which is the most benign variety of histiocytosis X comprising about (60% to 80%) of the cases and is primarily localized to bone. Histiocytosis X is a proliferative disorder of macrophages of unknown etiology causing cholesterol laden macrophages to collect in bone and soft tissues. The lesions may occur in any portion of the body including bones, skin, brain, lungs, lymph nodes, liver, and spleen. Although cholesterol deposition is prominent in macrophages and there are scattered eosinophils there is no defined abnormality related to cholesterol metabolism or peripheral eosinophilia. Histiocytosis represents three related diseases which range from benign to aggressive.

1) Eosinophilic granuloma which is a relatively benign self limiting unifocal disease involving lytic lesions of bones in children and young adults. The calvarium is most often affected, thereafter followed in decreasing order of frequency the mandible, spine, ribs, large long bones.

2) The second is Hand Schuller Christian disease which also affects children and young adults but is a multifocal disease involving bones and soft tissues and is accompanied by symptoms related to affected organ. The base of the skull is characteristically involved in (10%) of individuals producing the triad of proptosis, lytic bone lesions and diabetes insipidus. Lytic lesions in the jaw bone may yield a radiologic impression of floating teeth. (20%) of patients have lymphadenopathy and lung infiltrates. Deafness may be due to involvement of the external auditory canal and mastoid air cells. (30%) demonstrate hepatosplenomegaly. (40%) have bone lesions, half of which involve the skull.

3) The last entity is Letterer-Siwe disease which is an aggressive systemic disease affecting children less than two years old leading to failure to thrive. Multiple organ involvement culminates in massive hepatosplenomegaly, lymphadenopathy, leukopenia, thrombocytopenia, and widely scattered hemorrhagic skin lesions. Bony lesions may be multiple and widespread, but unlike the other two disease entities, bone lesions are not prominent. Progressive marrow replacement and pulmonary infiltration occasionally causes death.

References:
Rubin & Farber: A Pathologic Basis of Disease Processes 1988, 1336-1338.

Dahnert: A Radiology Review Manual, 1991, 56-57.

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