Answer: Langerhans cell histiocytosis

Findings:  

-Multifocal well-defined osteolytic masses in left femur and skull with mixed aggressive andnon-aggressive features

Differential Diagnosis:  

-Langerhans cell histiocystosis

-Polyostotic fibrous dysplasia

-Lymphoma / Ewing sarcoma

Discussion:  

Langerhans cell histiocytosis (LCH)

-Group of disorders involving abnormal proliferation of histiocytes in reticuloendothelial system

-Location: flat bones (65-70%), long bones (25-30%), spine (9%)

-Age: 60% in under age 10

-Monostotic (66-75%), polyostotic (25-64%)

-Early lesions at any site may appear highly aggressive

permeative, nongeographic, no sclerotic margin

periosteal reaction

cortical breakthrough, soft tissue mass

-Mature lesions: geographic, less aggressive

linear periosteal reaction

endosteal scalloping, minimal bone expansion

-Aggressive early phase of LCH may mimic other aggressive “small round blue cell” lesions such as Ewing sarcoma, osteomyelitis, neuroblastoma metastasis, and lymphoma
Late phase suggests nonaggressive or benign process

References:

Manaster BJ, Roberts CC, Petersilge CA. Diagnostic imaging. Musculoskeletal: Non-traumatic disease. Amirsys. 2010.