A rapidly expanding calvarial Langerhans cell histiocytosis with low Ki-67 in an adult: a challenging diagnosis on magnetic resonance imaging.

Langerhans cell histiocytosis may have a wide variety of clinical presentations. The growth of localised form is usually slow and it mainly involves bones such as the skull, femur, spine, ribs, mandible and pelvis in children. The spectrum of clinical manifestations and magnetic resonance imaging findings of the disease may sometimes mimic infections as well as benign and malignant tumours. In this report, we describe an unusual case of rapidly progressive calvarial Langerhans cell histiocytosis with a painful and tender mass in a 27-year-old man that created diagnostic and therapeutic challenges. We also discuss the relationship between Ki-67 proliferative index and the expansion of the disease. Magnetic resonance imaging studies revealed a bone defect on the left frontal bone with soft tissue components, showing a prominent expansion within a month. The clinical manifestation with the enhancement pattern of epidural soft tissue component on contrast-enhanced magnetic resonance imaging suggested an infection besides Langerhans cell histiocytosis. Immunohistochemical expression of Ki-67 antigen was 5%. The presence of a relatively low Ki-67 value indicates that the inflammatory response may have a more important role than local Langerhans cell proliferation in the aggressive clinical course and rapid expansion.