Imaging description

A 12-year-old boy presented with chronic pain of the right knee. The pain became gradually worse over a three-month period and increased with pressure and after long walks. There was no history of trauma. On physical examination, no swelling or redness of the right knee was noted. Mild external rotation of the right hip as well as limited active and passive internal rotation was seen on examination.

An X-ray of the right knee was unremarkable. An X-ray of the right hip showed a focal, well-defined, round osteolytic lesion with a diameter of approximately 1 cm in the inferior acetabulum (Fig. 86.1). An MRI and 18F-FDG PET were performed for further evaluation. T2 STIR images demonstrated a more extensive hyperintense lesion in the right ischium and an additional hyperintense lesion in the right femur (Fig. 86.2). These lesions showed enhancement on contrast-enhanced T1-weighted MR scans (not shown) and increased metabolic activity on 18F-FDG PET (Fig. 86.2). The diagnosis of Langerhans cell histiocytosis (LCH) was established by CT-guided biopsy and histopathology. Chemotherapy according to the LCH-III protocol was initiated and well tolerated by the patient. Follow-up simultaneous 18F-FDG PET/MR scans performed after treatment showed complete remission of both lesions. However, a new T2-hyperintense lesion was noted in the right ilium, which demonstrated normal 18F-FDG uptake (Fig. 86.3). It was assumed that this lesion had occurred after the initial 18F-FDG PET/MR scans and had been successfully treated as well. Thus, a conservative strategy without further systemic chemotherapy was chosen and a follow-up MRI after six months confirmed reduced lesion size.