Soft tissue Langerhans cell histiocytosis with secondary bone involvement in extremities: evolution of lesions in two patients.

Langerhans cell histiocytosis (LCH) is an uncommon disorder of unknown etiology with a wide spectrum of biological behavior. The most common sites of involvement are bone, skin, and lung. While osseous LCH can result in secondary soft tissue involvement, primary soft tissue LCH in extremities with secondary bone involvement is rare, and little is known about its natural course. We report two adult patients with multi-organ system LCH who had primary soft tissue lesions with secondary involvement of adjoining bones in lower extremities, and illustrate the important role played by various imaging modalities in detection and monitoring of treatment response. We also document spontaneous clinical resolution of LCH lesions without therapy, and positive response of these lesions to chemotherapy.