Coloured X-ray of lesions in the femur (thigh bone) of a 60 year old female patient with Erdheim-Chester disease (ECD). ECD is a rare blood cancer caused by the abnormal proliferation of histiocyte white blood cells. Accumulation and infiltration of the histiocytes into different tissues causes inflammation and damage. The long bones, such as the femur, are almost always affected and bone pain is the most common symptom. Treatments for ECD include chemotherapy and immunotherapy, although the prognosis is poor.