Fibrous dysplasia is a bone disease characterized by an osteoblastic dysfunction resulting in a fibrous replacement of the normal medullary bone. We describe the case of a 33-year-old who presented with low back pain irradiating to her right leg. Both the computed tomography scan and magnetic resonance imaging showed an osteolytic, multicystic lesion of the right hemi-sacrum with invasion of the right S1 forame...