Expansile lucent lesion within the neck of the left 10th rib with thin sclerotic margin and narrow zone of transition. Cortex is thinned and deficient in places. No soft-tissue mass outside the bone. No chondroid, osteoid or ground glass matrix. Major differential based on CT appearance would include fibrous dysplasia, enchondroma and aneurysmal bone cyst. Metastasis unlikely in absence of any history of malignancy.