Periosteal reaction may be classified as benign or aggressive based on the time course of the initiating process. Benign periosteal reaction is a low-grade chronic irritation which allows time for the formation of normal or near-normal cortex. The cortex will be thick and dense and has a wavy or uniform appearance. Benign periosteal reactions can be seen in callus formation in a fracture or with slowly growing tumours. Aggressive periosteal reaction is a rapid irritation process that does not allow the periosteum time to lay down and consolidate new bone to form normal cortex. The cortex may appear lamellated, amorphous, or sunburst-like. Aggressive periosteal reactions can not only be seen with malignant tumours, but also with more benign processes like infection, eosinophilic granuloma (Langerhans cell histiocytosis), aneurysmal bone cyst, osteoid osteoma, haemophilia, and trauma.