There are different types of “burning”. Some people described burning with the sensation of healing incisions from surgery, sometimes it is that itch you can’t reach to scratch. NEVER put anything down the cast to scratch it. Try a hair dryer on a cool setting. The only kind of “burning” that is concerning is from a nerve injury. If you have burning which is more like tingling (like when your foot falls asleep) that is new and getting worse, especially if there is new numbness in the toes, you should have the cast removed and the ankle examined.