Both Arms And My Chest Have Significant Sun Damage. My

Both arms and my chest have significant sun damage. My dermatologist prescribed Fluorouracil Cream, USP 5% and instructed me to apply it twice per day. He told me to start with only one arm and apply for 2 weeks, twice per day and then either do my chest or the other arm for the same duration as the first. Why can't I do all three areas at the same time?