The doctor's new treatment plan has ameliorated his condition significantly.

ameliorated 是动词 ameliorate 的过去分词形式。ameliorate 由前缀 a-（表示"加强"）和拉丁词根 melior（意为"更好"）组成，字面意思是"使变得更好"，即"改善、缓解"的意思。在句中表示"已经显著改善了"的状态。