Described herein are methods for reducing or maintaining the size of a tumor in a subject, where the method involves exposing the tumor to ionizing radiation and administering to the subject a modified hyaluronan or a pharmaceutically acceptable salt or ester. The use of the modified hyaluronan enhances or potentiates the effect of ionizing radiation used in cancer treatment. Additionally, the methods described herein prevent or reduces tumor regrowth in the subject after exposing the tumor to ionizing radiation and administration of the modified hyaluronan to the subject.