Abstract: The present invention provides a hydrogel for comprising a biodegradable polyphosphazene polymer, a radiation-sensitive diselenide cross-linker; and one or more payloads releasably loaded within the hydrogel. The present invention further provides methods for radiosensitizing target tissues such as tumors and providing sustained delivery of therapeutics triggered by irradiation. In another aspect, the present invention provides a method that includes: introducing the hydrogel, as describes herein, adjacent to malignant or marginal tissue; and administering radiation to the hydrogel, thereby disrupting the selenocystamine cross-linkers and releasing the one or more payloads.