The present invention provides radiosensitizer compositions, in controlled-release formulations or other acceptable formulations, particularly nitrohistidine radiosensitizer compositions, which may be administered by any suitable means including oral, intravenous, arterial infusion, intraperitoneal, intramuscular, subcutaneous, surgical, and topical. Optionally, radiosensitizer compositions may be formulated with other agents, including chemotherapy agents and agents that provide a synergistic radiosensitizing effect. Methods of potentiating radiotherapy cancer treatment of cancers in humans, particularly of astrocytomas, are also presented, wherein a radiosensitizer composition is administered and radiotherapy is directed to the site of the tumor. Chemotherapy regimens may also be used as adjuvant therapy.