An object of the present invention is to provide a method for improving the effect of radiation therapy in treating cancer such as melanoma, lymphoma, adenocarcinoma, and squamous cell carcinoma. A method for administering a radiation sensitizer represented by the formula (I) in a dose of 0.5 mg / kg to 8 mg / kg per dose. It is preferable to repeat the administration of the radiosensitizer and the irradiation immediately after the administration at a frequency of once or less per day and once or more per month. (In the formula, R1 represents an acyl residue of a fatty acid.) [Selection Diagram] None【課題】メラノーマ、リンパ腫、腺癌、扁平上皮がん等のがん治療において、放射線療法の効果を向上させる方法の提供。【解決手段】式（Ｉ）で表される放射線増感剤を、１回あたり０．５ｍｇ／ｋｇ以上、８ｍｇ／ｋｇ以下投与する方法。放射線増感剤の投与及び投与直後の放射線照射のセットを、１日に１回以下かつ１ヶ月に１回以上の頻度で繰り返すことが好ましい。（式中、Ｒ１は脂肪酸のアシル残基を表す。）【選択図】なし
