Linear accelerators producing Xrays of greater energy than 10MV are used in radiation therapy at many hospitals, although neutrons are known to be generated by such high energy X rays. Moreover, induction radioactivity is produced by neutrons in the irradiation room (the linear accelerator room). Exposures to gamma rays from residual radioactivity in the irradiation room can be harmful. In spite of many reports available on the production of neutrons, few investigations have been conducted on the assessment of residual radioactivity in the irradiation room. In the present study, radiation doses from residual radioactivity in the irradiation room were measured after 10MV X ray irradiation. The folloing results were obtained: (1) Residual radiations were predominantly gamma rays with the half life of about 1 to 2 minutes. The radiation 45dose in the irradiation room returned to the background level (the preirradiation level) in 15minutes after irradiation. (2) The amount of residual gamma rays increased with the Xray output, and, conversely, an increasing of residual gamma rays was noted with the closing of the X ray irradiation field. Finally, (3) Aliminium (Al) used in the treatment bed was found out to be activated by neutrons, suggesting that the replacement of Al with a nonactivating substance would eliminate exposure to gamma rays resulting from activated Al. Therefore, the use of beds constructed of material other than Al is recommended as a means of lessening the potentially harmful effects of gamma rays on radiologists, technicians and nureses involved in radiation therapy.

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