Response of LiF powder to 1 2 5 I photons

1 2 5 I seeds are being used to treat a variety of diseases, including brain tumors. However, the uncertainty in the energy response of LiF for 1 2 5 I photons relative to megavoltage radiation limits the accuracy of dosimetry with this material. An array of 12 40‐mCi 1 2 5 I seeds was used to irradiate LiF powder in throwaway capsules, and an ion chamber was used to measure the absorbed dose. A comparison of the response of these LiF dosimeters with the response of capsules given a similar dose of 6 0 Co radiation has yielded values of 1.39±0.03 (dose to water) and 1.32±0.03 (dose to muscle) for the LiF response for 1 2 5 I relative to that for 6 0 Co. Details of the procedure and sources of uncertainty are discussed.