Response of LiF powder to 125I photons

125I seeds are being used to treat a variety of diseases, including brain tumors. However, the uncertainty in the energy response of LiF for 125I photons relative to megavoltage radiation limits the accuracy of dosimetry with this material. An array of 12 40-mCi 125I 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 60Co 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 125I relative to that for 60Co. Details of the procedure and sources of uncertainty are discussed.