AN EMPIRICAL RELATIONSHIP FOR DETERMINING PHOTON-BEAM QUALITY IN TG-21 FROM A RATIO OF PERCENT DEPTH DOSES

A key component of the Radiological Physics Center's (RPC) on-site dos imetry review visits are photon beam calibrations for which determinat ion of the energy of the x rays is a key element. The ratio of ionizat ions, TPR20/TPR10, for a 10 cm x 10 cm field at depths of 20 and 10 cm for a constant SCD is used as a quantitative measure of beam quality in the Task Group 21 protocol. The RPC has measured both TPR20/TPR10 a nd the corresponding ratio of percent depth dose (D-20/D-10) at a cons tant SSD for 685 photon beams (4-25 MV) for most makes and models of a ccelerators. A strong correlation between TPR20/TPR10 and D-20/D-10 is presented which allows the determination of the TPR ratio from the me asurement of the ratio of percent depth doses. An analysis of the unce rtainty introduced in the TG-21 factors ((L) over bar/p, P-wall, P-rep l) caused by the spread in the measured data and translated into the d etermination of the TPR ratio results in an insignificant error (< 0.3 %). This empirical relationship provides an alternate technique for qu antifying the beam quality defined in the TG-21 protocol without surre ndering any loss of precision in output calibration. This technique ma y be found by those who calibrate at a fixed SSD to be an easier and q uicker method. (C) 1998 American Association of Physicists in Medicine .