Determination of consensus k _Q values for megavoltage photon beams for the update of IAEA TRS-398

The IAEA is currently coordinating a multi-year project to update the TRS-398 Code of Practice for the dosimetry of external beam radiotherapy based on standards of absorbed dose to water. One major aspect of the project is the determination of new beam quality correction factors, k _Q , for megavoltage photon beams consistent with developments in radiotherapy dosimetry and technology since the publication of TRS-398 in 2000. Specifically, all values must be based on, or consistent with, the key data of ICRU Report 90. Data sets obtained from Monte Carlo (MC) calculations by advanced users and measurements at primary standards laboratories have been compiled for 23 cylindrical ionization chamber types, consisting of 725 MC-calculated and 179 experimental data points. These have been used to derive consensus k _Q values as a function of the beam quality index TPR_20,10 with a combined standard uncertainty of 0.6%. Mean values of MC-derived chamber-specific [Formula: see text] factors for cylindrical and plane-parallel chamber types in ⁶⁰Co beams have also been obtained with an estimated uncertainty of 0.4%.