BACKGROUND AND PURPOSE: The normalisation depth for determination of output factors in photon fields has frequently been the depth of dose maximum. At high energies the contribution from contaminating electrons is significant at dose maximum and is critically dependent on the beam geometry parameters, which is why a larger depth should be preferred. MATERIALS AND METHODS: The effect of electron contamination was studied using a purging magnet to remove charged particles from the treatment head and a helium bag to minimise production between the head and the phantom. RESULTS: A depth of 10 cm was found to be beyond the range of the contaminating electrons for photon energies up to 20 MV (TPR(20)(10) = 0.772). However, at 50 MV (TPR(20)(10) = 0.810) contaminating electrons contribute 2-3% to the absorbed dose at 10 cm depth. CONCLUSIONS: 10 cm is recommended as both reference and normalisation depth for all megavoltage photon beam qualities, i.e. 60Co and X-rays from accelerators up to 50 MV.
BACKGROUND AND PURPOSE: The normalisation depth for determination of output factors in photon fields has frequently been the depth of dose maximum. At high energies the contribution from contaminating electrons is significant at dose maximum and is critically dependent on the beam geometry parameters, which is why a larger depth should be preferred. MATERIALS AND METHODS: The effect of electron contamination was studied using a purging magnet to remove charged particles from the treatment head and a helium bag to minimise production between the head and the phantom. RESULTS: A depth of 10 cm was found to be beyond the range of the contaminating electrons for photon energies up to 20 MV (TPR(20)(10) = 0.772). However, at 50 MV (TPR(20)(10) = 0.810) contaminating electrons contribute 2-3% to the absorbed dose at 10 cm depth. CONCLUSIONS: 10 cm is recommended as both reference and normalisation depth for all megavoltage photon beam qualities, i.e. 60Co and X-rays from accelerators up to 50 MV.