The absorbed doses to water and the TLD-100 signal contributions associated with the neutron contamination of a clinical 18Â MV photon beam

Neutrons produced by the photonuclear effect have to be considered as unwanted concomitants of radiotherapy photon beams. Therefore the response of TLD-100 to thermal neutrons via the L6i(n,α)3H reaction may disturb TLD-100 measurements of the absorbed dose to water DW in water (a) on the beam axis as part of the external quality control system by mailed TLD probes, and (b) at out-of-field positions utilizing their time-integrating properties. The present study aims at quantitatively estimating the possible disturbances and identifying the situations where TLD-100 has to be replaced by the TLD-600/TLD-700 system to get rid of the unwanted neutron induced TL signal. This work fills the gap of knowledge between on-axis and far out-of-field data. The absorbed doses to water and to a TLD-100 chip, caused by neutrons and photons at several positions in a water phantom, were determined using the EGSnrc and the FLUKA Monte Carlo codes, and for the H4e and H3 ions released in the L6i(n,α)3H reaction a relative TL efficiency of 0.3, compared with high-energy secondary electrons, has been adopted. It was found that (1) the total DW profile is negligibly neutron-affected up to 10cm off-axis distance, (2) the perturbation of the DW reading of a TLD-100 detector due to its neutron response can be neglected within the borders of the photon field, but becomes significant beyond the field edge, (3) the non-reference condition correction factor kNR for TLD-100-based DW measurements in water is accordingly affected at off-axis positions.