Radiotherapy out-of-field dosimetry: Experimental and computational results for photons in a water tank

The first objective of this work was to check and select a set of four kinds of passive photon, dosimeters (two thermo-luminescence dosimeter (TLD) types, one radiophotoluminescence (RPL) dosimeter and one optically stimulated luminescence (OSL) dosimeter) together with a common measurement protocol. Dosimeters were calibrated in a reference clinical linear acccelerator beam in a water tank at a reference facility at the Laboratoire National Henri Becquerel (CEA LIST/LNE LNHB, Saclay. Radiation qualities of 6, 12 and 20 MV were used with standard calibration conditions described in IAEA TRS 398 and non-standard conditions. Profile and depth dose ion chamber measurements were also made to provide reference values. Measurements were made in a water tank into which pipes could be inserted which held dosimeters in pre-determined and reproducible positions. The water tank was built to enable investigation of doses up to 60 cm from the beam axis. A first set of experiments was carried out with the beam passing through the tank. From this first experiment, penumbra and out-of-field dose profiles including water and collimator scatter and leakage were found over three orders of magnitude. Two further sets of experiments using the same experimental arrangement with the beam outside the tank, to avoid water scatter, were designed to measure collimator scatter and leakage by closing the jaws of the collimator. Depending on the energy, typical leakage and collimator scatter represents 10-40% and 30-50% of the total out-of-field doses respectively. It was concluded that all dosimeters can be used for out-of-field photon dosimetry. All show good uniformity, good reproducibility, and can be used down to low doses expected at distances remote from the subsequent radiotherapy target volume.