TLD EXTRAPOLATION MEASUREMENTS FOR ENTRANCE AND EXIT DOSE IN RADIOTHERAPY

If LIF TLD chips of three different thicknesses (0.230, 0.099 and 0.03 8 g.cm(-2)) are exposed in the same radiation held it is possible to e xtrapolate their dose reading to an infinitesimal thin detector size. This TLD extrapolation technique was used to determine the entrance an d exit surface dose on a solid water phantom in various photon beams o f radiotherapy treatment units. Both build-up and build-down effect co uld be assessed in dose measurements for telecaesium and megavoltage X ray beams from a medical linear accelerator while a rapid dose fair-o ff was observed at the phantom surface in a 120 kV(p) beam due to elec tron contamination from the lead glass treatment cone. One of the majo r advantages of the TLD extrapolation technique is that the dose can b e interpolated and extrapolated for all vital structures within the fi rst millimetre of the skin.