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If LiF TLD chips of three different thicknesses (0.230, 0.099 and 0.038 g.cm -2 ) are exposed in the same radiation field it is possible to extrapolate their dose reading to an infinitesimal thin detector size. This TLD extrapolation technique was used to determine the entrance and exit surface dose on a solid water phantom in various photon beams of radiotherapy treatment units. Both build-up and build-down effect could be assessed in dose measurements for telecaesium and megavoltage X ray beams from a medical linear accelerator while a rapid dose fall-off was observed at the phantom surface in a 120 kV P beam due to electron contamination from the lead glass treatment cone. One of the major advantages of the TLD extrapolation technique is that the dose can be interpolated and extrapolated for all vital structures within the first millimetre of the skin.