ENTRANCE AND EXIT DOSE MEASUREMENTS WITH SEMICONDUCTORS AND THERMOLUMINESCENT DOSIMETERS - A COMPARISON OF METHODS AND IN-VIVO RESULTS

Background and purpose: In order to compare diodes and TLD for in vivo dosimetry, systematic measurements of entrance and exit doses were pe rformed with semiconductor detectors and thermoluminescent dosemeters for brain and head and neck patients treated isocentrically with exter nal photon beam therapy. Material and methods: Scanditronix EDP-20 dio des and (LiF)-Li-7 thermoluminescent chips, irradiated in a 8 MV linac , were studied with similar build-up cap geometries and materials in o rder to assure an equivalent electronic equilibrium. Identical calibra tion methodology was applied to both detectors for the dose determinat ion in clinical conditions. Results: For the entrance dose evaluation over 249 field measurements, the ratio of the measured dose to the exp ected dose, calculated from tabulated tissue maximum ratios, was equal to 1.010+/-0.028 (1 S.D.) from diodes and 1.013+/-0.041 from thermolu minescent crystals. For the exit dose measurements, these ratios were equal to 0.998+/-0.049 and 1.016+/-0.070 for diodes and TLDs, respecti vely, after application of a simple inhomogeneity correction to the ca lculation of the expected exit dose. Conclusions: Thermoluminescence a nd semiconductors led to identical results for entrance and exit dose evaluation but TLDs were characterised by a lower reproducibility inhe rent to the TL process itself and to the acquisition and annihilation procedures.