P153] A comparison of different dosimetry systems for surface and buildup region dose measurements

Abstract

Purpose In this study, the surface doses obtained by optically stimulated luminescence (OSL), thermoluminescence (TLD) and markus parallel plane ion chamber were compared with TPS results and the availability of the systems in surface dosimetry was investigated. Methods The percentage depth dose (PDD) measurements of buildup region were made by TLD, OSL and markus pp ion chamber, at the surface, 1, 2, 5, 10 and 15 mm water equivalent depth at 100 cm source-detector distance for 5×5, 10×10, and 20×20 cm 2 field sizes.DD values were measured at the depth of 0.07 mm, which was suggested for surface dose measurements by ICRU Report 39. The effective measurement depth of each dosimeter was taken into account. The water equivalent thickness (WET) was considered to make an accurate comparison. Interpolations and extrapolations were performed for all systems to obtain the doses at the same depths. Under the same conditions, surface doses were calculated by TPS for different field sizes and were recorded. The buildup region doses measured by different dosimetry systems and calculated by TPS were compared for different field sizes. Results The overdoses occured in the buildup region were corrected for Markus chamber according to Gerbi’s method. The surface doses using 6 MV photon beams for 5×5, 10×10, and 20×20 cm 2 field sizes at 0.07 mm were found to be 14%, 24.75%, 13.17%, 7.74%; 19.69%, 31.78%, 21.63%, 11.04%; 30.87%, 43.80%, 28.71%, 13.61% for Markus chamber, TLD, OSL and TPS, respectively. Conclusions The lowest build-up region PDD values in all field sizes were obtained with the TPS. At 0.07 mm water equivalent depth, while the OSL and Markus pp. ion chamber results are close to each other, TLD results are higher in all field sizes. However, it is seen that the TLD gives closer results to Markus pp. ion chamber than OSL after 3 mm depth in all field sizes.