An investigation of the depth dose in the build-up region, and surface dose for a 6-MV therapeutic photon beam: Monte Carlo simulation and measurements

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An investigation of the depth dose in the build-up region, and surface dose for a 6-MV therapeutic photon beam: Monte Carlo simulation and measurements

The percentage depth doses in the build-up region and the surface doses for the 6-MV photon beams from a Varian Clinac 23EX medical linear accelerator was investigated for square field sizes of 5x5, 10x10, 15x15, and 20x20 cm2 using the EGS4nrc Monte Carlo (MC) simulation package. The depth dose was found to change rapidly in the build-up region, and the percentage surface dose increased proportionally with the field size from approximately 10% to 30%. The measurements were also taken using four common detectors: TLD chips, PFD dosimeter, parallel-plate and cylindrical ionization chamber, and compared with MC simulated data, which served as the gold standard in our study. The surface doses obtained from each detector were derived from the extrapolation of the measured depth doses near the surface and were all found to be higher than that of the MC simulation. The lowest and the highest over-responses in the surface dose measurement were found with the TLD chip and the CC13 cylindrical ionization chamber, respectively. Increasing the field size increased the percentage surface dose almost linearly in the various dosimeters and also in the MC simulation. Interestingly, the use of the CC13 ionization chamber eliminates the high gradient feature of the depth dose near the surface. The correction factors for the measured surface dose from each dosimeter for the square field sizes of between 5x5 and 20x20 cm2 are introduced.

Keywords: dose in build-up region / surface dose / Monte Carlo simulation / correction factor

Lukkana APIPUNYASOPON1, Somyot SRISATIT1 and Nakorn PHAISANGITTISAKUL2,3 1Department of Nuclear Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand 2Department of Physics, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand 3ThEP Center, CHE, 328 Si-Ayuttaya Road, Bangkok 10400, Thailand