PERCENTAGE DEPTH DOSE RECONSTRUCTION OF 60Co TELETHERAPHY UNIT WITH SSD VARIATION USING MAYNEORD FACTORS AND TAR METHODS BY MONTE CARLO EGSNRC SIMULATION

Total Body Irradiation (TBI) technique used for treating blood cancer requires an extended source-to-surface distance (SSD) that affects the distribution of percent depth dose (PDD). This study aims to reconstruct the PDD of 60Co teletherapy unit with SSD variations and analyze the reconstructed PDD against simulated PDD. The simulation of PDD data was performed using BEAMnrc and DOSRZnrc Monte Carlo code based on International Atomic Energy Agency (IAEA) phase space file at 72 cm of SSD. PDD data was collected for depths of 0 cm to 13 cm in a water phantom using a field size of 5 x 5 cm2 . PDD was reconstructed at extended SSDs of 167 cm and 210 cm by using Mayneord factor and Tissue-Air-Ratio (TAR) methods. The simulated and reconstructed PDD of 60Co unit were then analyzed by using gamma analysis. PDD resulted from the two reconstruction methods have a gamma analysis value of 100% at a tolerance of 5% - 5 mm compared to the simulated PDD. The standard deviation value of the reconstructed PDD to the simulated PDD is between 0.55% and 3.74%. The maximum value of absolute deviation from the reconstructed PDD to the simulated PDD was between 1.09% and 8.55%. There is no significant !"#$ depth shift as the SSD value increases. The TAR method is superior to the Mayneord factor based on gamma analysis values, deviation values, small shifts in !"#$, and dose change patterns. However, both reconstruction methods give good results compared to the simulation data.