Investigation of different target materials of medical linac on photon percentage depth-dose profiles: PHITS Monte Carlo study

Background: X-ray testing has been widely employed for examining internal structures in various fields since its discovery by Röntgen. Medical linear accelerators (linacs) are devices capable of generating X-rays using bremsstrahlung radiation. However, limited research exists on the properties of photons produced by different targets in the medical linac head. The aim of this study is to investigate the different target materials such as copper (Cu), gold (Au) and tungsten (W) on medical linac on photon percentage depth-dose (PDD) profiles using Particle and Heavy Ion Transport code System (PHITS). Methods: In this study, the PHITS Monte Carlo (MC) software was utilized to investigate the X-rays produced by copper, gold, and tungsten targets at varying incident energies of 10, 15, and 20 MeV. The linear energy transfer (LET) produced from these targets at various energies was examined and compared. The beam quality index of the simulated PDD profiles was investigated using the tissue tissue-phantom ratio at 20 and 10 cm depths (TPR20,10). In addition, an empirical model was developed to mimic the photon PDD profile curves using the buildup and tail functions. The buildup function, which is represented as dd2+n, incorporates a beam hardening factor (n), while the tail function, expressed as e−μd, adjusts the slope with the linear attenuation coefficient (μ). Results: The PHITS-simulated PDD is obtained and compared with the modified buildup and tail functions and the resultant PDD analysis shows increasing trends for copper with energy, while gold and tungsten maintained consistent edges. Copper exhibited the highest TPR values. Validation of the empirical model is achieved through comparison with simulated PDDs. Conclusions: In conclusion, among the target materials, copper demonstrates higher dose deposition as energy increases, whereas gold and tungsten maintain consistent PDD profiles. TPR20,10 values highlight copper’s superior beam penetration ability, and copper also exhibits the highest LET across all energy levels. The research includes a comparison between simulated PDDs and an empirical model, providing valuable insights for radiation therapy treatment planning.