Dosimetric determination of virtual photon energies in medical linear accelerator

Medical LINACs are generally produced at 6-15 MV real energy. There are also Medical LINACs at 6-10-15 MV real energies including 10 MV energy. However, the cost of LINACs rises with each additional energy added. One of the methods to increase the number of energies at LINACs is the virtual energies to be generated with different proportions of real energies. In this PhD thesis, we obtained, for the first time, a sufficient amount of virtual energy by LINACs with different proportions of real energies, and dosimetrically compared these virtual energies obtained with different proportions with the Treatment Planning data and investigated the clinical usage of these energies. The real energy contribution rates used for determining the virtual energies (to obtain the TPR20/10 values of 8, 10 and 12 MV photon energies in BJR 25), are used to obtain the TPR20cm and TPR10cm values of the 6, 10 and 15 MV photon energies in BJR 25. The TPR20/10 values obtained for virtual energies are found as 0.677 for 6 MV, 0.713 for 8 MV, 0.731 for 10 MV, 0.748 for 12 MV, and 0.763 for 15 MV. The TPR20/10 measurement assembly as explained in the reports numbered with IAEA-277 and IAEA-398 is used in measurements. In the first phase of the measurement, 10 units of RW-3 solid water phantom plates, each of which has a thickness of 1 cm, are laid onto the treatment table. In the second phase of the measurement, 10 more units of RW-3 solid water phantom plates, with a thickness of 1 cm, are added. By this way, the set-up is rearranged with the help of lasers. The TPR20/10 values of each energy is obtained by dividing the total TPR20 cm value obtained at 20 cm for that particular energy. Virtual energy assignments carried out by the ion chamber at LINAC are compared with the Treatment Planning System (TPS) data. The quality control checks undertaken for the real energies while setting up the TPS are also undertaken for the virtual energies. The TPR20/10 value for the 8 MV Near Virtual Energy ion chamber is 0,716, while the TPS TPR20/10 value is 0,714, with a difference of 0.3%. Another example is that the TPR20/10 value for the 12 MV Distant Virtual Energy ion chamber is 0,744, while the TPS TPR20/10 value is 0,748, with a difference of 0.5%. Both of these difference rates are lower than 2%, which is the acceptable rate of difference. In conclusion, in this study, it is dosimetrically demonstrated that a Medical LINAC is usable with 5 photon energies generated by adding 3 virtual photon energies (8 MV, 10 MV, 12 MV) to a Medical LINAC with 2 photon energies (6 MV and 15 MV), whereas a Medical LINAC is usable with 5 photon energies generated by adding 2 virtual photon energies (8 MV and 12 MV) to a Medical LINAC with 3 photon energies (6 MV, 10 MV, 15 MV), and that these virtual energies can safely be used for clinical treatments.