Photon energy synthesis method on medical linac for radiotherapy

Objective To propose an energy synthesis method that can fit photons with any known intermediate energies through high- and low-energy photons and compare it with the existing method. Methods Percentage depth dose (PDD) and off-center ratio (OCR) are two important characteristics that affect the calculation accuracy of the beam model. The study adopted the PDD and OCR of the gold standard data (6 and 15 MV beams) of Varian TrueBeam accelerator, and used the least squares fitting method to fit the intermediate energy (E_syn-10 MV). The E_syn-10 MV was compared with the gold standard data (TB-10 MV), and the proposed method was compared with other fitting method for validating the feasibility and validity of the method. The accuracy of the energy synthesis method was further verified by experiments with 3D water phantom and the actual cases of different tumor sites. Results Compared with the energy synthesis method that only considers PDD, the proposed method obtained the E_syn-10 MV with increased PDD root mean square error under each size of the field and reduced OCR root mean square error (especially for fields above 20 cm), and the root mean square errors of PDD and OCR were all less than 1.0% and 0.5%. The experimental results of the proposed method on 3D water phantom and the actual cases were also better than those of the energy synthesis method that only considers PDD. Conclusion The energy synthesis method that considers both PDD and OCR is superior to that only considering PDD, and the differences in PDD and OCR between E_syn-10 MV photons and TB-10 MV photons are trivial. The consistency of 3D dose distribution in different radiotherapy plans for 3D water phantom and the actual cases is high. Therefore, the synthetic energy photons can replace actual energy photons for clinical treatment.