Calculating weighting factors for mixing megavoltage photon beams to achieve desirable dose distribution in Radiotherapy

Introduction: Many studies have shown the effects of delivered dose distribution due to the incident photon energy on the tumor and healthy tissues. The ability to access the most appropriate radiation energy is essential to achieve the optimal treatment planning but there is a serious limitation in number of energies available on radiation therapy machines can restrict it. Materials and Methods: In this research, Electa linear accelerator with 6, 10 and 18MV photon energies and MATLAB software for analytical calculations were applied. The percentage depth dose (PDD) was measured for 6, 10 and 18MV energies in different depths with source-surface distance (SSD) of 100 cm2 for 4×4 to 40×40 cm2 fields. A double exponential function was fitted to the PDDs curves of 6 and 18MV using MATLAB software. Then, using the quality factor equation presented by LaRivere et al. and combination of 6 and 18MV energies, weighting factors to achieve desirable energies for the field of 10×10 cm2 were calculated. To verify the accuracy of this analytical method, PDD of 10MV energy was measured using ion chamber. The PDD obtained from dosimetry of 10MV energy, was compared with the results of mixing 6 and 18MV energies using the gamma index. Moreover, the dosimetric characteristics such as dmax, d50%, d80% and PDD10cm obtained from the mixed energy were compared with ones obtained from the measurement. Results: The value of the weighting factor of 6MV energy needed for the 10 ×10cm2 field to create dose distribution of 10MV energy using the mix was obtained as equal to 0.57. The obtained results from the mix of 6 and 18 MV energies and dosimetry of 10MV had good compatibility. The gamma index with two criteria of the percent dose difference (∆dM = 0.2cm) and distance to agreement (∆DM = 2%), expect at the points near the edges of the geometric fields, show values less than 1. Conclusion: The simultaneous use of the high and low photon energies with different weighting factors to achieve desirable energy makes possible the treatment of tumors located at various depths without the need for different modes of energy in the accelerator and leads to decrease in the costs of the equipment.