SU-E-T-422: Monte Carlo Simulations for Photon Energy Modulation in Radiotherapy

Purpose: A device to modulate photon energy during beam‐on time was designed and simulated with the Monte Carlo(MC) method. Method and Materials:A device named energy modulator was a cylinder made of aluminum. The frame of that device was connected with 20 tubes through which mercury could be injected or drained. The mercury thickness inside the device could modulate the photon energy of beam penetrated. Monte Carlo simulations of 6 and 10 MV linac using the EGSnrc code were conducted to evaluate the performance of the device. After the design study, a flattening filter of the linac was replaced with the energy modulator in the MC simulations. At least 5 billion histories were followed for each simulation to create phase‐space files at 100 cm SSD. The thickness of mercury inside the energy modulator varied from 2 mm to 40 mm at the field size of 10 cm by 10 cm. The changes of beam quality were expressed in terms of percentage depth dose (PDD) at the depth of 10 cm and 20 cm in water. Results: As increasing the thickness of mercury, the PDD of 6 MV beam at 10 cm and 20 cm was changed from 63.8% to 68.2% and from 34.5% to 40.2%, respectively. The PDD of 10 MV beam was changed from 68.7% to 73.1% at the depth of 10 cm and 40.2% to 45.7% at the depth of 20 cm. Conclusions: The results of this study showed the feasibility of the energy modulation during beam‐on time, which could be advantageous in dose conformity.