Guangyao Sun

SU‐E‐T‐752: Energy Spectrum and Fluence Reconstruction of High Energy Photon Beams Based on Analytical Double‐Source Model

Purpose: To improve the accuracy on unfolding photon energy spectra and fluence for dose calculation, by use of measured percentage depth dose and off‐axis ratio, an analytical double‐source model considered both photonbeam and electron contamination was constructed, an improvement of Schiff formula and finite‐size pencil beam (FSPB) model. Methods: For energy spectrum reconstruction, several regression algorithms were used to obtain the optimum solution. It was also compared with the mono‐source model that ignored the electron contamination. For energy fluence reconstruction, the double‐source FSPB was developed by considering the energy spectra of photonbeam and electron contamination, and was also reconstructed by above algorithms. Results: A representative testing sample, the AAPM 55# report as an 18MVs AECL Therac‐20 medical linac, was used. The comparison between the double‐source model and mono‐source model in energy spectrum reconstruction found that there was a great improvement in accuracy; the mean error of double‐source model was about 0.015% while the mono‐source models was about 0.1%. We found that there are two peaks at 2 and 3 MeV in the photon energy spectra and the peak of the electron energy was in the low energy. We also found that if ignoring the electron contamination, the peak of the photon energy spectra could drift to the lower energy, and the mean error became great. We also obtained the accurate energy fluence, only about 0.01% mean error in the field. Conclusions: This study has revealed the great improvement of Schiff formula and FSPB model on energy spectrum and fluence of photonbeam and electron contamination by reconstruction method, an analytical double‐source model. Our finding will help know more about detail of medical linac, and improve the dose accuracy in dose calculation of radiotherapy by use of this reconstructed information. Supported by the National Natural Science Foundation under grant No.30900386 and the Anhui Provincial Natural Science Foundation under grantNo. 090413095 and 11040606Q55.

SU‐E‐T‐711: Monte Carlo Finite‐Size Pencil Beam Dose Calculation Method Based on Energy Spectra and Fluence Reconstruction

Purpose: Dose calculation is one of the core functions in Treatment Planning System (TPS). Finite‐Size Pencil Beam(FSPB) method is used in the clinic TPS widely, with the need of calculated time and inverse plan design. Yet the results of the traditional FSPB method may disagree with the practical situation due to lack of practical information of a given medical accelerator such as energy spectra and fluence. Methods: This study mainly focused on the shortage of traditional FSPB method, developing a new photondose calculation based on the Monte Carlo Finite Size Pencil Beam (MCFSPB) in the Advanced/Accurate Radiotherapy System (ARTS). Based on the MC simulation and the technology of medical accelerator energy spectrum reconstruction, a new pencil beam kernel model was constructed. In the condition of the filters influence, fluence reconstruction was also a part of MCFSPB model. With the convolution of fluence and MCFSPB kernel, energy deposition of body (or phantom) would be known. Results: Based on the above studies, we designed the MCFSPB method and implemented it with the visual c++ development tool. With several tests including the comparison among the AAPM55 Report sample, the results showed that the average error in the field size was less than 0.5% in the homogeneous phantom and less than 2% in the heterogeneous phantom. Conclusions: This method met the clinical criteria, and would be expected to be used as a fast and accurate dose calculation engine for clinic TPS. Supported by the National Natural Science Foundation under grant No.30900386 and the Anhui Provincial Natural Science Foundation under grant No. 090413095 and 11040606Q55.