Room: Exhibit Hall | Forum 7
Purpose: To investigate small-field dosimetry for an Elekta MR-linac system using Monte Carlo (MC) simulations.
Methods: The EGSnrc/BEAMnrc code was used to simulate the Elekta MR-linac treatment head. The percentage depth-doses (PDDs) and off-axis dose profiles (OARs) were calculated for field sizes of 1 x 1 to 10 x 10 cmÂ² using the DOSXYZnrc code, and output factors (OPFs) were obtained by the egs_chamber code. The OARs and OPFs were calculated at depth of 10 cm in water at a source-axis-distance of 143.5 cm. The water-to-air stopping power ratios (SPRs) were also calculated by MC simulations. In addition, OPFs were calculated for detectors with different sensitive volumes: ionization chambers of PTW-31021 (0.07 cmÂ³), and PTW-31022 (0.016 cmÂ³) and a diode of PTW-60018 (0.25 mmÂ³). The beam quality conversion factor, kâ‚?Q,Bâ‚Ž, for ionization chambers was also obtained. The OPFs for detectors were compared with those in water and also those measured by Oâ€™Brien et al.
Results: The accuracy of the simulated beam was validated with agreement within 2%, by comparing MC-calculated PDDs and OCRs with those measured by Oâ€™Brien et al. The OPFs with the chambers decreased rapidly for small fields depending on the sensitive volume. The calculated OPFs for the PTW-31022 agreed within 2% with those in water down to a 1 x 1 cmÂ² field. The OPFs for PTW-31021 and PTW-31022 chambers and the PTW-60018 diode agreed within 1% to those measured by Oâ€™Brien et al. The difference in SPRs between 1 x 1 and 10 x 10 cmÂ² fields was within 0.5%. Furthermore, kâ‚?Q,Bâ‚Ž values for PTW-31021 and PTW-31022 chambers were 0.977 and 0.995, respectively.
Conclusion: We simulated the Elekta MR-linac treatment head by the MC method. Calculated PDDs, OCRs, and OPFs were in good agreement with those measured by Oâ€™Brien et al.