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Monte Carlo Study of Small-Field Dosimetry for An Elekta MR-Linac System

M Yano*, F Araki , T Ohno , Kumamoto University, Kumamoto, 43


(Sunday, 7/14/2019) 4:00 PM - 4:30 PM

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 cm² 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 cm³), and PTW-31022 (0.016 cm³) and a diode of PTW-60018 (0.25 mm³). The beam quality conversion factor, k�Q,B₎, for ionization chambers was also obtained. The OPFs for detectors were compared with those in water and also those measured by O’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 O’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 cm² field. The OPFs for PTW-31021 and PTW-31022 chambers and the PTW-60018 diode agreed within 1% to those measured by O’Brien et al. The difference in SPRs between 1 x 1 and 10 x 10 cm² fields was within 0.5%. Furthermore, k�Q,B₎ 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 O’Brien et al.


Monte Carlo, Magnetic Fields, Dosimetry


TH- External beam- photons: Monte Carlo

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