Monte Carlo study of small-field dosimetry for an ELEKTA Unity MR-Linac system

This study aims to investigate the magnetic field effect in small-field dosimetry for an ELEKTA Unity MR-Linac system using Monte Carlo (MC) simulations. The EGSnrc/BEAMnrc code was used to simulate the MR-Linac treatment head. The percentage depth-doses (PDDs) and off-axis dose profiles were calculated for field sizes of 2 cm x 2 cm and 10 cm x 10 cm using the DOSXYZnrc code, while the output factors were calculated with the egs_chamber code. The dose profiles and output factors were calculated at a depth of 10 cm in water with a 143.5 cm source-axis-distance. The output factors for the detectors with different sensitive volumes were compared with previously measured values. The k(Q,B) was also calculated. The accuracy of the simulated beam was validated by comparing the MC-calculated PDDs and dose profiles with previously measured values, which agreed within 2% except the penumbra region. The output factors of the ion chambers decreased rapidly depending on the sensitive volumes of small fields. The output factors of the PTW-3102 agreed within 2% with those in a water slab down to a 1 cm x 1 cm field. The output factors of PTW-31021 and PTW-31022 chambers and PTW-60018 diode agreed within 2% with previously measured values. The k(Q,B) values for PTW-30013, PTW-31010, PTW-31021, and PTW-31022 chambers were 0.974 +/- 0.14%, 0.976 +/- 0.26%, 0.977 +/- 0.21%, and 0.995 +/- 0.36%, respectively. The calculated output factors for the detectors showed good agreement with previously measured values. The magnetic field effect was small in ionization chambers with the small cavity radius.