Session Item

This study is to investigate the surface dose in a 1.5T MR-Linac and compare the measured data with those collected without the magnetic field.

7 MV flattening filter free beams with field size ranging from 2×2 to 57×22 cm² at gantry 0 degree (G0) were delivered by an Elekta 1.5T MR-Linac. A plane-parallel ionization chamber (PTW 34045 Advanced Markus) placed in solid water phantom was used for measuring the surface doses, which were normalized to Dmax at depth of 1.3 cm. For evaluating the performance of Advanced Markus chamber in MR environment, percentage depth dose (PDD) for 10x10 cm² field was measured and compared with data collected in water using a microDiamond detector. Surface dose was also measured with EBT3 Gafchromic film and solid water phantom. The films were placed on the surface and at depth of dmax, perpendicular to the radiation beam at G0. Water was filled in between the film and phantom to avoid any air gap. Dose calculation on virtual phantom was performed by Monaco 5.40 with a calculation grid of 1mm and exported for analysis.

As shown in figure 1, the surface dose changes from 12.8% to 16.6% while the field size varies from 2x2 to 57x22 cm², which is much less than the variation of surface dose without the magnetic field. Calculated surface dose by Monaco is systematically higher than our measurements using chamber but shows a similar trend as field size varies. PDDs measured with Advanced Markus chamber in solid water phantom agree well with data collected by the microDiamond detector in water. Especially for the data with the magnetic field, the deviation at most depth is less than 0.5% except the region from depth 25 mm to 40 mm, which is probably due to the air gap between phantom slabs that caused the decrease of chamber reading. The surface dose measured by the film was averaged based on three color channels and is very close to the result of chamber with less than 1% deviation on average, which is reasonable after taking the thickness of entrance foil of Advanced Markus chamber and polyester base of EBT3 film into account.

Compared with measurements before the magnetic field was on, the surface dose with magnetic field for field size larger than 8x8 cm² is much smaller because of the purging of the majority of lepton contamination. For small field size (less than 8x8 cm²), the larger surface dose with magnetic field could be due to the skewed average direction of the secondary particles. The change of surface dose with field size with magnetic field is not as drastic as conventional Linac while the surface dose of maximum field size is 16.6% for the 1.5T MR-Linac.