Room: AAPM ePoster Library
Purpose: To evaluate surface dose with the use of EBT3 Gafchromic films placed in a water phantom surface in the presence of strong magnetic field 1.5T.
Methods: EBT3 film was placed in a phantom water surface 10cm above isocenter so that the B field is in parallel with the film surface. The water level surface was accurately defined with the use of the PTW MR-water tank. The water surface tension retains the film in surface and in direct contact with water with no signs of airgaps. Several field sizes were delivered with gantry at 0 degrees. For each field size EBT3 Gafchromic films were developed. Films scanned with Epson scanner while the film calibration dose curve was performed with the tripe channel method using FilmQApro software. A Farmer-type ionization chamber was used to evaluate the output of MR-linac, considering the B field correction factors. The same field sizes where computed in MONACO TPS and surface doses were calculated under specific parametrization.
Results: Doses measured in films with this setup were found to be around 42% of the prescribed dose at isocenter for all fields sizes. For MONACO calculations options dose to water, fine grid space and 0.5% uncertainty per control point were used, while the accurate selection of the measuring points found to be very critical since the steep dose gradient in surface generates uncertainties.
Conclusion: The prescribed EBT3 film set up could be performed during the MRlinac commissioning. The setup is moderately easy and these measurements could be useful for the surface dose evaluation. Films are placed in the surface in the absence of air while film transparency enables the visualization of any presence of air cavities. A further work could follow with oblique incident beams and exit surface dose for a more comprehensive investigation.
Funding Support, Disclosures, and Conflict of Interest: I am an Elekta employee.