Suitability of EBT3 GafChromic film for quality assurance in MR-guided radiotherapy at 0.35 T with and without real-time MR imaging

Abstract

The 0.35 T Co MRIdian system (ViewRay Inc., Mountain View) has been in clinical use in our institution since May 2016. For quality assurance (QA) of dose delivery and end-to-end testing for this machine, a reliable dosimeter is required. However, it is possible that a magnetic field may cause perturbations to dosimetry measurements. For static magnetic fields, there is conflicting information in the literature concerning EBT film behaviour, while for real-time MR imaging such information is not available at all. The purpose of this study was to investigate the suitability of EBT3 GafChromic film for MRIdian QA, both with and without real-time MR imaging. EBT3 film sheets were irradiated in water using the MRIdian and a conventional linear accelerator (Linac) for reference. Dose calibration measurements were first performed up to 8 Gy for both machines. The MRIdian measurements were performed with and without real-time MR imaging. Second, film sheets were irradiated at seven different angles with respect to the B-field. Optical density and dose values were analysed for the three colour channels. In both the film dose-response and B-field orientation measurements, the mean dose values were within the 1% uncertainty range of prescribed dose values for the red and green channels, for both machines. There were no dose deviations detected between the MRIdian and Linac film measurements, nor for different B-field orientations. In addition, the film dose-response measurements during real-time imaging were within 1.5% of the reference Linac measurements. EBT3 GafChromic film can be used for absolute dosimetry during real-time MR imaging independent of its orientation in the B-field. This makes it a suitable dosimeter for patient-specific QA measurements and end-to-end testing of 0.35 T MRI-radiotherapy devices.

Multichannel Film Dosimetry for Quality Assurance of Intensity Modulated Radiotherapy Treatment Plans Under 0.35 T Magnetic Field

“…As a result, the findings of our study support the findings of the authors who studied EBT3 film characteristics under 0.35 T magnetic field and also our results show that signal scan method with multichannel analysis achieved reliable results under low tesla magnetic field [20][21][22].…”

“…They found small dose uncertainties under the magnetic fields at the rate of -2.1%, whereas relative dose distribution remained acceptable. Similarly, Barten et al found a small deviation of less than 1.5% in EBT3 dose response after irradiation under 0.35 T using radiofrequency energy at on and off mode [22]. Findings of Delfs et al and Barten et al were within the same deviation line.…”

“…In addition to all these, OD evaluation of radiochromic film under magnetic field needs more study. There are several researches on that topic [18][19][20][21][22].…”

“…On the contrary, EBT3 active layer is coated with symmetric 125 matte polyesters [10,12]. Therefore, EBT3 film seems to be more suitable than EBT2 under presence of magnetic field [18][19][20][21][22]. Cusumano et al experimentally studied estimating the impact of a 0.35 T transverse magnetic field on dose distribution in presence of tissue-air and tissue-lung interfaces [21].…”

“…As a consequence, physical characteristics, end-to-end (E2E) results, optical density (OD) consistency, dose distribution measurements in presence of tissue-air and tissue-lung interfaces with film and patient specific IMRT QA response of Gafchromic™ EBT2/3 films under magnetic field are interesting areas for research [17][18][19][20][21][22].…”