Room: Exhibit Hall | Forum 4
Purpose: To investigate the influence of magnetic field on dosimetric response of radiochromic films by studying their optical absorbance spectrum and net optical density.
Methods: EBT3 radiochromic film samples were irradiated at different doses ranging from 0.5 to 50 Gy using 6 MV flattening filter free (FFF) x-rays generated by MRIdianâ„¢ MRI-guided radiotherapy system in the presence of B=0.35 T magnetic field. Films from the same batch were irradiated at corresponding dose levels using 6 MV FFF beam from a TrueBeamâ„¢ (i.e. B=0) linac for comparison. The absorption spectra of the films were acquired using an optical spectrometer over 450-800 nm wavelength range with 2 nm resolution. The net optical density of the films was also measured using a flatbed scanner.
Results: The net absorbance spectrum of the films, for both irradiation modalities, resulted in two absorption bands centered at 636 nm and 585 nm, characteristic of the EBT3 model. A visible decrease ~2-5% in the peak net absorbance was observed for the samples irradiated with doses > 2 Gy using the MRIgRT system. When the films irradiated beyond 2 Gy, were analyzed using the scanner average reduction in ~5%, 2% and 3% in the net optical density was observed for red, green, and blue channels, respectively. The under-response of the films in MRIgRT can lead to up to ~10% under estimation of the dose if a generic calibration curve based on B=0 is used.
Conclusion: The spectral shapes (in terms of the position of the absorption peaks) remain the same for films irradiated in the presence or absence of a magnetic field. However, the presence of magnetic field during irradiation causes a decrease in the optical density of the films that has a dosimetric consequence if a correct calibration curve is not utilized.
Funding Support, Disclosures, and Conflict of Interest: This work is supported by the Department of Radiation Oncology of the Washington University School of Medicine.