title: Technical Note: Magnetic field effects on Gafchromic-film response in MR-IGRT.
abstract: OBJECTIVE
  Magnetokinetic changes may affect crystal orientation and polymerization within the active layer of radiochromic film (RCF). This effect is investigated in a magnetic resonance image-guided radiotherapy unit within the context of film dosimetry.
  
  
  METHODS
  Gafchromic EBT2 RCF was irradiated in a 30 × 30 × 30 cm3solid water phantom using a Co-60 MRI guided radiotherapy system (B = 0.35 T) under normal operating conditions, and under the exact conditions and setup without a magnetic field. Fifteen 20.3 × 25.4 cm2EBT2 film sheets were placed at three different depths (d = 0.5, 5, and 10 cm) using five different treatment plans. The plans were computed using the MRIdian (ViewRay, Inc.) treatment planning system to deliver doses between 0 and 17.6 Gy. Films were analyzed before and after irradiation to obtain the net optical density (netOD) for each color channel separately. Scanning electron microscope (SEM) images were obtained to compare the active layer of selected samples.
  
  
  RESULTS
  The results indicated that the red channel netOD decreased between 0.013 and 0.123 (average of 0.060 ± 0.033) for doses above 2.8 Gy, with a linear increase in this effect for higher doses. Green channel netOD showed similar results with a decrease between 0.012 and 0.105 (average of 0.041 ± 0.027) for doses above 3.5 Gy. The blue channel showed the weakest effect with a netOD decrease between 0.013 and 0.029 (average of 0.020 ± 0.006) for doses above 8.0 Gy. SEM images show changes in crystal orientation within active layer in RCF exposed in a magnetic field.
  
  
  CONCLUSIONS
  The presence of a magnetic field affects crystal orientation and polymerization during irradiation, where netOD decreased by an average of 8.7%, 8.0%, and 4.3% in the red, green, and blue channels, respectively. The under response was dependent on dose and differed by up to 15% at 17.6 Gy.
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Reynoso
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Austen
authors.initials: A
authors.lastname: Curcuru
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Olga
authors.initials: O
authors.lastname: Green
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Sasa
authors.initials: S
authors.lastname: Mutic
authors.affiliation: Department of Radiation Oncology, New York University Langone Medical Center, New York, New York 10016.
authors.forename: Indra J
authors.initials: IJ
authors.lastname: Das
authors.affiliation: Department of Radiation Oncology, Washington University School of Medicine, St. Louis, Missouri 63110.
authors.forename: Lakshmi
authors.initials: L
authors.lastname: Santanam
mesh_terms: D004867:Equipment Design; D005373:Film Dosimetry; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging; D019047:Phantoms, Imaging; D061089:Radiotherapy, Image-Guided