TH-CD-BRA-06: Magnetic Field Effects On Gafchromic-Film Response in MR-IGRT

Authors


Abstract

Purpose:

To investigate the effects of magnetic fields in radiochromic films (RCF). Magnetokinetic changes may affect crystal orientation and polymerization within active layer of RCF, these effects are investigated in Magnetic Resonance Image-guided Radiotherapy (MR-IGRT).

Methods:

Gafchromic EBT2 RCF were irradiated in a 30×30×30 cm3 solid water phantom using a ViewRay MRIdian Co-60 MRI guided radiotherapy system (B=0.35 T). Fifteen 20.3×25.4 cm2 EBT2 film sheets were placed at three different depths (d=0.5, 5 and 10 cm) and irradiated using 5 different treatment plans. The plans were computed using the MRIdian treatment planning system to deliver 2, 4, 6, 8, and 10 Gy at a depth of 10 cm. The films were scanned using an Epson Expression 10000 XL flat-bed document scanner in transmission mode. Films were processed before and after irradiation to obtain a 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 show the red channel netOD decreases between 1.3–12.3 % (average of 5.95 %) 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 1.2–10.5 % (average of 4.09 %) for doses above 3.5 Gy. Blue channel showed the weakest effect between 1.3–2.9 % (average of 1.94 %) for doses above 8.0 Gy. SEM images show changes in crystal orientation within active layer in RCF exposed in a magnetic field.

Conclusion:

The presence of a magnetic field affects crystal orientation and polymerization during irradiation, decreasing netOD by an average of 5.95 % in the red channel. The under response is dependent on dose and differs by up to 12.3 % at 17.6 Gy. The results show that magnetokinetic effects should be carefully considered in MR-IGRT.

Ancillary