Magnetic fields are causing small, but significant changes of the radiochromic EBT3 film response to 6 MV photons

Abstract

The optical density (OD) of EBT3 radiochromic films (Ashland Specialty Ingredients, Bridgewater, NJ, USA) exposed to absorbed doses to water up to D = 20 Gy in magnetic fields of B = 0.35 and 1.42 T was measured in the three colour channels of an Epson Expression 10000XL flatbed scanner. A 7 cm wide water phantom with fixed film holder was placed between the pole shoes of a constant-current electromagnet with variable field strength and was irradiated by a 6 MV photon beam whose axis was directed at right angles with the field lines. The doses at the film position at water depth 5 cm were measured with a calibrated ionization chamber when the magnet was switched off and were converted to the doses in presence of the magnetic field via the monitor units and by a Monte Carlo-calculated correction accounting for the slight change of the depth dose curves in magnetic fields. In the presence of the 0.35 and 1.42 T fields small negative changes of the OD values at given absorbed doses to water occurred and just significantly exceeded the uncertainty margin given by the stochastic and the uncorrected systematic deviations. This change can be described by a +2.1% change of the dose values needed to produce a given optical density in the presence of a 1.42 T field. The thereby modified OD versus D function remained unchanged irrespective of whether the original short film side-the preference direction of the monomer crystals of the film-was directed parallel or orthogonal to the magnetic field. The 'orientation effect', the difference between the optical densities measured in the 'portrait' or 'landscape' film positions on the scanner bed caused by the reflection of polarised light in the scanner's mirror system, remained unaltered after EBT3 film exposure in magnetic fields. An independent optical bench investigation of EBT3 films exposed to doses of 10 and 20 Gy at 0.35 and 1.42 T showed that the direction of the electric vector of polarised light experiencing the largest transmission through EBT3 films remained unaltered after film exposure in the magnetic fields. The observed small modification of the OD versus D curve of the radiochromic film EBT3 in the range up to 20 Gy and 1.42 T, hardly exceeding the experimental uncertainty margin, numerically confirms other recent studies on EBT3 film. A stronger magnetic field effect had been observed with the previous product EBT2 exposed to Co gamma radiation at 0.35 T.

Technical Note: Experimental verification of magnetic field‐induced beam deflection and Bragg peak displacement for MR‐integrated proton therapy

“…The obtained range in Bragg peak displacement of 1–10 mm corresponds well with previous theoretical studies in a homogeneous magnetic field in water . As opposed to a previous study for photons, no systematic differences in absolute dose were found between the film measurements with and without magnetic field, which may be caused by the fact that film dosimeters were oriented perpendicular to the magnetic field in this study.…”

Optical imaging method to quantify spatial dose variation due to the electron return effect in an MR‐linac

“…There are demonstrated effects of magnetic fields on Gafchromic‐film response . Reynoso et al used scanning electron microscopy to assess changes in the crystal orientation and polymerization within the active layer of radiochromic film after exposure to a 0.35 T magnetic field of a similar MR‐IGRT system.…”

“…There are demonstrated effects of magnetic fields on Gafchromic-film response. 22 Reynoso et al 45 used scanning electron microscopy to assess changes in the crystal orientation and polymerization within the active layer of radiochromic film after exposure to a 0.35 T magnetic field of a similar MR-IGRT system. After comparing films irradiated with and without the magnetic field, the results of their work concluded that there was a dose-dependent under response of the radiochromic film in the presence of the magnetic field.…”

“…Two-dimensional dose distributions can be measured with radiochromic films with reasonable accuracy, but require strict physical and temporal protocols for irradiation and readout; films have been used to measure ERE in limited scenarios. [21][22][23] Gel-based three-dimensional dosimeters, such as PRESAGEâ radiochromic plastic and FOX gels, are insensitive to magnetic field effects; [24][25][26] however, they are not generally regarded as user-friendly, utilize toxic materials, 27 and limited literature is available in regards to using them to quantify ERE with respect to commissioning MR-IGRT treatment planning system (TPS) data. 28 An intriguing approach to measuring ERE in millimeter-scale cavities using electron paramagnetic resonance imaging (EPRI) has recently been published, 29 however it is unclear if this type of measurement can be translated to centimeter-scale cavities or exit beam measurements.…”

A feasibility study for high‐resolution silicon array detector performance in the magnetic field of a permanent magnet system

“…EBT2 film has shown a decrease in optical density (OD) of up to 15% due to changes in the orientation of monomer crystals within the active layer under the influence of a 0.35 T magnetic field . However, for EBT3, the difference between 0 T and 1.42 T was only just outside of uncertainty: a decrease in OD of 0.8%, as well as a 2.1% shift toward higher doses for corresponding OD values …”