Low-density gel dosimeter for measurement of the electron return effect in an MR-linac

Abstract

Radiation therapy in the presence of a strong magnetic field is known to cause regions of enhanced and reduced dose at interfaces of materials with varying densities, in a phenomenon known as the electron return effect (ERE). In this study, a novel low-density gel dosimeter was developed to simulate lung tissue and was used to measure the ERE at the lung-soft tissue interface. Low-density gel dosimeters were developed with Fricke xylenol orange gelatin (FXG) and ferrous oxide xylenol orange (FOX) gels mixed with polystyrene foam beads of various sizes. The gels were characterized based on CT number, MR signal intensity, and uniformity. All low-density gels had CT numbers roughly equivalent to lung tissue. The optimal lung-equivalent gel formulation was determined to be FXG with <1 mm polystyrene beads due to the higher signal intensity of FXG compared to FOX and the higher uniformity with the small beads. Dose response curves were generated for the optimal low-density gel and conventional FXG. The change in spin-lattice relaxation rate (R1) before and after irradiation was linear with dose for both gels. Next, phantoms consisting of concentric cylinders with low-density and conventional FXG were created to simulate the lung-soft tissue interface. The phantoms were irradiated in a conventional linear accelerator (linac) and in a linac combined with a 1.5 T magnetic resonance imaging (MRI) unit (MR-linac) to measure the effects of the magnetic field on the dose distribution. Hot and cold spots were observed in the dose distribution at the boundaries between the gels for the phantom irradiated in the MR-linac but not the conventional linac, consistent with the ERE.

Machine QA for the Elekta Unity system: A Report from the Elekta MR‐linac consortium

“…This effect has been termed the electron return effect (ERE) and has been investigated extensively. [46][47][48][49][50][51][52][53] If electrons exiting into air are not reabsorbed, they spiral along the B-field which has been termed the electron streaming effect (ESE). This primarily occurs where there are oblique tissue angles.…”

Radiation Dosimetry by Use of Radiosensitive Hydrogels and Polymers: Mechanisms, State-of-the-Art and Perspective from 3D to 4D

“…Electrons entering a low-density tissue may be re-directed toward the high-density region by the Lorentz force, resulting in hot spots of the dose in the high-density region. This, so-called electron return effect was observed by use of a two-compartment FXG gel, where a lower density was obtained by use of polystyrene beads [ 156 ]. While some signal change in Fricke gel dosimeters was visually observed during radiation [ 157 ], the temporal uncertainty is too large for quantifiable real-time 4D dosimetry with inline MRI readout [ 93 ].…”

High-resolution entry and exit surface dosimetry in a 1.5 T MR-linac

“…Previous investigations of near-surface dose in a transverse MR-linac have used dosimeters with varying effective point of measurement (EPOM). These include radiochromic film [6,14,15], PTW 31,021 Semiflex 3D [16], optically stimulated luminescence dosimeters (OSLDs) [17][18][19], thermoluminescent dosimeters (TLDs) [3], metal-oxide-semiconductor field-effect transistor (MOSFET) [20], gel [21], PTW 60019 microDiamond [6,14,16], and a PTW 34045 Advanced Markus chamber [6,12]. Parallel-plate chambers, such as the Advanced Markus chamber, are commonly used for dose measurements in the build-up region on conventional linacs.…”