Investigating the effect of a magnetic field on dose distributions at phantom-air interfaces using PRESAGE^®3D dosimeter and Monte Carlo simulations

Abstract

Dosimetric quality assurance (QA) of the new Elekta Unity (MR-linac) will differ from the QA performed of a conventional linac due to the constant magnetic field, which creates an electron return effect (ERE). In this work we aim to validate PRESAGE® dosimetry in a transverse magnetic field, and assess its use to validate the research version of the Monaco TPS of the MR-linac. Cylindrical samples of PRESAGE® 3D dosimeter separated by an air gap were irradiated with a cobalt-60 unit, while placed between the poles of an electromagnet at 0.5 T and 1.5 T. This set-up was simulated in EGSnrc/Cavity Monte Carlo (MC) code and relative dose distributions were compared with measurements using 1D and 2D gamma criteria of 3% and 1.5 mm. The irradiation conditions were adapted for the MR-linac and compared with Monaco TPS simulations. Measured and EGSnrc/Cavity simulated profiles showed good agreement with a gamma passing rate of 99.9% for 0.5 T and 99.8% for 1.5 T. Measurements on the MR-linac also compared well with Monaco TPS simulations, with a gamma passing rate of 98.4% at 1.5 T. Results demonstrated that PRESAGE® can accurately measure dose and detect the ERE, encouraging its use as a QA tool to validate the Monaco TPS of the MR-linac for clinically relevant dose distributions at tissue-air boundaries.

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.…”

Improvement in sensitivity of radiochromic 3D dosimeter based on rigid polyurethane resin by incorporating tartrazine

“…In the clinical practice, 3D dosimetry is not only performed by moving ionization chambers or silicon diodes or thermoluminescent detectors (TLD) in the water phantom but also chemical dosimetry systems such as polymer gels, radiochromic gels, and radiochromic plastics have been developed, recently [14,15]. These exhibit physical phenomena to radiation that changes their properties (e.g., optical absorption or scattering, X-ray absorption, NMR, or acoustic properties) [16,17]. Their changes are quantified and imaged by readout systems, which typically use magnetic resonance imaging (MRI) and optical CT systems.…”

EPR imaging of magnetic field effects on radiation dose distributions around millimeter-size air cavities

“…Recent experimental studies have shown the applicability of EBT3 radiochromic film [20] as well as 3D detectors such as radiochromic plastic, radiochromic gel and polymer gel [21], [22] for the purpose of relative dosimetry in the presence of strong magnetic fields. More recently, Presage® radiochromic plastic was tested for relative dosimetry at dosimeter-air interfaces [23], [24] and showed good agreement with Monte Carlo (MC) radiation transport simulations. However, several authors have shown the limited use of radiochromic film and Presage® dosimeters within the first millimeter from the dosimeter edge due to different kinds of artefacts [25], [26], [23].…”

“…More recently, Presage® radiochromic plastic was tested for relative dosimetry at dosimeter-air interfaces [23], [24] and showed good agreement with Monte Carlo (MC) radiation transport simulations. However, several authors have shown the limited use of radiochromic film and Presage® dosimeters within the first millimeter from the dosimeter edge due to different kinds of artefacts [25], [26], [23]. Besides, these edge artefacts limit the maximum surface-tovolume ratio of such dosimeters and therefore restrict the minimum analyzable detector size.…”