The effects of a high frequency electromagnetic field (HF EMF) in the terahertz (THz) range on biological systems is the subject of ongoing investigation. With a beamline range of 0.5 THz - 20 THz, the THz/Far-IR beamline at the Australian Synchrotron is ideally suited to explore possible THz effects. We have developed a technique to achieve precise estimation of the amount of absorbed energy when using diverse cell types at THz frequencies. The approach involves evaluating the overall incident beam power, the determination of the frequency and photon dispersion, and evaluating the relative contribution of the frequency ranges. The sample depth is an important component of the evaluation. Since higher THz frequencies have shallower sample penetration depths, parts of the sample are being exposed to not only different total THz energy doses, but to a different frequency profile. Our technique achieves accurate estimation of the exposure profile.
For the past few decades, fibre-optic dosimeters (FODs) have been a focus of research for dosimetry with LINACs, owing to a unique set of advantageous qualities: compact dosimeter sizes, an all optical composition (i.e. no wires or electronics around their sensitive volume), real-time response proportional to the absorbed dose-rate in their sensitive volumes and direct water equivalence. Such a set of qualities makes FODs “near-correctionless” for dosimetry with LINACs, such that they have been recommended as in vivo dosimeters and small field dosimeters. Further, their scintillation and luminescence response mechanisms are not affected by magnetic fields. Given this set of qualities, FODs are attractive candidates for dosimetry with MRI-LINACs. This mini-review aims to provide an overview of FODs to the wider medical physics community, and present the current challenges and opportunities for FODs given previous investigations into MRI-LINAC dosimetry.
The Australian MRI-Linac prototype radiotherapy system has been shown to generate significant entry skin or surface dose increases. This arises from electron contamination focusing toward the isocenter caused by the 1 T MRI field being in-line with the x-ray beam. The aim of this study is to present accurate Monte Carlo modeling of these skin dose changes and to compare them with previous experimental measurements. Accurate skin dose modeling will improve confidence in the pathway forward to treatment planning for clinical trials. A COMSOL Multiphysics model of the Australian MRI-Linac system was used to generate a 3D magnetic field map to be used in corresponding Geant4 Monte Carlo simulations. The Geant4 simulations included the x-ray source (6 MV Linac), multileaf collimators (MLCs), and a 30 cm × 30 cm × 30 cm water phantom located with its front surface at the beam isocenter. Simulations were performed with a source to surface distance (SSD) of 1,819 mm for nominal field sizes 2