Validation of a Monte Carlo simulation for Microbeam Radiation Therapy on the Imaging and Medical Beamline at the Australian Synchrotron

Abstract

Microbeam Radiation Therapy (MRT) is an emerging cancer treatment modality characterised by the use of high-intensity synchrotron-generated x-rays, spatially fractionated by a multi-slit collimator (MSC), to ablate target tumours. The implementation of an accurate treatment planning system, coupled with simulation tools that allow for independent verification of calculated dose distributions are required to ensure optimal treatment outcomes via reliable dose delivery. In this article we present data from the first Geant4 Monte Carlo radiation transport model of the Imaging and Medical Beamline at the Australian Synchrotron. We have developed the model for use as an independent verification tool for experiments in one of three MRT delivery rooms and therefore compare simulation results with equivalent experimental data. The normalised x-ray spectra produced by the Geant4 model and a previously validated analytical model, SPEC, showed very good agreement using wiggler magnetic field strengths of 2 and 3 T. However, the validity of absolute photon flux at the plane of the Phase Space File (PSF) for a fixed number of simulated electrons was unable to be established. This work shows a possible limitation of the G4SynchrotronRadiation process to model synchrotron radiation when using a variable magnetic field. To account for this limitation, experimentally derived normalisation factors for each wiggler field strength determined under reference conditions were implemented. Experimentally measured broadbeam and microbeam dose distributions within a Gammex RMI457 Solid Water® phantom were compared to simulated distributions generated by the Geant4 model. Simulated and measured broadbeam dose distributions agreed within 3% for all investigated configurations and measured depths. Agreement between the simulated and measured microbeam dose distributions agreed within 5% for all investigated configurations and measured depths.

Toward personalized synchrotron microbeam radiation therapy

“…For in vivo experiments, a single column of unidirectional microbeams was used. The complete beam configuration parameters for cell and animal experiments are shown in Table 1, also found in Dipuglia et al 26 .…”

“…In vitro experiments, dosimetry and treatment verification. The dosimetric protocol for SBB and MRT at the IMBL is outlined in previous publications [25][26][27][28][29][30][31] . Briefly, it involves characterizing the pre-filtered SBB uniform in custom designed RMI-457 Gammex Solid Water ® phantoms (Gammex-RMI, Middleton, WI, USA) using a PinPoint ionization chamber (IC) (PTW 31014, Freiburg, Germany), calibrated to a traceable standard.…”

“…Experimental SBB IC measurements were compared with Geant4 radiation transport simulations modelling the experimental set-up for quality assurance purposes 27 . The Geant4 simulation (version 9.6, patch 4, and using the Livermore Polarized Physics List to model EM interactions) described by Dipuglia et al 26 was used to evaluate the MRT peak and valley doses at tumor depth. X-Tream measurements were made at a 12.5 mm reference depth in the Solid Water ® phantom with precise quantification and tuning of the peak and valley dose using the vertical motor translation speed.…”

Study of the X-ray radiation interaction with a multislit collimator for the creation of microbeams in radiation therapy

“…It is worth mentioning that, at the time of writing, the phantom dimension was chosen to be similar to the dimension used by different research groups (Day et al, 2020;Dipuglia et al, 2019). Even if this geometry may not correspond to a realistic clinical scenario, the phantom is adequate to answer the scientific question of this work.…”

“…For the Imaging and Medical Beamline (IMBL) of the Australian Synchrotron, two MC models have recently been presented. Dipuglia et al (2019) developed a detailed MC model of the entire IMBL, including the electron transport inside the wiggler source, for the generation of phase space files as input for dose calculation. Day et al (2020) also presented a reliable IMBL model, where an analytical approach is employed to calculate the generated polychromatic spectrum used in MC simulations for the dose calculation.…”