Experimental benchmarking of Monte Carlo simulations for radiotherapy dosimetry using monochromatic X-ray beams in the presence of metal-based compounds

The local dose deposition obtained in X-ray radiotherapy can be increased by the presence of metal-based compounds in the irradiated tissues. This finding is strongly enhanced if the radiation energy is chosen in the kiloelectronvolt energy range, due to the proximity to the absorption edge. In this study, we present a MC application developed with the toolkit Geant4 to investigate the dosimetric distribution of a uniform monochromatic X-ray beam, and benchmark it against experimental measurements. Two validation studies were performed, using a commercial PTW RW3 water-equivalent slab phantom for radiotherapy, and a custom-made PMMA phantom conceived to assess the influence of high atomic number compounds on the dose profile, such as iodine and gadolinium at different concentrations. An agreement within 9% among simulations and experimental data was found for the monochromatic energies considered, which were in the range of 30–140 keV; the agreement was better than 5% for depths <60 mm. A dose enhancement was observed in the calculations, corresponding to the regions containing the contrast agents. Dose enhancement factors (DEFs) were calculated, and the highest values were found for energies higher than the corresponding K-edges of iodine and gadolinium. The in-silico results are in line with the empirical findings, which suggest that Geant4 can be satisfactorily used as a tool for the calculation of the percentage depth dose (PDD) at the energies considered in this study in the presence of contrast agents

Monte Carlo computations for radiotherapy with the use of dedicated processors

The project we present foresees the development of a hardware processor dedicated to real time Monte Carlo computations for radiotherapy treatment planning. Treatment planning systems are nowadays based on empirical methods which can lead to errors in the localization of the area to be irradiated of the order of centimeters. The use of Monte Carlo techniques permits to reach a more adequate precision but with the drawback of a computing time which is too high with respect to the requirements of a radiotherapy center. We propose to implement time consuming algorithms directly in hardware circuits based on programmable logic devices in order to speed up the computation. The hardware is based on a 'Configurable Computer', a PCI master board housing one or more high performance fPGA and a RAM large enough for the application

A Geant4 study on the comparison of the absorption of low energy X-rays in water and PMMA phantoms

The aim of this study was to validate a Geant4 Monte Carlo application for dosimetry measurements at low energies when using water and PMMA phantoms. To measure the water-equivalence in plastic materials, relative dosimetry can be performed by deploying the calculation of depth dose profiles and output factors. We simulated a quasi-uniform monochromatic X-ray beam, with dimension of 2x2 cm 2 , perpendicularly impinging on a phantom, with energy ranging from 30 to 140 keV. A comparison of the attenuation of the beam after passing through slabs of PMMA and/or water is presented and validated against experimental measurement performed at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The use of PMMA as a substitute of water at low energies is not recommended for dosimetric studies as the depth dose profiles show discrepancies of at least 3%

Geant4 simulations for microbeam radiation therapy (MRT) dosimetry

Radiation therapy is one of the techniques most commonly used in the treatment of various types of tumors. The microbeam radiation therapy (MRT) is a very promising variant, which exploits the property that tissues can tolerate high doses of radiation in small volumes. The effectiveness of MRT is well represented by the peak-to-valley dose ratios (PVDRs), which are one of the crucial parameters associated with the outcome of the treatment. In this study, we investigate on the factors that influence PVDRs, such as different beam energies and geometries. MRT experiments typically employ rectangular (planar) microbeams of different sizes, but, for convenience of analysis, preliminary computations have been performed also using arrays of cylindrical microbeams. This work shows that the shape of the impinging irradiation field largely influences the dose distribution. It highlights that a bundle of larger microbeams, with a small separation, produces more scattered radiation and therefore lower PVDRs. The study of how dose distributions vary with different setups and irradiation parameters is an essential step in enhancing the comparability of experimental data and simulation results

Microdosimetry for Microbeam Radiation Therapy (MRT) : theoretical calculations using the Monte Carlo toolkit

Radiation therapy is widely used in the treatment of very different types of cancer. Recent developments in this field are aiming at delivering high doses to the target volume while sparing the surrounding healthy tissues. The microbeam radiation therapy (MRT) is a new kind of radiotherapy which could be used for treating infantile brain tumors, as other kinds of radiotherapy would be extremely dangerous to the normal brain development. MRT is carried out using an array of parallel microbeams of synchrotron-wiggler-generated X-rays. In this work, Monte Carlo simulations using the Geant4 toolkit are carried out to estimate the dose deposition on a 20-cm-diameter, 20-cm-long cylindrical PMMA phantom, mimicking an infantile head. A set of physics processes is implemented in Geant4 to extend the range of validity of electromagnetic interactions down to 250 eV. The dose distribution in MRT is computed to prepare the treatment planning of preclinical trials. Primary photon histories are simulated for the different experimental setups, scoring the dose in cylindrical shells. We used cylindrical monoenergetic microbeams of 50, 100 and 150 keV and one microbeam with energies sampled from the measured spectrum at the ESRF ID17 beamline. The depth- and lateral-dose profiles have been studied, and for a few typical cases, the simulation results have been compared with those obtained with other codes

The GEANT4 toolkit for microdosimetry calculations : application to microbeam radiation therapy (MRT)

Theoretical dose distributions for microbeam radiation therapy (MRT) are computed in this paper using the GEANT4 Monte Carlo (MC) simulation toolkit. MRT is an innovative experimental radiotherapy technique carried out using an array of parallel microbeams of synchrotron‐wiggler‐generated x rays. Although the biological mechanisms underlying the effects of microbeams are still largely unknown, the effectiveness of MRT can be traced back to the natural ability of normal tissues to rapidly repair small damages to the vasculature, and on the lack of a similar healing process in tumoral tissues. Contrary to conventional therapy, in which each beam is at least several millimeters wide, the narrowness of the microbeams allows a rapid regeneration of the blood vessels along the beams’ trajectories. For this reason the calculation of the “valley” dose is of crucial importance and the correct use of MC codes for such purposes must be understood. GEANT4 offers, in addition to the standard libraries, a specialized package specifically designed to deal with electromagnetic interactions of particles with matter for energies down to 250 eV. This package implements two different approaches for electron and photon transport, one based on evaluated data libraries, the other adopting analytical models. These features are exploited to cross‐check theoretical computations for MRT. The lateral and depth dose profiles are studied for the irradiation of a 20 cm diameter, 20 cm long cylindrical phantom, with cylindrical sources of different size and energy. Microbeam arrays are simulated with the aid of superposition algorithms, and the ratios of peak‐to‐valley doses are computed for typical cases used in preclinical assays. Dose profiles obtained using the GEANT4 evaluated data libraries and analytical models are compared with simulation results previously obtained using the PENELOPE code. The results show that dose profiles computed with GEANT4's analytical model are almost indistinguishable from those obtained with the PENELOPE code, but some noticeable differences appear when the evaluated data libraries are used

The effect of beam polarization in Microbeam Radiation Therapy (MRT) : Monte Carlo simulations using Geant4

Microbeam Radiation Therapy (MRT) is an innovative experimental technique potentially able to overcome the limitations of conventional radiotherapy for infantile brain tumors. Its effectiveness seems to be related to the ability of normal tissues to tolerate a very high radiation dose in small volumes, resulting in the preservation of the tissues' architecture. The effectiveness of MRT is well represented by peak-to-valley dose ratios (PVDRs), which are one of the crucial parameters associated with the outcome of the treatment. We present Geant4 Monte Carlo calculations of the dose distribution deposited by planar polarized microbeams with micrometric resolution. The simulation of the beam polarization, made possible by different libraries included in Geant4, is a crucial step in enhancing the comparability of experimental data and simulation results

Dose calculation for radiotherapy treatment planning using Monte Carlo methods on FPGA based hardware

This work describes a fast Monte Carlo Machine for dose calculation in radiotherapy treatment planning on FPGA based hardware. When performing Monte Carlo simulations of the radiation dose delivered to the human body, the Compton interaction is simulated. The inputs to the system are the energy and the normalized direction vectors of the incoming photon. The energy and the direction vectors of the scattered photon and the scattered electron are calculated. The energy distribution by the scattered electron along its path in a voxel space is then calculated which can be used to construct maps of dose distribution in real time