David Bolst

Correction factors to convert microdosimetry measurements in silicon to tissue in 12C ion therapy

Silicon microdosimetry is a promising technology for heavy ion therapy (HIT) quality assurance, because of its sub-mm spatial resolution and capability to determine radiation effects at a cellular level in a mixed radiation field. A drawback of silicon is not being tissue-equivalent, thus the need to convert the detector response obtained in silicon to tissue. This paper presents a method for converting silicon microdosimetric spectra to tissue for a therapeutic 12C beam, based on Monte Carlo simulations. The energy deposition spectra in a 10 μm sized silicon cylindrical sensitive volume (SV) were found to be equivalent to those measured in a tissue SV, with the same shape, but with dimensions scaled by a factor κ equal to 0.57 and 0.54 for muscle and water, respectively. A low energy correction factor was determined to account for the enhanced response in silicon at low energy depositions, produced by electrons. The concept of the mean path length [Formula: see text] to calculate the lineal energy was introduced as an alternative to the mean chord length [Formula: see text] because it was found that adopting Cauchys formula for the [Formula: see text] was not appropriate for the radiation field typical of HIT as it is very directional. [Formula: see text] can be determined based on the peak of the lineal energy distribution produced by the incident carbon beam. Furthermore it was demonstrated that the thickness of the SV along the direction of the incident 12C ion beam can be adopted as [Formula: see text]. The tissue equivalence conversion method and [Formula: see text] were adopted to determine the RBE10, calculated using a modified microdosimetric kinetic model, applied to the microdosimetric spectra resulting from the simulation study. Comparison of the RBE10 along the Bragg peak to experimental TEPC measurements at HIMAC, NIRS, showed good agreement. Such agreement demonstrates the validity of the developed tissue equivalence correction factors and of the determination of [Formula: see text].

3D Silicon Microdosimetry and RBE Study Using

This paper presents a new version of the 3D mesa Bridge microdosimeter comprised of an array of 4248 silicon cells fabricated on 10 μm thick n-type silicon-on-insulator substrate. This microdosimeter has been designed to overcome limitations existing in previous generation silicon microdosimeters and it provides well-defined sensitive volumes and high spatial resolution. The charge collection characteristics of the new 3D mesa microdosimeter were investigated using the ANSTO heavy ion microprobe, utilizing 5.5 MeV He2 + ions. Measurement of microdosimetric quantities allowed for the determination of the relative biological effectiveness of 290 MeV/u and 350 MeV/u 12C heavy ion therapy beams at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. The microdosimetric RBE obtained showed good agreement with the tissue-equivalent proportional counter. Utilizing the high spatial resolution of the SOI microdosimeter, the LET spectra for 70 MeV 12C+6 ions, like those present at the distal edge of 290 and 350 MeV/u beams, were obtained as the ions passed through thin layers of polyethylene film. This microdosimeter can provide useful information about the lineal energy transfer (LET) spectra downstream of the protective layers used for shielding of electronic devices for single event upset prediction.

^{12}{\rm C}

In this paper we report a Geant4 simulation study to investigate the characteristic prompt gamma (PG) emission in a water phantom for real-time monitoring of the Bragg peak (BP) during proton beam irradiation. The PG production, emission spatial correlation with the BP, and position preference for detection with respect to the BP have been quantified in different PG energy windows as a function of proton pencil-beam energy from 100 to 200MeV. The PG response to small BP shifts was evaluated using a 2cm-thick slab with different human body materials embedded in a water phantom. Our results show that the prominent characteristic PG emissions of 4.44, 5.21 and 6.13MeV exhibit distinctive correlation with the dose deposition curve. The accuracy in BP position identification using these characteristic PG rays is highly consistent as the beam energy increases from 100 to 200MeV. There exists a position preference for PG detection with respect to the BP position, which has a strong dependence on the proton beam energy and PG energies. It was also observed that a submillimeter shift of the BP position can be realized by using PG signals. These results indicate that the characteristic PG signal is sensitive and reliable for BP tracking. Although the maximization of the PG measurement associated with the BP is difficult, it can be optimized with energy and detection position preferences.

Ion of Different Energies

Purpose: This work aims to characterize a proton pencil beam scanning (PBS) and passive double scattering (DS) systems as well as to measure parameters relevant to the relative biological effectiveness (RBE) of the beam using a silicon on insulator (SOI) microdosimeter with well‐defined 3D sensitive volumes (SV). The dose equivalent downstream and laterally outside of a clinical PBS treatment field was assessed and compared to that of a DS beam. Methods: A novel silicon microdosimeter with well‐defined 3D SVs was used in this study. It was connected to low noise electronics, allowing for detection of lineal energies as low as 0.15 keV/μm. The microdosimeter was placed at various depths in a water phantom along the central axis of the proton beam, and at the distal part of the spread‐out Bragg peak (SOBP) in 0.5 mm increments. The RBE values of the pristine Bragg peak (BP) and SOBP were derived using the measured microdosimetric lineal energy spectra as inputs to the modified microdosimetric kinetic model (MKM). Geant4 simulations were performed in order to verify the calculated depth‐dose distribution from the treatment planning system (TPS) and to compare the simulated dose‐mean lineal energy to the experimental results. Results: For a 131 MeV PBS spot (124.6 mm R90 range in water), the measured dose‐mean lineal energy Symbol increased from 2 keV/μm at the entrance to 8 keV/μm in the BP, with a maximum value of 10 keV/μm at the distal edge. The derived RBE distribution for the PBS beam slowly increased from 0.97 ± 0.14 at the entrance to 1.04 ± 0.09 proximal to the BP, then to 1.1 ± 0.08 in the BP, and steeply rose to 1.57 ± 0.19 at the distal part of the BP. The RBE distribution for the DS SOBP beam was approximately 0.96 ± 0.16 to 1.01 ± 0.16 at shallow depths, and 1.01 ± 0.16 to 1.28 ± 0.17 within the SOBP. The RBE significantly increased from 1.29 ± 0.17 to 1.43 ± 0.18 at the distal edge of the SOBP. Symbol. No Caption available. Conclusions: The SOI microdosimeter with its well‐defined 3D SV has applicability in characterizing proton radiation fields and can measure relevant physical parameters to model the RBE with submillimeter spatial resolution. It has been shown that for a physical dose of 1.82 Gy at the BP, the derived RBE based on the MKM model increased from 1.14 to 1.6 in the BP and its distal part. Good agreement was observed between the experimental and simulation results, confirming the potential application of SOI microdosimeter with 3D SV for quality assurance in proton therapy.

Characterization of prompt gamma-ray emission with respect to the Bragg peak for proton beam range verification: A Monte Carlo study

In this work, we used the Monte Carlo-based Geant4 simulation toolkit to calculate the ambient dose equivalents due to the secondary neutron field produced in a new projected proton therapy facility. In particular the facility geometry was modeled in Geant4 based on the CAD design. Proton beams were originated with an energy of 250MeV in the gantry rooms with different angles with respect to the patient; a fixed 250MeV proton beam was also modeled. The ambient dose equivalent was calculated in several locations of interest inside and outside the facility, for different scenarios. The simulation results were compared qualitatively to previous work on an existing facility bearing some similarities with the design under study, showing that the ambient dose equivalent ranges obtained are reasonable. The ambient dose equivalents, calculated by means of the Geant4 simulation, were compared to the Australian regulatory limits and showed that the new facility will not pose health risks for the public or staff, with a maximum equivalent dose rate equal to 7.9mSv/y in the control rooms and maze exit areas and 1.3·10-1mSv/y close to the walls, outside the facility, under very conservative assumptions. This work represents the first neutron shielding verification analysis of a new projected proton therapy facility and, as such, it may serve as a new source of comparison and validation for the international community, besides confirming the viability of the project from a radioprotection point of view.

Characterization of proton pencil beam scanning and passive beam using a high spatial resolution solid‐state microdosimeter

Heavy-particle therapy such as carbon ion therapy is currently very popular because of its superior conformality in terms of dose distribution and higher Relative Biological Effectiveness (RBE). However, carbon ion beams produce a complex mixed radiation field, which needs to be fully characterised. In this study, the fragmentation of a 290 MeV/u primary carbon ion beam was studied using the Geant4 Monte Carlo Toolkit. When the primary carbon ion beam interacts with water, secondary light charged particles (H, He, Li, Be, B) and fast neutrons are produced, contributing to the dose, especially after the distal edge of the Bragg peak.

Neutron shielding for a new projected proton therapy facility: A Geant4 simulation study

Silicon-on-insulator (SOI) microdosimeters offer a promising method for routine quality assurance (QA) for hadron therapy due to their ease of operation and high spatial resolution. However, one complication which has been shown previously is that the traditional use of the mean chord length, [Formula: see text], calculated using Cauchys formula, for SOI devices in clinical carbon ion fields is not appropriate due to the strong directionality of the radiation field. In a previous study, we demonstrated that the mean path length, [Formula: see text], which is the mean path of charged particles in the sensitive volume (SV), is a more appropriate method to obtain microdosimetric quantities and biological relevant values, namely the relative biological effectiveness (RBE) by means of the microdosimetric kinetic model. The previous work, which was limited to mono-energetic [Formula: see text] ion beams typical of heavy ion therapy (HIT), is extended here to investigate the [Formula: see text] in a pristine proton beam as well as for spread out Bragg peaks (SOBP) for both proton and carbon ion clinical beams. In addition, the angular dependence of the SOI device for a number of different SV designs is also investigated to quantify the effects which the alignment has on the [Formula: see text]. It is demonstrated that the [Formula: see text] can be accurately estimated along the depth of a pristine or SOBP using the energy deposition spectra for both proton and [Formula: see text] ion beams. This observation allows a quick and accurate estimation of the [Formula: see text] for experimental use.

Contributions of secondary fragmentation by carbon ion beams in water phantom: Monte Carlo simulation

BACKGROUND The aim of this study was to measure the microdosimetric distributions of a carbon pencil beam scanning (PBS) and passive scattering system as well as to evaluate the relative biological effectiveness (RBE) of different ions, namely 12 C, 14 N, and 16 O, using a silicon-on-insulator (SOI) microdosimeter with well-defined 3D-sensitive volumes (SV). Geant4 simulations were performed with the same experimental setup and results were compared to the experimental results for benchmarking. METHOD Two different silicon microdosimeters with rectangular parallelepiped and cylindrical shaped SVs, both 10 μm in thickness were used in this study. The microdosimeters were connected to low noise electronics which allowed for the detection of lineal energies as low as 0.15 keV/μm in tissue. The silicon microdosimeters provide extremely high spatial resolution and can be used for in-field and out-of-field measurements in both passive scattering and PBS deliveries. The response of the microdosimeters was studied in 290 MeV/u 12 C, 180 MeV/u 14 N, 400 MeV/u 16 O passive ion beams, and 290 MeV/u 12 C scanning carbon therapy beam at heavy ion medical accelerator in Chiba (HIMAC) and Gunma University Heavy Ion Medical Center (GHMC), Japan, respectively. The microdosimeters were placed at various depths in a water phantom along the central axis of the ion beam, and at the distal part of the Spread Out Bragg Peak (SOBP) in 0.5 mm increments. The RBE values of the pristine Bragg peak (BP) and SOBP were derived using the microdosimetric lineal energy spectra and the modified microdosimetric kinetic model (MKM), using MKM input parameters corresponding to human salivary gland (HSG) tumor cells. Geant4 simulations were performed in order to verify the calculated depth-dose distribution from the treatment planning system (TPS) and to compare the simulated dose-mean lineal energy to the experimental results. RESULTS For a 180 MeV/u 14 N pristine BP, the dose-mean lineal energy yD¯ obtained with two types of silicon microdosimeters started from approximately 29 keV/μm at the entrance to 92 keV/μm at the BP, with a maximum value in the range of 412 to 438 keV/μm at the distal edge. For 400 MeV/u 16 O ions, the dose-mean lineal energy yD¯ started from about 24 keV/μm at the entrance to 106 keV/μm at the BP, with a maximum value of approximately 381 keV/μm at the distal edge. The maximum derived RBE10 values for 14 N and 16 O ions were found to be 3.10 ± 0.47 and 2.93 ± 0.45, respectively. Silicon microdosimetry measurements using pencilbeam scanning 12 C ions were also compared to the passive scattering beam. CONCLUSIONS These SOI microdosimeters with well-defined three-dimensional (3D) SVs have applicability in characterizing heavy ion radiation fields and measuring lineal energy deposition with sub-millimeter spatial resolution. It has been shown that the dose-mean lineal energy increased significantly at the distal part of the BP and SOBP due to very high LET particles. Good agreement was observed for the experimental and simulation results obtained with silicon microdosimeters in 14 N and 16 O ion beams, confirming the potential application of SOI microdosimeter with 3D SV for quality assurance in charged particle therapy.

Optimisation of the design of SOI microdosimeters for hadron therapy quality assurance

Due to the high LET and dense ionisation tracks associated with ions, microdosimetric approaches have been used in carbon ion therapy to assess field quality and calculate radiobiological quantities for a variety of cell lines. There is however a lack of instrumentation for simple and routine use in a clinical environment, important for determination of RBE which provides accurate treatment planning and delivery in hadron therapy. In this study, a 10 μm thick silicon microdosimeter with 3D sensitive volumes has been used to investigate the effect of motion on the RBE and field quality of a typical 12C ion therapy beam. For a passively scattered 290 MeV/u 12C beam with 6 cm spread-out Bragg peak (SOBP), variations in biological dose along the SOBP were observed, as well as a significant changes to particle LET when incident on a moving target.

The relative biological effectiveness for carbon, nitrogen, and oxygen ion beams using passive and scanning techniques evaluated with fully 3D silicon microdosimeters

The rationale for utilizing the prompt gamma (PG) signal for in vivo proton beam range verification is such that the PG fall-off distribution along the beam path is associated with the dose profile in the Bragg peak (BP) distal fall-off region. Quantitative characterization of this association, particularly with respect to the BP, is of great importance to assess its limitation and aid in the development of a clinically reliable PG imaging system to maximize PG detection. In this work we investigate the angular dependence of PG detection with respect to the BP for in vivo beam range verification in proton radiation therapy. Geant4 Monte Carlo simulations have been used to study the energy spectral and spatial characteristics of the PG signal from high-energy proton beam irradiations. A cylindrical water phantom (φ30 cm × 50 cm) with an ideal detecting cylinder (φ100 cm × 50 cm) coaxially surrounding the phantom has been used in the simulation. The angular dependence of PG detection as a function of beam energy and PG energy has been characterized with respect to the BP. Our results show that there exists an angular preference for PG detection, which has a strong dependence on the beam energy. As the beam energy increases, the longitudinal angular preference for PG detection becomes increasingly backward with respect to the BP position. This implies that the detector with sufficient longitudinal angular coverage is desired for the BP tracking, especially for the Spread-Out Bragg Peak tracking.