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 C 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 Cauchy's 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 incidentC ion beam can be adopted as [Formula: see text]. The tissue equivalence conversion method and [Formula: see text] were adopted to determine the RBE, calculated using a modified microdosimetric kinetic model, applied to the microdosimetric spectra resulting from the simulation study. Comparison of the RBE 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].
An improved biological weighting function (IBWF) is proposed to phenomenologically relate microdosimetric lineal energy probability density distributions with the relative biological effectiveness (RBE) for the in vitro clonogenic cell survival (surviving fraction = 10%) of the most commonly used mammalian cell line, i.e. the Chinese hamster lung fibroblasts (V79). The IBWF, intended as a simple and robust tool for a fast RBE assessment to compare different exposure conditions in particle therapy beams, was determined through an iterative global-fitting process aimed to minimize the average relative deviation between RBE calculations and literature in vitro data in case of exposure to various types of ions from 1H to 238U. By using a single particle- and energy- independent function, it was possible to establish an univocal correlation between lineal energy and clonogenic cell survival for particles spanning over an unrestricted linear energy transfer range of almost five orders of magnitude (0.2 keV µm−1 to 15 000 keV µm−1 in liquid water). The average deviation between IBWF-derived RBE values and the published in vitro data was ∼14%. The IBWF results were also compared with corresponding calculations (in vitro RBE10 for the V79 cell line) performed using the modified microdosimetric kinetic model (modified MKM). Furthermore, RBE values computed with the reference biological weighting function (BWF) for the in vivo early intestine tolerance in mice were included for comparison and to further explore potential correlations between the BWF results and the in vitro RBE as reported in previous studies. The results suggest that the modified MKM possess limitations in reproducing the experimental in vitro RBE10 for the V79 cell line in case of ions heavier than 20Ne. Furthermore, due to the different modelled endpoint, marked deviations were found between the RBE values assessed using the reference BWF and the IBWF for ions heavier than 2H. Finally, the IBWF was unchangingly applied to calculate RBE values by processing lineal energy density distributions experimentally measured with eight different microdosimeters in 19 1H and 12C beams at ten different facilities (eight clinical and two research ones). Despite the differences between the detectors, irradiation facilities, beam profiles (pristine or spread out Bragg peak), maximum beam energy, beam delivery (passive or active scanning), energy degradation system (water, PMMA, polyamide or low-density polyethylene), the obtained IBWF-based RBE trends were found to be in good agreement with the corresponding ones in case of computer-simulated microdosimetric spectra (average relative deviation equal to 0.8% and 5.7% for 1H and 12C ions respectively).
Microdosimetric energy depositions have been suggested as a key variable for the modeling of the relative biological effectiveness (RBE) in proton and ion radiation therapy. However, microdosimetry has been underutilized in radiation therapy. Recent advances in detector technology allow the design of new mico-and nano-dosimeters. At the same time Monte Carlo simulations have become more widely used in radiation therapy. In order to address the growing interest in the field, a microdosimetric extension was developed in TOPAS. The extension provides users with the functionality to simulate microdosimetric spectra as well as the contribution of secondary particles to the spectra, calculate microdosimetric parameters, and determine RBE with a biological weighting function approach or with the microdosimetric kinetic model. Simulations were conducted with the extension and the results were compared with published experimental data and other simulation results for three types of microdosimeters, a spherical tissue equivalent proportional counter (TEPC), a cylindrical TEPC and a solid state microdosimeter. The corresponding microdosimetric spectra obtained with TOPAS from the plateau region to the distal tail of the Bragg curve generally show good agreement with the published data.
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 N and O ion beams, confirming the potential application of SOI microdosimeter with 3D SV for quality assurance in charged particle therapy.
Microdosimetry is an extremely useful technique, used for dosimetry in unknown mixed radiation fields typical of space and aviation, as well as in hadron therapy. A new silicon microdosimeter with 3D sensitive volumes has been proposed to overcome the shortcomings of the conventional Tissue Equivalent Proportional Counter. In this article, the charge collection characteristics of a new 3D mesa microdosimeter were investigated using the ANSTO heavy ion microprobe utilizing 5.5 MeV and 2 MeV ions. Measurement of the microdosimetric characteristics allowed for the determination of the Relative Biological Effectiveness of the heavy ion therapy beam at the Heavy Ion Medical Accelerator in Chiba (HIMAC), Japan. Well-defined sensitive volumes of the 3D mesa microdosimeter have been observed and the microdosimetric RBE obtained showed good agreement with the TEPC. The new 3D mesa "bridge" microdosimeter is a step forward towards a microdosimeter with fully free-standing 3D sensitive volumes.
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