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].

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.

Purpose: Microdosimetry is a vital tool for assessing the microscopic patterns of energy deposition by radiation, which ultimately govern biological effect. Solid-state, silicon-on-insulator microdosimeters offer an approach for making microdosimetric measurements with high spatial resolution (on the order of tens of micrometers). These high-resolution, solid-state microdosimeters may therefore play a useful role in characterizing proton radiotherapy fields, particularly for making highly resolved measurements within the Bragg peak region. In this work, we obtain microdosimetric measurements with a solid-state microdosimeter (MicroPlus probe) in a clinical, spot-scanning proton beam of small spot size. Methods: The MicroPlus probe had a 3D single sensitive volume on top of silicon oxide. The sensitive volume had an active cross-sectional area of 250 lm 9 10 lm and thickness of 10 lm. The proton facility was a synchrotron-based, spot-scanning system with small spot size (r % 2 mm). We performed measurements with the clinical beam current (%1 nA) and had no detected pulse pile-up. Measurements were made in a water-equivalent phantom in water-equivalent depth (WED) increments of 0.25 mm or 1.0 mm along pristine Bragg peaks of energies 71.3 MeV and 159.9 MeV, respectively. For each depth, we measured lineal energy distributions and then calculated the dose-weighted mean lineal energy, y D . The measurements were repeated for two field sizes: 4 9 4 cm 2 and 20 9 20 cm Conclusions: We performed microdosimetric measurements with a novel solid-state, silicon-oninsulator microdosimeter in a clinical spot-scanning proton beam of small spot size and unmodified beam current. For all of the proton field sizes and energies considered, the measurements of y D were in agreement with expected trends. Furthermore, we obtained measurements with a spatial resolution of 10 lm in the beam direction. This spatial resolution greatly exceeded that possible with a conventional gaseous tissue-equivalent proportional counter and allowed us to perform a high-resolution investigation within the Bragg peak region. The MicroPlus probe is therefore suitable for applications in proton radiotherapy.

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.