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 ¹² C beam, based on Monte Carlo simulations. The energy deposition spectra in a 10 μ m 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 〈 l P a t h 〉 to calculate the lineal energy was introduced as an alternative to the mean chord length 〈 l 〉 because it was found that adopting Cauchy’s formula for the 〈 l 〉 was not appropriate for the radiation field typical of HIT as it is very directional. 〈 l P a t h 〉 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 ¹² C ion beam can be adopted as 〈 l P a t h 〉 . The tissue equivalence conversion method and 〈 l P a t h 〉 were adopted to determine the RBE ₁₀ , calculated using a modified microdosimetric kinetic model, applied to the microdosimetric spectra resulting from the simulation study. Comparison of the RBE ₁₀ 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 〈 l P a t h 〉 .

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 ¹²C, ¹⁴N, and ¹⁶O, 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 ¹²C, 180 MeV/u ¹⁴N, 400 MeV/u ¹⁶O passive ion beams, and 290 MeV/u ¹²C 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 ¹⁴N pristine BP, the dose‐mean lineal energy 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 ¹⁶O ions, the dose‐mean lineal energy 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 ¹⁴N and ¹⁶O ions were found to be 3.10 ± 0.47 and 2.93 ± 0.45, respectively. Silicon microdosimetry measurements using pencilbeam scanning ¹²C 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 ¹⁴N and ¹⁶O 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 He ²⁺ and 2 MeV H ⁺ ions. Measurement of the microdosimetric characteristics allowed for the determination of the Relative Biological Effectiveness of the ¹² C 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.

New 10-μm-thick silicon microdosimeters utilizing 3-D technology have been developed and investigated in this paper. The TCAD simulations were carried out to understand the electrical properties of the microdosimeters' design. A charge collection study of the devices was performed using 5.5-MeV He ²⁺ ions which were raster scanned over the surface of the detectors and the charge collection median energy maps were obtained and the detection yield was also evaluated. The devices were tested in a 290 MeV/u carbon ion beam at the Heavy Ion Medical Accelerator in Chiba (HIMAC) in Japan. Based on the microdosimetric measurements, the quality factor and dose equivalent out of field were obtained in a mixed radiation field mimicking the radiation environment for spacecraft in deep space.