The relative biological effectiveness (RBE) of protons is highly variable and difficult to quantify. However, RBE is related to the local ionization density, which can be related to the physical measurable dose weighted linear energy transfer (LET D ). The aim of this study was to validate the LET D calculations for proton therapy beams implemented in a commercially available treatment planning system (TPS) using microdosimetry measurements and independent LET D calculations (Open-MCsquare (MCS)).The TPS (RayStation v6R) was used to generate treatment plans on the CIRS-731-HN anthropomorphic phantom for three anatomical sites (brain, nasopharynx, neck) for a spherical target (Ø = 5 cm) with uniform target dose to calculate the LET D distribution. Measurements were performed at the University Medical Center Groningen proton therapy center (Proteus Plus, IBA) using a µ + -probe utilizing silicon on insulator microdosimeters capable of detecting lineal energies as low as 0.15 keV µm −1 in tissue. Dose averaged mean lineal energy γ D depth-profiles were measured for 70 and 130 MeV spots in water and for the three treatment plans in water and an anthropomorphic phantom. The γ D measurements were compared to the LET D calculated in the TPS and MCS independent dose calculation engine. D • γ D was compared to D • LET D in terms of a gamma-index with a distance-to-agreement criteria of 2 mm and increasing dose difference criteria to determine the criteria for which a 90% pass rate was accomplished.Measurements of D • γ D were in good agreement with the D • LET D calculated in the TPS and MCS. The 90% passing rate threshold was reached at different D • LET D difference criteria for single spots (TPS: 1% MCS: 1%), treatment plans in water (TPS: 3% MCS: 6%) and treatment plans in an anthropomorphic phantom (TPS: 6% MCS: 1%).We conclude that D • LET D calculations accuracy in the RayStation TPS and open MCSquare are within 6%, and sufficient for clinical D • LET D evaluation and optimization. These findings remove an important obstacle in the road towards clinical implementation of D • LET D evaluation and optimization of proton therapy treatment plans.
Novelty and significanceThe dose weighed linear energy transfer (LET D ) distribution can be calculated for proton therapy treatment plans by Monte Carlo dose engines. The relative biological effectiveness (RBE) of protons is known to vary with the LET D distribution. Therefore, there exists a need for accurate calculation of PAPER RECEIVED
Abstract-The authors describe techniques to make fast, subnanosecond time resolution solid-state detector systems using sensors with 3D electrodes, current amplifiers, constant-fraction comparators or fast wave-form recorders, and some of the next steps to reach still faster results.
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.
3D-detectors, with electrodes penetrating through the entire substrates have drawn great interests for high energy physics and medical imaging applications. Since its introduction by C. Kenney et al in 1995, many laboratories have begun research on different 3D-detector structures to simplify and industrialise the fabrication process. SINTEF MiNaLab joined the 3D collaboration in 2006 and started the first 3D fabrication run in 2007. This is the first step in an effort to fabricate affordable 3D-detectors in small to medium size production volumes. The first run was fully completed in February 2008 and preliminary results are promising. Good p-n junction characteristics have been shown on selected devices at the chip level with a leakage current of less than 0.5 nA per pixel. Thus SINTEF is the second laboratory in the world after the Stanford Nanofabrication Facility that has succeeded in demonstrating full 3D-detectors with active edge. A full 3D-stacked detector system were formed by bump-bonding the detectors to the ATLAS readout electronics, and successful particle hit maps using an Am-241 source were recorded. Most modules, however, showed largely increased leakage currents after assembly, which is due to the active edge and pspray acting as part of the total chip pn-junction and not as a depletion stop. This paper describes the first fabrication and the encountered processing issues. The preliminary measurements on both the individual detector chips and the integrated 3D-stacked modules are discussed. A new lot has now been started on p-type wafers, which offers a more robust configuration with the active edge acting as depletion stop instead of part of the pn-junction.
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