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内容記述 |
[Purpose] Multi-ion therapy can increase the dose-averaged linear energy transfer (LET<SUB>d</SUB>) within the target volume. For the verification of the LET<SUB>d</SUB> predicted by treatment planning system (TPS), the measurement accuracy of silicon-on-insulator (SOI) microdosimeter connected with MicroPlus2 was evaluated.[Methods] For the measurements of the LET spectra in target volume, several plans were created with TPS using carbon, oxygen, and neon ion beams: phantom plans with cubic target in water, and patient-specific quality assurance (PsQA) plans for the patients with head and neck tumors or bone and soft tissue sarcomas. The signal readout electronics called MicroPlus2 connected with SOI microdosimeter was used to measure the LET spectra of therapeutic ion beams. The noise level of the MicroPlus2 was low and the shaping time was 0.3 μs. It was connected to cylindrical 25 sensitive volumes with a thickness of 5 μm and a diameter of 30 μm. Firstly, the dependence of beam intensity was evaluated with phantom plans. Secondly, regarding the accuracy of LET<SUB>d</SUB> in water derived from the measured spectra by SOI detector, two conversion methods were compared; one was conventional method based on a fixed mass stopping power ratio (MSPR) and the other was the energy-dependent MSPR-based conversion method.[Results] A good agreement of up to 4.2% was obtained in the comparison of the LET<SUB>d</SUB> of 6 ports measured at 7 depths with the minimum and typical beam intensities (three times higher than the minimum beam intensity). In this comparison, the LET<SUB>d</SUB> variation of multiple measurements was larger (up to 10.66%) than the variation due to the beam intensity difference. While the mean squared error was approximately 4.9% in the comparison between SOI measurements with energy-dependent MSPR-based conversion method and TPS calculations of PsQA plans (6 ports, 12 depths), the mean squared error was approximately 6.4% in the comparison with a conventional method. [Conclusions] The utilization of MicroPlus2 and the energy-dependent MSPR-based conversion method have achieved better agreement between the measurements and TPS-calculated LET<SUB>d</SUB> with typical beam intensity. However, repeated measurements by SOI microdosimeter are necessary to reduce statistical uncertainties. Following the commissioning results, tolerance level of 10%/3mm was determined for LET measurements in PsQA. A clinical trial of multi-ion therapy has begun, and LET measurements with SOI microdosimeter have been implemented as PsQA. |
