Hideaki Nishiuchi

Beam commissioning of the new proton therapy system for University of Tsukuba

Beam commissioning of the new proton therapy system for University of Tsukuba was started in September 2000. The present system employs a synchrotron with a maximum energy of 250 MeV and two rotating gantries. The beam was successfully accelerated up to 250 MeV and transported to the irradiation nozzles. The position of the beam extracted from the synchrotron was confirmed to be very stable and sufficient flatness for the irradiation area was realized by using the dual ring double scattering method developed at University of Tsukuba. Furthermore, synchrotron operation triggered by patient respiration signal was succeeded.

Reduction of the number of stacking layers in proton uniform scanning

Uniform scanning with a relatively large beam size can improve beam utilization efficiency more than conventional irradiation methods using scatterers and can achieve a large-field, long-range and large spread-out Bragg peak (SOBP). The SOBP is obtained by energy stacking in uniform scanning, but its disadvantage is that the number of stacking layers is large, especially in the low-energy region, because the Bragg peak of the pristine beam is very sharp. We applied a mini-ridge filter to broaden the pristine Bragg peak up to a stacked layer thickness of 1 or 2 cm in order to decrease the number of stacking layers. The number of stacking layers can be reduced to 20% or less than that in the case of pristine beam stacking. Although the distal falloff of the SOBP is deteriorated by applying the mini-ridge filter, we can improve the distal falloff to that of pristine beam stacking by introducing the distal filter to the irradiation of the most distal layer. Uniform scanning in combination with mini-ridge filter use can more than double the beam utilization efficiency over that of passive irradiation techniques.

Beam commissioning of a multi-purpose compact ion synchrotron

W-MAST (Wakasa-wan Energy Research Center Multipurpose Accelerator with Synchrotron and Tandem) consists of a tandem accelerator with 5 MV terminal voltage and a compact ion synchrotron. The beam commissioning was started at the beginning of 1999 and was successfully accomplished in March 2000. A proton beam of 10 MeV was injected from the tandem accelerator and accelerated by the ion synchrotron to 200 MeV during 0.6 s. In order to reduce the space charge induced tune spread, the second order harmonic component was superimposed onto the accelerating radio frequency field with an untuned accelerating cavity having FINEMET cores. The betatron tune during the acceleration was measured and controlled to improve the acceleration efficiency.

Dual ring multilayer ionization chamber and theory-based correction technique for scanning proton therapy

PURPOSE To develop a multilayer ionization chamber (MLIC) and a correction technique that suppresses differences between the MLIC and water phantom measurements in order to achieve fast and accurate depth dose measurements in pencil beam scanning proton therapy. METHODS The authors distinguish between a calibration procedure and an additional correction: 1-the calibration for variations in the air gap thickness and the electrometer gains is addressed without involving measurements in water; 2-the correction is addressed to suppress the difference between depth dose profiles in water and in the MLIC materials due to the nuclear interaction cross sections by a semiempirical model tuned by using measurements in water. In the correction technique, raw MLIC data are obtained for each energy layer and integrated after multiplying them by the correction factor because the correction factor depends on incident energy. The MLIC described here has been designed especially for pencil beam scanning proton therapy. This MLIC is called a dual ring multilayer ionization chamber (DRMLIC). The shape of the electrodes allows the DRMLIC to measure both the percentage depth dose (PDD) and integrated depth dose (IDD) because ionization electrons are collected from inner and outer air gaps independently. RESULTS IDDs for which the beam energies were 71.6, 120.6, 159, 180.6, and 221.4 MeV were measured and compared with water phantom results. Furthermore, the measured PDDs along the central axis of the proton field with a nominal field size of 10 × 10 cm(2) were compared. The spread out Bragg peak was 20 cm for fields with a range of 30.6 and 3 cm for fields with a range of 6.9 cm. The IDDs measured with the DRMLIC using the correction technique were consistent with those that of the water phantom; except for the beam energy of 71.6 MeV, all of the points satisfied the 1% dose/1 mm distance to agreement criterion of the gamma index. The 71.6 MeV depth dose profile showed slight differences in the shallow region, but 94.5% of the points satisfied the 1%/1 mm criterion. The 90% ranges, defined at the 90% dose position in distal fall off, were in good agreement with those in the water phantom, and the range differences from the water phantom were less than ±0.3 mm. The PDDs measured with the DRMLIC were also consistent with those that of the water phantom; 97% of the points passed the 1%/1 mm criterion. CONCLUSIONS It was demonstrated that the new correction technique suppresses the difference between the depth dose profiles obtained with the MLIC and those obtained from a water phantom, and a DRMLIC enabling fast measurements of both IDD and PDD was developed. The IDDs and PDDs measured with the DRMLIC and using the correction technique were in good agreement with those that of the water phantom, and it was concluded that the correction technique and DRMLIC are useful for depth dose profile measurements in pencil beam scanning proton therapy.