Takuji Furukawa

Advanced RF-KO slow-extraction method for the reduction of spill ripple

Two advanced RF-knockout (RF-KO) slow-extraction methods have been developed at HIMAC in order to reduce the spill ripple for accurate heavy-ion cancer therapy: the dual frequency modulation (FM) method and the separated function method. As a result of simulations and experiments, it was verified that the spill ripple could be considerably reduced using these advanced methods, compared with the ordinary RF-KO method. The dual FM method and the separated function method bring about a low spill ripple within standard deviations of around 25% and of 15% during beam extraction within around 2 s, respectively, which are in good agreement with the simulation results.

Optimization of Spiral-Wobbler System for Heavy-Ion Radiotherapy

We propose an amplitude modulation wobbler-scatterer system, a so-called spiral-wobbler system, to produce a large irradiation field in a relatively short irradiation port for heavy-ion radiation therapy. The wobbler parameters, such as the wobbler-radius-modulation function, the wobbler radial frequency and the rotational frequency, are optimized and the uniformity of the irradiation field is estimated as the function of irradiation time. By using a wobbler radial frequency of 23 Hz, and a wobbler rotational frequency of 59 Hz, an irradiation field with a uniformity of ±3% can be obtained for an irradiation duration of 1 s. The spiral-wobbler system is applicable to respiration-gated irradiation.

Source of spill ripple in the RF-KO slow-extraction method with FM and AM

The RF-knockout (RF-KO) slow-extraction method with frequency modulation (FM) and amplitude modulation (AM) has brought high-accuracy irradiation to the treatment ofa cancer tumor moving with respiration, because ofa quick response to beam start/stop. However, a beam spill extracted from a synchrotron ring through RF-KO slowextraction has a huge ripple with a frequency of around 1 kHz related to the FM. The spill ripple will disturb the lateral dose distribution in the beam scanning methods. Thus, the source ofthe spill ripple has been investigated through experiments and simulations. There are two tune regions for the extraction process through the RF-KO method: the extraction region and the diffusion region. The particles in the extraction region can be extracted due to amplitude growth through the transverse RF field, only when its frequency matches with the tune in the extraction region. For a large chromaticity, however, the particles in the extraction region can be extracted through the synchrotron oscillation, even when the frequency does not match with the tune in the extraction region. Thus, the spill structure during one period ofthe FM strongly depends on the horizontal chromaticity. They are repeated with the repetition f of the FM, which is the very source ofthe spill ripple in the RF-KO method. r 2002 Elsevier Science B.V. All rights reserved.

Nuclear-interaction correction of integrated depth dose in carbon-ion radiotherapy treatment planning

In treatment planning of charged-particle therapy, tissue heterogeneity is conventionally modeled as water with various densities, i.e. stopping effective densities ρ(S), and the integrated depth dose measured in water (IDD) is applied accordingly for the patient dose calculation. Since the chemical composition of body tissues is different from that of water, this approximation causes dose calculation errors, especially due to difference in nuclear interactions. Here, we propose and validate an IDD correction method for these errors in patient dose calculations. For accurate handling of nuclear interactions, ρ(S) of the patient is converted to nuclear effective density ρ(N), defined as the ratio of the probability of nuclear interactions in the tissue to that in water using a recently formulated semi-empirical relationship between the two. The attenuation correction factor Φ(w)(p), defined as the ratio of the attenuation of primary carbon ions in a patient to that in water, is calculated from a linear integration of ρ(N) along the beam path. In our treatment planning system, a carbon-ion beam is modeled to be composed of three components according to their transverse beam sizes: primary carbon ions, heavier fragments, and lighter fragments. We corrected the dose contribution from primary carbon ions to IDD as proportional to Φ(w)(p), and corrected that from lighter fragments as inversely proportional to Φ(w)(p). We tested the correction method for some non-water materials, e.g. milk, lard, ethanol and water solution of potassium phosphate (K2HPO4), with un-scanned and scanned carbon-ion beams. In un-scanned beams, the difference in IDD between a beam penetrating a 150 mm-thick layer of lard and a beam penetrating water of the corresponding thickness amounted to -4%, while it was +6% for a 150 mm-thick layer of 40% K2HPO4. The observed differences were accurately predicted by the correction method. The corrected IDDs agreed with the measurements within ±1% for all materials and combinations of them. In scanned beams, the dose estimation error in target dose amounted to 4% for a 150 mm-thick layer of 40% K2HPO4. The error is significantly reduced with the correction method. The planned dose distributions with the method agreed with the measurements within ±1.5% of target dose for all materials not only in the target region but also in the plateau and fragment-tail regions. We tested the correction method of IDD in some non-water materials to verify that this method would offer the accuracy and simplicity required in carbon-ion radiotherapy treatment planning.

A Patient-Specific QA Procedure for Moving Target Irradiation in Scanned Ion Therapy

Three-dimensional (3D) pencil-beam scanning technique has been utilized since 2011 in NIRS-HIMAC. Beam delivery system and treatment planning software (TPS) require dosimetric patient-specific QA to check each individual plan. Any change in the scanned beams will result in a significant impact on the irradiation dose. Therefore, patient-specific QA for moving target irradiation requires additional procedure. In an additional QA for moving target irradiation, we placed 2D ionization chamber on the PMMA plate tilted with respect to the beam axis. The PMMA plate was set on the stage of the moving phantom. The moving phantom was moved according to patient data. We measured the dose distribution for both the static target and the moving target. We compared the results for the moving target with those for the static targets by means of a gamma index analysis. In the additional patient-specific QA, the gamma analysis between the moving and static targets showed the good agreement. We confirmed that this new technique was a beneficial QA procedure for moving target irradiation.

Development of QA System for the Rotating Gantry for Carbon Ion Therapy at NIRS

At the National Institute of Radiological Sciences (NIRS), we have been developing the rotating-gantry system for the carbon-ion radiotherapy. This system is equipped with a three-dimensional pencil beam scanning irradiation system. To ensure the treatment quality, calibration of the primary dose monitor, range check, dose rate check, machine safety check, and some mechanical tests should be performed efficiently. For this purpose, we have developed a measurement system dedicated for quality assurance (QA) of this gantry system. The ion beam’s dose output are calibrated by measurement using an ionization chamber. A Farmer type ionization chamber is inserted into the center of a water equivalent phantom. The thickness of the phantom could be changed so that employ both calibration of the output at entrance and output checking at center of the irradiation field. The ranges of beams are verified using a scintillator and a CCD camera system. From the taken images, maximum gradient points are determined by some image processing and compared with reference data. In this paper, we describe consideration of the daily QA for the rotating-gantry.