Shinichi Minohara

Biophysical characteristics of HIMAC clinical irradiation system for heavy-ion radiation therapy

PURPOSE The irradiation system and biophysical characteristics of carbon beams are examined regarding radiation therapy. METHODS AND MATERIALS An irradiation system was developed for heavy-ion radiotherapy. Wobbler magnets and a scatterer were used for flattening the radiation field. A patient-positioning system using X ray and image intensifiers was also installed in the irradiation system. The depth-dose distributions of the carbon beams were modified to make a spread-out Bragg peak, which was designed based on the biophysical characteristics of monoenergetic beams. A dosimetry system for heavy-ion radiotherapy was established to deliver heavy-ion doses safely to the patients according to the treatment planning. A carbon beam of 80 keV/microm in the spread-out Bragg peak was found to be equivalent in biological responses to the neutron beam that is produced at cyclotron facility in National Institute Radiological Sciences (NIRS) by bombarding 30-MeV deuteron beam on beryllium target. The fractionation schedule of the NIRS neutron therapy was adapted for the first clinical trials using carbon beams. RESULTS Carbon beams, 290, 350, and 400 MeV/u, were used for a clinical trial from June of 1994. Over 300 patients have already been treated by this irradiation system by the end of 1997.

Respiratory gated irradiation system for heavy-ion radiotherapy

PURPOSE In order to reduce the treatment margin of the moving target due to breathing, we developed a gated irradiation system for heavy-ion radiotherapy. METHODS AND MATERIALS The motion of a patient due to respiration is detected by the motion of the body surface around the chest wall. A respiratory sensor was developed using an infrared light spot and a position-sensitive detector. A timing signal to request a beam is generated in response to the respiration waveform, and a carbon beam is extracted from the synchrotron using a RF-knockout method. CT images for treatment planning are taken in synchronization with the respiratory motion. For patient positioning, digitized fluoroscopic images superimposed with the respiration waveform were used. The relation between the respiratory sensor signal and the organ motion was examined using digitized video images from fluoroscopy. The performance of our gated system was demonstrated by using the moving phantom, and dose profiles were measured in the direction of phantom motion. RESULTS The timing of gate-on is set at the end of the expiratory phase, because the motion of the diaphragm is slower and more reproducible than during the inspiratory phase. The signal of the respiratory sensor shows a phase difference of 120 milliseconds between lower and upper locations on the chest wall. The motion of diaphragm is delayed by 200 milliseconds from the respiration waveform at the lower location. The beam extraction system worked according to the beam on/off logic for gating, and the gated CT scanner performed well. The lateral penumbra size of the dose profile along the moving axis was distinguishably decreased by the gated irradiation. The ratio of the nongated to gated lateral fall-off was 4.3, 3.5, and 2. 0 under the stroke of 40.0, 29.0, and 13.0 mm respectively. CONCLUSION We developed a total treatment system of gated irradiation for heavy-ion radiotherapy. We found that with this system the target margin along the body axis could be decreased to 5-10 mm although the target moved twice or three times. Over 150 patients with lung or liver cancer had already been treated by this gated irradiation system by the end of July 1999.

Breathing‐synchronized radiotherapy program at the University of California Davis Cancer Center

In this paper we present a complete description of the breathing synchronized radiotherapy (BSRT) system, which has been jointly developed between the University of California Davis Cancer Center and Varian Associates. BSRT is a description of an emerging radiation oncology procedure, where simulation, CT scan, treatment planning, and radiation treatment are synchronized with voluntary breath-hold, forced breath-hold, or breathing gating. The BSRT system consists of a breathing monitoring system (BMOS) and a linear accelerator gating hardware and software package. Two methods, a video camera-based method and the use of wraparound inductive plethysmography (RespiTrace), generate the BMOS signals. The BMOS signals and the synchronized fluoroscopic images are simultaneously recorded in the simulation room and are later analyzed to define the ideal treatment point (ITP) where organ motion is stationary. The BMOS signals at ITP can be used to gate a CT scanner or a linear accelerator to maintain the same organ configuration as in the simulation. The BSRT system allows breath-hold or gating. This dual role allows the system to be applicable for a variety of patients, i.e., the breath-hold method for those patients who can maintain and reproduce the ITP, and the forced breath-hold or gating method for those who are not ideal for voluntary breath-hold.

Heavy ion synchrotron for medical use —HIMAC project at NIRS-Japan—

Abstract A heavy ion synchrotron complex for medical use is being constructed at Chiba, Japan. General feature and present status of this project are described.

Design study of a raster scanning system for moving target irradiation in heavy-ion radiotherapy

A project to construct a new treatment facility as an extension of the existing heavy-ion medical accelerator in chiba (HIMAC) facility has been initiated for further development of carbon-ion therapy. The greatest challenge of this project is to realize treatment of a moving target by scanning irradiation. For this purpose, we decided to combine the rescanning technique and the gated irradiation method. To determine how to avoid hot and/or cold spots by the relatively large number of rescannings within an acceptable irradiation time, we have studied the scanning strategy, scanning magnets and their control, and beam intensity dynamic control. We have designed a raster scanning system and carried out a simulation of irradiating moving targets. The result shows the possibility of practical realization of moving target irradiation with pencil beam scanning. We describe the present status of our design study of the raster scanning system for the HIMAC new treatment facility.

A CT calibration method based on the polybinary tissue model for radiotherapy treatment planning

A method to establish the relationship between CT number and effective density for therapeutic radiations is proposed. We approximated body tissues to mixtures of muscle, air, fat and bone. Consequently, the relationship can be calibrated only with a CT scan of their substitutes, for which we chose water, air, ethanol and potassium phosphate solution, respectively. With simple and specific corrections for non-equivalencies of the substitutes, a calibration accuracy of 1% will be achieved. We tested the calibration method with some biological materials to verify that the proposed method would offer the accuracy, simplicity and specificity required for a standard in radiotherapy treatment planning, in particular with heavy charged particles.

Performance of the NIRS fast scanning system for heavy‐ion radiotherapy

PURPOSE A project to construct a new treatment facility, as an extension of the existing HIMAC facility, has been initiated for the further development of carbon-ion therapy at NIRS. This new treatment facility is equipped with a 3D irradiation system with pencil-beam scanning. The challenge of this project is to realize treatment of a moving target by scanning irradiation. To achieve fast rescanning within an acceptable irradiation time, the authors developed a fast scanning system. METHODS In order to verify the validity of the design and to demonstrate the performance of the fast scanning prior to use in the new treatment facility, a new scanning-irradiation system was developed and installed into the existing HIMAC physics-experiment course. The authors made strong efforts to develop (1) the fast scanning magnet and its power supply, (2) the high-speed control system, and (3) the beam monitoring. The performance of the system including 3D dose conformation was tested by using the carbon beam from the HIMAC accelerator. RESULTS The performance of the fast scanning system was verified by beam tests. Precision of the scanned beam position was less than +/-0.5 mm. By cooperating with the planning software, the authors verified the homogeneity of the delivered field within +/-3% for the 3D delivery. This system took only 20 s to deliver the physical dose of 1 Gy to a spherical target having a diameter of 60 mm with eight rescans. In this test, the average of the spot-staying time was considerably reduced to 154 micros, while the minimum staying time was 30 micros. CONCLUSIONS As a result of this study, the authors verified that the new scanning delivery system can produce an accurate 3D dose distribution for the target volume in combination with the planning software.

Relationship between CT number and electron density, scatter angle and nuclear reaction for hadron-therapy treatment planning

The precise conversion of CT numbers to their electron densities is essential in treatment planning for hadron therapy. Although some conversion methods have already been proposed, it is hard to check the conversion accuracy during practical therapy. We have estimated the CT numbers of real tissues by a calculational method established by Mustafa and Jackson. The relationship between the CT numbers and the electron densities was investigated for various body tissues as well as some tissue-equivalent materials used for a conversion to check the accuracy of the current conversion methods. The result indicates a slight disagreement at the high-CT-number region. A precise estimation of the multiple scattering, nuclear reaction and range straggling of incident particles has been considered as being important to realize higher-level conformal therapy in the future. The relationship between these parameters and the CT numbers was also investigated for tissues and water. The result shows that it is sufficiently practical to replace these parameters for real tissues with those for water by adjusting the density.

Dosimetry and Measured Differential W Values of Air for Heavy Ions

Heavy-ion irradiation systems were designed and constructed at two cyclotron facilities in Japan for use in various fields of radiation physics and radiation biology. A 135 MeV/u carbon beam as well as 12 MeV/u carbon and helium-3 beams were first used in experiments. We have established a systematic method for heavy-ion dosimetry at both high and low incident energies involving measurements of fluences. We also obtained differential W values (w) of air for those beams by comparing the results of fluence measurement dosimetry with ionization chamber dosimetry. The differential W values of air were found to be 36.2 +/- 1.0, 34.5 +/- 1.0, and 33.7 +/- 0.9 eV for 6.7 MeV/u carbon ions, 10.3 MeV/u 3He ions, and 129.4 MeV/u carbon ions, respectively. The w value for high-energy heavy ions approaches the W value for high-energy electron or photon beams. In ionization chamber dosimetry for a heavy-ion beam, we found a track-size effect. A difference in the track sizes of heavy ions in the gas and solid phases affected the output current of the ion chamber in the case of high-energy heavy ions.

Estimating uncertainties of the geometrical range of particle radiotherapy during respiration

PURPOSE To propose a method for estimating uncertainties of the range calculation in particle radiotherapy associated with patient respiration. MATERIALS AND METHODS A set of sequential CT images at every 0.2 s was reconstructed from continuous X-ray projection data accumulated by dynamic helical scanning. At the same time that CT data was acquired, the respiratory signal of the patient and the X-ray on/off signal on CT scanner were recorded. Each CT image was timed according to the phase of respiration waveform. Conversion of the CT number to the water equivalent path length (WEL) was performed with our treatment planning system that included a conversion table. As an illustration, the CT images of a patient with liver cancer at the right upper lobe were analyzed. The geometric size of the liver and WELs from body surface to isocenter were measured in each CT image. RESULTS Variations of WEL from body surface to isocenter at the anterior-posterior and posterior-anterior direction were 6.2 mm and 18.9 mm, respectively. Liver size changed by 35.2 mm. However, these variations were shown to be considerably reduced by gated irradiation. CONCLUSIONS A method using sequential CT images with respiration waveform was proposed. It appeared to be useful in evaluating the uncertainties of the range calculation associated with patient breathing.