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Fujio Araki

Publications and source records attributed to Fujio Araki.

15 recordsLinked to original sources
PubMed2006

Monte Carlo study of a Cyberknife stereotactic radiosurgery system.

This study investigated small-field dosimetry for a Cyberknife stereotactic radiosurgery system using Monte Carlo simulations. The EGSnrc/BEAMnrc Monte Carlo code was used to simulate the Cyberknife treatment head, and the DOSXYZnrc code was implemented to calculate central axis depth-dose curves, off-axis dose profiles, and relative output factors for various circular collimator sizes of 5 to 60 mm. Water-to-air stopping power ratios necessary for clinical reference dosimetry of the Cyberknife system were also evaluated by Monte Carlo simulations. Additionally, a beam quality conversion factor, kQ, for the Cyberknife system was evaluated for cylindrical ion chambers with different wall material. The accuracy of the simulated beam was validated by agreement within 2% between the Monte Carlo calculated and measured central axis depth-dose curves and off-axis dose profiles. The calculated output factors were compared with those measured by a diode detector and an ion chamber in water. The diode output factors agreed within 1% with the calculated values down to a 10 mm collimator. The output factors with the ion chamber decreased rapidly for collimators below 20 mm. These results were confirmed by the comparison to those from Monte Carlo methods with voxel sizes and materials corresponding to both detectors. It was demonstrated that the discrepancy in the 5 and 7.5 mm collimators for the diode detector is due to the water non-equivalence of the silicon material, and the dose fall-off for the ion chamber is due to its large active volume against collimators below 20 mm. The calculated stopping power ratios of the 60 mm collimator from the Cyberknife system (without a flattening filter) agreed within 0.2% with those of a 10 X 10 cm2 field from a conventional linear accelerator with a heavy flattening filter and the incident electron energy, 6 MeV. The difference in the stopping power ratios between 5 and 60 mm collimators was within 0.5% at a 10 cm depth in water. Furthermore, kQ values for the Cyberknife system were in agreement within 0.3% with those of the conventional 6 MV-linear accelerator for the cylindrical ion chambers with different wall material.

Algorithms↗
PubMed2005

[A comparison of the helmet output factors for five Gamma-Knife units.].

A radiophotoluminescent (RPL) glass rod dosimeter (GRD) and a small active volume p-type silicon diode detector are used for the measurement of the helmet output factors from five Gamma-Knife units, which include four Model B units and a Model C unit. The output factors for the five units measured with the GRD from 14, 8 and 4 mm helmets relative to the 18 mm helmet are 0.984 +/- 0.003, 0.951 +/- 0.003 and 0.884 +/- 0.006, respectively. Similarly, the corresponding output factors measured with the p-type silicon diode detector are 0.983 +/- 0.002, 0.952 +/- 0.003 and 0.867 +/- 0.015, respectively. The output factors are corrected with the end effect for each helmet of the five units. The end effect time for the four Model B units ranges from 4 sec for the 18 mm helmet to 2 sec for the 4 mm helmet. The results for the Model C unit are within 1 sec for all the helmets. The output factors for the five units obtained from both detectors are in good agreement with the values in a recent publication and the values recommended by Elekta, the device manufacturer, except for that of the 4 mm helmet measured with the GRD. The average GRD output factor for the 4 mm helmet is 1.6% higher than Elekta's value, 0.870, but is in good agreement with the published value which was measured using small active volume detectors. The helmet output factors for the five Gamma-Knife units measured with the GRD agree within measurement deviation.

Glass↗
PubMed2005

[Clinical calibration dosimetry in JSMP-01: measurements using Farmer-type cylindrical ion chambers.].

The Japan Society of Medical Physics (JSMP) Task Group published Standard dosimetry of absorbed dose in external beam radiotherapy (Standard dosimetry 01) as a new high-energy photon and electron dosimetry protocol in 2002. In this study, we present Standard dosimetry 01 as the JSMP-01 protocol for the convenience of users. This protocol is based on using an ion chamber having a (60)Co absorbed dose to water calibration coefficient, N(D,w), which is calculated from a (60)Co exposure calibration coefficient, N(c). We present dose comparisons between a reference chamber and various Farmer-type cylindrical chambers with different wall materials. The absorbed dose to water was compared at the calibration depths of 5 cm for a (60)Co beam, 10 cm for photons, and d(c) = 0.6 R(50) - 0.1 (cm) for electrons according to JSMP-01. The JARP chamber in the Kyushu Regional Center which meets third-order standards in Japan was used as the reference chamber. The absorbed dose to water for the Farmer-type chambers determined according to JSMP-01 agreed with that for the JARP chamber within 1% for photon and electron beams. The doses obtained by JSMP-01 and the Japan Association of Radiological Physics protocol (JARP-86) were also compared for photon and electron beams. For the Farmer-type chambers with photon beams, JSMP-01 results were up to 1.5% higher than JARP-86 results. For electron beams JSMP-01 results were higher than JARP-86 results by 1.3-2.8%.

Calibration↗
PubMed2005

[Clinical calibration dosimetry in JSMP-01: measurements using plane-parallel ion chambers.].

The Japan Society of Medical Physics (JSMP) Task Group published Standard dosimetry of absorbed dose in external beam radiotherapy (Standard dosimetry 01) as a new high-energy photon and electron dosimetry protocol in 2002. In this study, we present Standard dosimetry 01 as the JSMP-01 protocol for the convenience of users. This protocol is based on using an ion chamber having a (60)Co absorbed dose to water calibration coefficient, N(D,w), which is calculated from a (60)Co exposure calibration coefficient, N(c). We present dose comparisons between a reference chamber and various plane-parallel chambers. The absorbed dose to water was compared at the calibration depth of 5 cm for a (60)Co beam and d(c) = 0.6R(50) - 0.1 (cm) for electron beams according to JSMP-01. The absorbed dose to water calibration coefficients, [N(D,w)](Co) and [N(D,w)](18E), for the plane-parallel chambers were also determined by (60)Co and electron beam cross-calibrations using a reference chamber. The dose for the plane-parallel chambers derived from [N(D,w)](Co) and [N(D,w)](18E) was compared to that for the reference chamber using electron beams. The JARP chamber in the Kyushu Regional Center which meets third-order standards in Japan was used as the reference chamber. The doses for the plane-parallel chambers determined according to JSMP-01 agreed with that for the JARP chamber within 1% and 2% for (60)Co and electron beams, respectively. For electron beams, the doses for the plane-parallel chambers calculated from [N(D,w)](Co) and [N(D,w)](18E) were within 1.5% and 1.0% compared to those for the JARP chamber, respectively, except for the Exradin A10 chamber.

Calibration↗
PubMed2003

Measurements of Gamma-Knife helmet output factors using a radiophotoluminescent glass rod dosimeter and a diode detector.

A radiophotoluminescent (RPL) glass rod dosimeter (GRD) and a small active volume p-type silicon diode detector are used for the measurement of the output factors from Gamma-Knife fields. The GRD system consists of small rod-shaped glass chip detectors and an automatic readout device. The output factors measured with the GRD from the 14, 8 and 4 mm helmets relative to the 18 mm helmet are 0.981, 0.942 and 0.877, respectively. Similarly, the corresponding output factors measured with the p-type silicon diode detector are 0.980, 0.949 and 0.867, respectively. The output factors are corrected for the end effect for each helmet. The output factors obtained from both detectors are in good agreement with the values in a recent publication and the values recommended by Elekta, the manufacturer. The directional dependence of these detectors is also measured. For the Gamma-Knife angle ranging from 6 to 36 degrees in the y-z plane of the stereotactic space, the measured angular dependence of the GRD is approximately 1.0% at a 4 MV x-ray beam. The response of the silicon diode detector indicates approximately 3-4% directional dependence for the same angular range for a 6 MV x-ray beam. The Gamma-Knife helmet output factors measured with the silicon diode detector are corrected for angular dependence.

Film Dosimetry↗
PubMed2002

Comparison of high-energy photon and electron dosimetry for various dosimetry protocols.

The American Association of Physicists in Medicine Task Group 51 (TG-51) and the International Atomic Energy Agency (IAEA) published a new high-energy photon and electron dosimetry protocol, in 1999 and 2000, respectively. These protocols are based on the use of an ion chamber having an absorbed-dose to water calibration factor with a 60Co beam. These are different from the predecessors, the TG-21 and IAEA TRS-277 protocols, which require a 60Co exposure or air-kerma calibration factor. The purpose of this work is to present the dose comparison between various dosimetry protocols and the AAPM TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. The absorbed-dose to water calculated according to the Japanese Association of Radiological Physics (JARP), International Atomic Energy Agency Technical Report Series No. 277 (IAEA TRS-277) and No. 398 (IAEA TRS-398) protocols is compared to that calculated using the TG-51 protocol. For various Farmer-type chambers in photon beams, TG-51 is found to predict 0.6-2.1% higher dose than JARP. Similarly, TG-51 is found to be higher by 0.7-1.7% than TRS-277. For electron beams TG-51 is higher than JARP by 1.5-3.8% and TRS-277 by 0.2-1.9%. The reasons for these differences are presented in terms of the cavity-gas calibration factor, Ngas, and a dose conversion factor, Fw, which converts the absorbed-dose to air in the chamber to the absorbed-dose to water. The ratio of cavity-gas calibration factors based on absorbed-dose to water calibration factors, N60Co(D,w), in TG-51 and cavity-gas calibration factors which are equivalent to absorbed-dose to air chamber factors, N(D,air), based on the IAEA TRS-381 protocol is 1.008 on average. However, the estimated uncertainty of the ratio between the two cavity-gas calibration factors is 0.9% (1 s.d.) and consequently, the observed difference of 0.8% is not significant. The absorbed-dose to water and exposure or air-kerma calibration factors are based on standards traceable to the National Institute of Standards and Technology (NIST). In contrast, the absorbed-dose to water determined with TRS-398 is in good agreement with TG-51 within about 0.5% for photon and electron beams.

Air↗
PubMed2002

Dosimetry and mechanical accuracy of the first rotating gamma system installed in North America.

The purpose of this paper is to present the dosimetry and mechanical accuracy of the first rotating gamma system (RGS) installed in North America for stereotactic radiosurgery. The data were obtained during the installation, acceptance test procedure, and commissioning of the unit. The RGS unit installed at UC Davis Cancer Center (RGSu) has modifications on the source and collimator bodies from the earlier version of the Chinese RGS (RGSc). The differences between these two RGSs are presented. The absolute dose at the focal point was measured in a 16-cm-diam acrylic phantom using a small volume chamber, which was calibrated at the University of Wisconsin Accredited Dosimetry Calibration Laboratory (UW-ADCL). The dose in acrylic was then converted to a dose in water. A collimator output factor from each of the four different collimator sizes ranging from 4, 8, 14, and 18 mm was measured with (1) a smaller volume chamber and (2) approximately 3.0 mm x 3.0 mm x 1.0 mm TLD chips in the same acrylic phantom. The Gafchromic films were used for the dose profile, collimator output factor, and mechanical/radiation field isocentricity measurements. The TLD chips were processed in-house whereas Gafchromic films were processed both at the UW-ADCL and in-house. The timer error, timer accuracy, and timer linearity were also determined. The dose profiles were found to be similar between RGSc and RGSu. The 4 mm collimator output factor of the RGSu was approximately 0.6, similar to that from RGSc, in comparison to 0.8 in the report for a Leksell Model U Gamma-Knife. The mechanical/radiation field isocentricity for RGSc and RGSu is found to be similar and is within 0.3 mm in both X and Y directions. In the Z direction, the beam center of the RGSu is shifted toward the sources by 0.75 mm from the mechanical isocenter whereas no data are available for RGSc. Little dosimetric difference is found between RGSu and RGSc. It is reported that RGSc has the same dosimetric and mechanical characteristics as the Model U Gamma-Knife. Therefore, RGSu should be capable of achieving stereotactic radiosurgery with the same degree of dosimetric and mechanical accuracy as with the Gamma-Knife.

California↗
PubMed2000

Determination of overall perturbation factors for plane-parallel ionization chambers in electron beams.

Most dosimetry protocols recommend the use of plane-parallel chambers for dose determination in electron beams with energies below 10 MeV. The new IAEA TRS 381 (1997) protocol includes the overall perturbation factor p(Q) that consists of the in-scattering correction factor p(cav) (or P(repl)) and the wall correction factor p(wall) (or P(wall)). In this work, p(Q) for the commonly applied NACP, PTW/Roos and PTW/Markus plane-parallel chambers was determined experimentally. For the NACP plane-parallel chamber, p(Q) was obtained by comparison with a cylindrical Farmer chamber, while for the PTW/Roos and PTW/Markus chambers it was obtained by comparison with the NACP chamber. The values of p(Q) for these plane-parallel chambers were measured as a function of mean electron energies E(z) from 1.7 MeV to 11.5 MeV. It was found that for the NACP and PTW/Roos chambers, p(Q) is independent of energy down to E(z) =1.7 MeV, while for the PTW/Markus chamber it shows a systematic and exponential drop of about 2% with decreasing energy down to E(z) = 2.7 MeV. However, the decrease of p(Q) for E(z) =1.7 MeV was not exponential.

Journal Article↗