Field-size dependence of doses of therapeutic carbon beams

To estimate the physical dose at the center of spread-out Bragg peaks (SOBP) for various conditions of the irradiation system, a semi-empirical approach was applied. The dose at the center of the SOBP depends on the field size because of large-angle scattering particles in the water phantom. For a small field of 5x5 cm2, the dose was reduced to 99.2, 97.5, and 96.5 % of the dose used for the open field in the case of 290, 350, and 400 MeV/n carbon beams, respectively. Based on the three-Gaussian form of the lateral dose distributions of the carbon pencil beam, which has previously been shown to be effective for describing scattered carbon beams, we reconstructed the dose distributions of the SOBP beam. The reconstructed lateral dose distribution reproduced the measured lateral dose distributions very well. The field-size dependencies calculated using the reconstructed lateral dose distribution of the therapeutic carbon beam agreed with the measured dose dependency very well. The reconstructed beam was also used for irregularly shaped fields. The resultant dose distribution agreed with the measured dose distribution. The reconstructed beams were found to be applicable to the treatment-planning system

LET dependence of oxidative DNA base damage in 2-deoxyguanine aqueous solution irradiated with heavy ions

To evaluate the contribution of indirect action mediated by OH radicals in biological effects of high LET radiations, we examined the production of 8-hydroxy-2-deoxyguanosine (8-OHdG) in 2-deoxyguanosine aqueous solution using various ion species with an LET range from 20 to 300 keV/mm. The 8-OHdG yield decreased with increasing LET. In the hypoxic irradiation condition, the yield showed constant or rather increasing tendency above about 100 keV/mm, which is consistent with an oxygen-in-the-track hypothesis to explain the diminishment of oxygen effect.The 2nd Asia-Pacific Symposium on Radiation Chemistry (APSRC-2008

Corss-calibration of ionization chambers in proton and carbon beams

The calibration coefficinets of a parallel plate ionization chamber are examined by comparing the coefficients obtained through three methods: a calculation from a Co-60 calibration coefficient, N[D,w,Co-60], a cross-calibration of a parallel plate ionization chamber using a cylindrical ionization chamber at the plateau region of a mono-energetic beam and a cross-calibration of the chamber using a cylindrical chamber at the middle of the SOBP of the therapeutic beams, This paper also examines reference conditions for determining absorbed dose to water in the cases of therapeutic carbon and proton beams, In the dose calibration procedure recommended by IAEA, irradiation fields should be larger than 10 cm in diameter and the water phantom should extend by at least 5 cm beyond each side of the field. These recommendations are experimentally verified for proton and carbon beams. For proton beams, the calibration coefficients obtained by these three methods epproximately agreed. For carbon beams, the calibration coefficients obtained by the second method were about 1.0 % larger thanthose obtained by the third method, and the calibration coefficinets obtained by cross-calibration using 290 MeV/u beams were 0.5% lower than those obtained using 400 MeV/u beams. The calibration coefficinet obtained by the first method agreed roughly with the results obtained by SOBP beams

LET dependence of the formation of oxidative damage 8-hydroxy-2\u27-deoxyguanosine(8-OHdG)in 2\u27-deoxyguanosine aqueous solution irradiated with heavy ions

To evaluate the contribution of indirect action mediated by OH radicals in biological effects of high LET radiations, we examined the production of 8-hydroxy-20-deoxyguanosine (8-OHdG) in 20-deoxyguanosine aqueous solution using various ion species with an LET range from 20 to 300 keV/mm. The 8-OHdG yield decreased with increasing LET. In the hypoxic irradiation condition, the yield showed constant or rather increasing tendency above about 100 keV/mm, which is consistent with an oxygen-in-the-track hypothesis to explain the diminishment of oxygen effect

Comparative Study Between Glass Dosimeter and Thermoluminescent Dosimeter for Postal Dose Audit System

Thermoluminescent dosimeter (TLD) has been used for postal dose audit by IAEA, ESTRO and RPC. TLD is small enough to handle easily and have fine accuracy up to these days. However, the accuracy of radiotherapy has become more and more progressed and the requirement of quality assurance is increasing. We\u27d like to suggest glass dosimeter (GDR) for postal dose audit tool. GDR (Chiyoda technol Co.) is radiation-induced photoluminescence (RPL) detector. RPL origins are created by irradiation and these origins are excited by UV-ray results in orange RPL emission. This RPL emission is proportional to the amount of irradiation dose. The advantages of GDR compared with TLD are repeatable readout results in reduction of random error and little fading results in reduction of systematic error. Feasibility study using GRD were done at NIRS using linac (6MV and 10MV) and Co-60 machines. The standard deviation was about 1.2 %(N=180) which seems pretty well than conventional TLD method. We are going to confirm the accuracy of TLD using same measurement condition to make sure the GRD is the superior tool for postal dose audit.World Congress on Medical Physics and Biomedical Engineering 200

Positional Dependence of the CT number use of a Cone-beam CT Scanner for an Electron Density Phantom in Particle Beam Therapy

We evaluated the CT number accuracy in determining a CT calibration method for treatment planning by use of a 256 multi-slice CT (MSCT) scanner. An electron density phantom, which extends full length in the longitudinal direction, was scanned by the 256 MSCT scanner in a single rotation. We inserted four types of samples (air, 100 % ethanol, 40 wt% aqueous K2HPO4, and water) into the phantom. The regions of interest were set for all image slices, and the average CT number was calculated in the transverse and longitudinal sections. For the transverse image direction, the CT number became lower toward the center of the phantom with almost all samples. The causes of these phenomena might be attributed to the effects of scattered radiation and those of beam hardening due to the high-atomic-number material (40 wt% aqueous K2HPO4). However, the phenomena were too complicated to allow us to determine their causes only from the present studies. Meanwhile, for increasing the accuracy of the CT number calibration, a single sample should be inserted, and this step should be repeated with different samples. For the longitudinal direction, the CT number for a 40 wt % aqueous K2HPO4 sample increased by 30 HU from the anode to the cathode side due to variations in the X-ray energy caused by the heel effect. The caveat is that the CT number varies in the longitudinal direction when a CT scanner with a wide beam width is used

Dose contributions from large-angle scattered particles in therapeutic carbon beams

In carbon therapy, doses at center of spread-out Bragg peaks depend on field size. For a small field of 5*5 cm2, the central dose reduces to 96% of the central dose for the open field in case of 400 MeV/n carbon beam. Assuming the broad beam injected to the water phantom is made up of many pencil beams, the transverse dose distribution can be reconstructed by summing the dose distribution of the pencil beams. We estimated dose profiles of this pencil beam through measurements of dose distributions of broad uniform beams blocked half of the irradiation fields. The dose at a distance of a few cm from the edge of the irradiation field reaches up to a few percent of the central dose. From radiation quality measurements of this penumbra, the large-angle scattered particles were found to be secondary fragments which have lower LET than primary carbon beams. Carbon ions break up in beam modifying devices or in water phantom through nuclear interaction with target nuclei. The angular distributions of these fragmented nuclei are much broader than those of primary carbon particles. The transverse dose distribution of the pencil beam can be approximated by a function of the three-Gaussian form. For a simplest case of mono-energetic beam, contributions of the Gaussian components which have large mean deviations become larger as the depth in the water phantom increases. ©2007 American Association of Physicists in Medicin

Induced Radionuclides and Their Activity Concentration in Gel Dosimeters Irradiated by Carbon Ion Beam

Radioactivity was measured in a micellar gel dosimeter, a polymer gel dosimeter, and water was irradiated by carbon ion beams at various beam energy conditions. Monte Carlo simulation was also performed to estimate the radioactivity. Short-lived positron-emitting nuclides were observed immediately after irradiation, but they decayed rapidly into the background. At 24 h post-irradiation, the dominant measured radioactivity was of 7Be. The simulation also showed minor activity of 24Na and 3H; however, they were not experimentally observed. The measured radioactivity was independent of the type of gel dosimeter under all irradiation conditions, suggesting that the radioactivity was induced by the interaction of carbon ions with water (the main component of the gel dosimeters). The ratio between the simulated and measured radioactivity was within 0.9–1.5. The activity concentration of 7Be was found to be less than 1/10 of the value derived using the exemption concept proposed by the International Atomic Energy Agency. This result should be applicable to irradiated gel dosimeters containing mainly water and 0–4 wt.% C and 0–1.7 wt.% N

Induced Radionuclides and Their Activity Concentration in Gel Dosimeters Irradiated by Carbon Ion Beam

Radioactivity was measured in a micellar gel dosimeter, a polymer gel dosimeter, and water was irradiated by carbon ion beams at various beam energy conditions. Monte Carlo simulation was also performed to estimate the radioactivity. Short-lived positron-emitting nuclides were observed immediately after irradiation, but they decayed rapidly into the background. At 24 h post-irradiation, the dominant measured radioactivity was of 7Be. The simulation also showed minor activity of 24Na and 3H; however, they were not experimentally observed. The measured radioactivity was independent of the type of gel dosimeter under all irradiation conditions, suggesting that the radioactivity was induced by the interaction of carbon ions with water (the main component of the gel dosimeters). The ratio between the simulated and measured radioactivity was within 0.9–1.5. The activity concentration of 7Be was found to be less than 1/10 of the value derived using the exemption concept proposed by the International Atomic Energy Agency. This result should be applicable to irradiated gel dosimeters containing mainly water and 0–4 wt.% C and 0–1.7 wt.% N