Yumiko Ohno

Dose distribution of a 125 keV mean energy microplanar x‐ray beam for basic studies on microbeam radiotherapy

A multislit collimator was designed and fabricated for basic studies on microbeam radiation therapy (MRT) with an x-ray energy of about 100 keV. It consists of 30 slits that are 25 μm high, 30 mm wide, and 5 mm thick in the beam direction. The slits were made of 25 μm-thick polyimide sheets that were separated by 175 μm-thick tungsten sheets. The authors measured the dose distribution of a single microbeam with a mean energy of 125 keV by a scanning slit method using a phosphor coupled to a charge coupled device camera and found that the ratios of the dose at the center of a microbeam to that at midpositions to adjacent slits were 1050 and 760 for each side of the microbeam. This dose distribution was well reproduced by the Monte Carlo simulation code PHITS.

Measurement of electron density in dual-energy x-ray CT with monochromatic x rays and evaluation of its accuracy.

Information on electron density is important for radiotherapy treatment planning in order to optimize the dose distribution in the target volume of a patient. At present, the electron density is derived from a computed tomography (CT) number measured in x-ray CT scanning; however, there are uncertainties due to the beam hardening effect and the method by which the electron density is converted from the CT number. In order to measure the electron density with an accuracy of +/-1%, the authors have developed dual-energy x ray CT using monochromatic x rays. They experimentally proved that the measured linear attenuation coefficients were only a few percent lower than the theoretical ones, which led to an accuracy within 2% for the electron density. There were three factors causing inaccuracy in the linear attenuation coefficient and the electron density: the influence of scattered radiation, the nonlinearity in the detector response function, and a theoretical process to derive the electron density from the linear attenuation coefficients. The linear attenuation coefficients of water were experimentally proved to differ by 1%-2% from the theoretical one even when the scattering effect was negligible. The nonlinearity of the response function played an important role in correcting the difference in the linear attenuation coefficient. Furthermore, the theoretical process used for deriving the electron density from the linear attenuation coefficients introduces about 0.6% deviation from the theoretical value into the resultant electron density. This deviation occurs systematically so that it can be corrected. The authors measured the electron densities for seven samples equivalent to soft tissue in dual-energy x-ray CT, and finally obtained them with an accuracy of around +/-1%.

Dosimetry for a microbeam array generated by synchrotron radiation at SPring-8

A microbeam array was formed with a multi-slit collimator (MSC) for research on radiation therapy (MRT). Kodak EDR2 film was used to measure the dose distribution of the microbeam array. The calibration curve of optical density of the film with respect to a dose was established using a standard Farmer chamber and (60)Co gamma-ray source. The peak dose of 3.6 Gy/s at the maximum was derived from the film dosimetry using the calibration curve. The uncertainty was estimated to be 5% which was mainly attributed to the uncertainty of the calibration. It was found that the ionization chamber used for monitoring the dose during the MRT experiments gave lower dose by about 30% than the dose derived from the film dosimetry.

Gross and partial ionization cross sections in 6-MeV/amu bare-ion collisions with methane

Gross and partial ionization cross sections of CH4 molecules were measured in 6-MeV/amu H+, He2+, C6+, Ne10+ and Ar18+ ion impact. The gross ionization cross sections in light H+ and He2+ ion impact were found to be proportional to square of projectile charge q in accord with the first Born approximation. As the projectile charge was increased, the dependence became weaker than q2. From mass-spectroscopic measurements, the cross sections for CH4+ and CH3+ ion production were also found to obey the q2 dependence in low charge ion impact. In high charge ion impact, their dependence, however, became weak compared with q2 scaling. The fragmented ions, such as CH2+, CH+ and C+, showed charge dependence steeper than CH4+ and CH3+ ions. Multiply charged carbon ions up to C5+ were also observed.

Isomer effect on ionization processes in collisions of 6-MeV/amu bare ions with C3H6 molecules

Gross and partial ionization cross sections for C3H6 isomers, propene and cyclopropane, have been measured under impact of 6 MeV/amu fully stripped ions with charge state q = 1, 2, 6,10, and 18. The gross ionization cross sections have been found to depend weakly on the projectile charge q compared with the q2 dependence, especially for highly-charged projectiles. The cross sections for cyclopropane have been found to be slightly larger than those for propene in all projectiles studied. The most prominent product was parent C3H6+ ions in cyclopropane, whereas C3H5+ ion dominated C3H6+ ion in propene, indicating an isomer effect on fragmentation processes. Mechanisms relevant to the production of fragmented ions are also discussed.