Presentation Evaluation of the depth dose distribution using a graphite calorimeter and ionization chambers in 290 MeV/n carbon ion beam

坂間, 誠

In radiotherapy, the ionization chambers are usually used in the dosimetry and this method needs the beam quality correction factors to obtain the absorbed dose to water for used beam quality. These factors are composed of the stopping power ratios, the w values and the perturbation factors for the ionization chamber. The factors for the heavy-ion beams have large uncertainties and are assumed to be constant values for all conditions in the IAEA TRS 398 protocol. In this study, the depth dose distributions of carbon ion beam were measured by the developed graphite calorimeter and also by ionization chambers following the IAEA protocol. Comparisons of absorbed dose to graphite obtained by the graphite calorimeter and the ionization chambers are discussed in order to evaluate the depth dependence of the beam quality correction factors in the therapeutic carbon ion beam. In 290 MeV/n carbon ion beam, a cylindrical chamber (PTW type 30011, Farmer) and a plane-parallel chamber (PTW Advanced Markus) calibrated by the secondary standard dosimetry laboratory were used. The beam quality correction factors were obtained by the comparisons and the beam quality information from Monte Carlo simulation. The depth dose distributions in graphite obtained by the ionization chambers were agreed well without the Bragg peak region. The differences of the distributions obtained by the chambers and calorimeter changed slightly with the depth. The relative doses of the chambers were overestimated comparing with that of calorimeter. Therefore, the relative beam quality correction factors will become smaller increasing the depth before the Bragg peak. Relative dose measurements were performed using the calorimetry and ionization chamber dosimetry in the carbon ion beam. The calorimetry in Bragg peak region will be complicated due to the steep dose gradient, the influence of fragment particles and other correction factors. The difference of relative dose distributions between ionization chambers and the calorimeter is larger increasing the depth. The change of relative beam quality correction factors will be influenced by the w value in air in the heavy ions. More detailed information about depth dependence of the beam quality correction factors will be needed to sophisticate the dosimetry of carbon ion beam.
The 15th International Congress of Radiation Research (ICRR 2015)

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