Measurement of Lineal Energy Spectra at Different Penetration Depths Along a Radioactive 9C-Ion Beam

Abstract：

It is expected that a radioactive ⁹C-ion beam as a type of double irradiation source can increase the curative effect when applying it to heavy-ion cancer therapy,and it has been proved in our previous radiobiological experiment that the relative biological effectiveness (RBE) at the depths around the Bragg peak of a ⁹C beam was greater than that of a therapeutic ¹²C beam at least by a factor of 2. To characterize the physical basis of a ⁹C ion beam applying to radiotherapy, a spherical tissue-equivalent proportional counter (TEPC) was used to measure the lineal energy (y) spectra at different depths along a radioactive ⁹C-ion beam in this work. Based on the measured spectra, the distribution of dose-averaged lineal energy in depth was derived.Moreover, the measured lineal energy spectra were converted into the distributions of LET in absorbed dose at the different depths. The distribution of dose-averaged LET in depth, therefore, has been obtained for the ⁹C beam. The dose-averaged LETs measured with the TEPC and our preceding parellel-plate proportional counter (PPPC) were compared. It has been found that both the measurements agreed well with each other at the entrance channel of the ⁹C beam while the dose-averaged LETs by the TEPC were larger than those by the PPPC around the Bragg peak, i.e. the stopping region of the incident ⁹C ions. This coincides with the expectation that all of the delayed alpha particles and protons,which are emitted isotropically during the decays of the ⁹C ions, can be included by means of the TEPC measurement. The data sets obtained in this work provide a substantial physical basis for revealing the advantages of ⁹C beams in cancer therapy.