Microdosimetric characteristics and effective RBE value evaluation of the beam of Taiwan's first heavy ion therapy facility

The effectiveness of heavy ion therapy has attracted global attention. Taiwan's first heavy ion therapy center, which has started clinical treatments in May 2023, is the 14th operational heavy ion therapy center in the world. This study used a tissue-equivalent proportional counter (TEPC) to assess the microdosimetric parameters of three different energy beams at the heavy ion facility, in order to evaluate representative beam quality indicators such as the relative RBE (Radiation Biological Effectiveness) of the heavy ion radiation field. The maximum absorbed dose for the three energy beams were found to occur at the d_max (Bragg peak) position. After the d_max, secondary particles continue to deposit energy at deeper depths, but clinical treatment planning systems may overlook the dose contribution from these secondary particles. The effective RBE values of all three energy beams at their respective d_max positions were found to be not necessarily the highest. The effective RBE values for the evaluated heavy ion therapy facility ranged from approximately 3.1 to 3.4. The evaluation methods and related results of this study can serve as a reference for dose assessment in clinical heavy ion therapy centers.