Design of bi-material bolus for carbon ion therapy using dose optimization method

Material and Methods: The biomaterial bolus consists of Cycowood and brass. Since the probability of nuclear fragmentation is different between low-Z and high-Z materials with the same water equivalent length, the shape of Bragg curves depends on the material through which carbon ions pass. Then we have developed an accurate dose calculation model taking into account the effect by using an interpolation method based on measured data. We used a dose optimization technique to design the optimized shape of the bi-material bolus. To confirm the effectiveness of the design method, we designed and manufactured an optimized bolus (Fig. 1) using our method and another conventionally designed bolus for comparison for a target model simulating a para-spinal tumor (Fig.2). We measured dose distribution in water of protons passing through the beam delivery devices (ridge filter with the ripple filter) of HIMAC 290 MeV/u carbon beam line and either of the two boluses using a 96-ch parallel plate ionization chamber.