Response of superheated bubble detector for therapeutic swift light ions

Elevating energy loss of swift charged particles toward range end shapes unique and favorable dose distribution for treating deep-seated tumors while sparing surrounding normal tissues. The benefit is further enhanced on heavy charged particles such as carbon by their less scatting and resultant shaper dose localization associated with increasing relative biological effectiveness (RBE). Modern biological models used in carbon ion radiotherapy consider microscopic energy distribution of incident ion to estimate its RBE. The accurate and precise modeling of the RBE is the requisite for the success of the ion beam therapy, and its biological confirmation is sometimes preferred especially at the commissioning of the facility itself or newly installed techniques even though the biological experiment is time-and cost consuming and easily suffered from inaccuracy and/or imprecision. The aim of this study is to investigate the response of a superheated bubble detector (SBD) for therapeutic light regarding their microscopic energy information in order to estimate the RBE. Local energy deposition by incident radiation stimulates bubble cores in superheated status distributed in the medium, then the cores expand to visible bubbles through phase transition. The transition is considered to be triggered by the amount of deposited energy up to micrometer scale.This evokes that the SBDs can be used as a microdosimetric detector. In this study, we produced SBD in house with butane gas in gelatin medium encapsulated in a cylinder piston, then investigated its response for 1H-160MeV, 12C-290MeV/n and 40Ar-500MeV/n beams at the HIMAC (Heavy Ion Medical Accelerator in Chiba) of NIRS, Japan. The number of produced bubbles were counted on X-ray CT images 3 dimensionally. The net increase of the bubbles showed good linearity on dose up to 100 mGy. The efficiency of the production of the bubbles in terms of dose was further enhanced by the increase of LET. This tendency is different from conventional radiation detectors such as scintillators or ionization chambers, in line with the general RBE response. The overall response was studied and its analogy to biological response was investigated by the RBE model.The 15th International Congress of Radiation Research (ICRR 2015