SU-E-T-105: The LET Dependence of Liquid Ionization Chambers (LICs) in High-LET Beams

LICs are novel detectors for radiotherapy: the higher density of the medium allows to build them with a smaller sensitive volume, making them appealing in particle therapy. With RBE varying along the depth dose curve (DDC) and with the rising interest in dose/LET-painting, verifying the LET is becoming more important. Nevertheless, while the LET distributions for different ionizing particles have been calculated, they have never been directly measured in realistic therapeutic beams. Our interest in LICs is based on the characterization of the beam quality in terms of LET. It has been shown in earlier works that the extrapolation of the linear portion of the voltage curve yields an intercept with the x-axis that depends on LET. The quantitative establishment of this method, however, depends on how accurately recombination effects are taken into account.

Due to the higher density of charge carriers produced in the liquid, LICs have high recombination effects: general recombination effects, involving pairs belonging to different tracks (dose rate dependent), and initial recombination between ion-electron pairs belonging to the same incident particle event (LET dependent). To perform this study we propose a two-dimensional array of LICs, composed by a 16×8 matrix of 2×2 mm² pixels, which gives a fine spatial resolution on the plane.

Voltage curves have been measured for proton, carbon and oxygen beams available at the HIT facility in Heidelberg for different energies and dose rates. After correcting the curves for general recombination losses using the Three Voltage Method, we have indications of dose rate independence, indicating successful correction.

Further investigations are foreseen to quantify the LET dependence along the DDC, where different LET values are expected. A comparison with simulated dose averaged LET values will give quantitative information about 2D LET distributions for different beam species.