Real-time measurement of two-dimensional LET distributions of proton beams using scintillators

Objective. The linear energy transfer (LET) of proton therapy beams increases rapidly from the Bragg peak to the end of the beam. Although the LET can be determined using analytical or computational methods, a technique for efficiently measuring its spatial distribution has not yet been established. Thus, the purpose of this study is to develop a technique to measure the two-dimensional LET distribution in proton therapy in real time using a combination of multiple scintillators with different quenching. Approach. Inorganic and organic scintillator sheets were layered and irradiated with proton beams. Two-color signals of the CMOS sensor were obtained from the scintillation light and calibration curves were generated using LET. LET was calculated using Monte Carlo simulations as LETt and LETd weighted by fluence and dose, respectively. The accuracy of the calibration curve was evaluated by comparing the calculated and measured LET values for the 200 MeV monoenergetic and spread-out Bragg peak (SOBP) beams. LET distributions were obtained from the calibration curves. Main results. The deviation between the calculated and measured LET values was evaluated. For both LETt and LETd, the deviation in the plateau region of the monoenergetic and SOBP beams tended to be larger than those in the peak region. The deviation was smaller for LETd. In the obtained LETd distribution, the deviation between the calculated and measured values agreed within 3% in the peak region, while the deviation was larger in other regions. Significance. The LET distribution can be measured with a single irradiation using two scintillator sheets. This method may be effective for verifying LET in daily clinical practice and for quality control.