Clinical application of ionization density dependence of the glow curve for measuring linear energy transfer in therapeutic proton beams

Linear energy transfer (LET) is related to the relative biological effect caused by particle therapy. LET is a candidate parameter for optimizing proton therapy treatment plans by considering the biological effects. Therefore, being able to measure LET in clinical sites has become crucial. The purpose of this study was to develop a feasible LET measurement system for clinical therapeutic proton therapy by using the ionization density dependence of the heating-induced fluorescence (LiF):Mg,Ti glow curve in the high-temperature region. The integrated thermoluminescent (TL) intensity over the specified high-temperature region after exposure to a proton beam was divided by that after exposure to a photon beam to obtain the high-temperature ratio (HTR). The relationship between the HTR and LET was determined. TL dosimeters (TLDs) were irradiated with the same dose at several depths of a water phantom using three types of radiation: a⁶⁰Co gamma source at the National Measurement Laboratory of the Institute of Nuclear Energy Research, a 6-MV photon beam, and a 190-MeV proton beam at Chang Gung Memorial Hospital. The LET dependence on depth was simulated using a Monte Carlo code called TOPAS (version 3.0.1). The LET values ranged from 1 to 10 keV/μm in estimations using LiF:Mg,Ti for a proton beam. The LET measurement system was developed by comparing TL signals caused by proton beams with those caused by ⁶⁰Co gamma rays or 6-MV X-rays. A system incorporating Linac X-rays would be most convenient for the majority of hospitals. Therefore, TLDs can not only be dosimeters but also LET detectors in proton therapeutic sites.