Simultaneous determination of the dose and linear energy transfer (LET) of carbon-ion beams using radiochromic films

Abstract

Radiochromic films are useful as dosimeters in high-precision radiotherapy owing to their high spatial resolution. However, when a particle beam is measured using a radiochromic film, the dose cannot be estimated accurately because the film darkness varies with variations in linear energy transfer (LET). This paper proposes a novel method for estimating the LET and the dose based on the film darkness. In this method, after a high-LET particle beam, such as a carbon-ion beam, was incident on the film, the film was digitized and its net optical density was determined. Further, the non-linearity of the film response curve between the dose and the darkness, depending on LET, was used. Then, calibration curves were created using 290 MeV u−1 mono energetic carbon-ion beams. We used LETs of 20, 50, 100, and 150 keV µm−1 and a physical dose of 2–14 Gy. The calibration curves were approximated for each LET using a quadratic function. The correlations between the coefficients of the quadratic function and the LET were also obtained. To verify the proposed method, the films were irradiated under 12 different conditions corresponding to various depths and doses. Four depths of −2, −5, −10, and −20 mm with respect to the Bragg peak, and three different preset values were used for the film measurements. The films were analyzed in four groups, where each group comprised films irradiated at the same depth. The LETs obtained from the film analysis, ordered from the upstream of the beam, were 20, 41, 56, and 97 keV µm−1, and the doses for the lowest preset value were 3.95, 4.07, 4.03, and 3.99 Gy for the four groups. The LETs obtained from the film analysis increased toward the Bragg peak, and the doses measured in the ionization chamber were almost equal to 4 Gy.

Dual‐storage phosphor proton therapy dosimetry: Simultaneous quantification of dose and linear energy transfer

“…Their applicability in describing physical effects such as the experimental under‐responses is a choice that seems to be detector dependent. For instance, Christensen et al, had shown that track‐averaged LET values were more accurate in predicting the magnitude of proton quenching of plastic scintillators 48 whereas other authors have shown that dose‐averaging was a more accurate measure for other dosimeters such as OSLDs 25 and radiochromic films 26,49 . As such, it is important for us to determine the more accurate averaging scheme (dose‐ or track‐averaging) that is more relevant to BaFBr 0.85 I 0.15 :Eu 2+ dosimeters.…”

“…For instance, Christensen et al, had shown that track-averaged LET values were more accurate in predicting the magnitude of proton quenching of plastic scintillators 48 whereas other authors have shown that dose-averaging was a more accurate measure for other dosimeters such as OSLDs 25 and radiochromic films. 26,49 As such, it is important for us to determine the more accurate averaging scheme (dose-or track-averaging) that is more relevant to BaFBr 0.85 I 0.15 :Eu 2+ dosimeters. From the experimental under-response values of BaFBr 0.85 I 0.15 :Eu 2+ , we attempted to restore the original SOBP curve by normalizing it with their expected under-response [Fig.…”

“…Proton LET is notoriously difficult to measure directly with only a very limited number of devices commercially available such as the tissue‐equivalent proportionality counter (TEPC), 19 silicon spectrometer, 20 or CR‐39 etch detectors 21 . Other authors have presented creative strategies for simultaneous proton dose and LET measurements such as the use of organic scintillators, 22 thermoluminescent dosimeters 23,24 (TLDs), Al 2 O 3 :C OSLD 25 detectors, and Gafchromic EBT3 films 26 . In this work, we present a novel solution to this problem by proposing the use of a proton energy dependent high Z eff storage phosphor (Z eff = 49, BaFBr 0.85 I 0.15 :Eu 2+ ) in conjunction with a near‐proton energy independent low Z eff storage phosphor (Z eff = 18, KCl:Eu 2+ ).…”

Improved simultaneous LET and dose measurements in proton therapy

“…The proton therapy community increasingly focuses on the relative biological effectiveness (RBE) and the effect of the linear energy transfer (LET) distributions in treatment plans 1 . Several detectors, such as semiconductors 2 , gas counters 3 , and radiochromic films 4 , have been proposed to measure the LET of hadrons at conventional dose-rates. Nevertheless, dosimetry in hadron beams remains challenging because of the typical under-response due to the ionization quenching of otherwise dose-rate independent detectors 5 – 7 .…”

Linear energy transfer-independent calibration of radiochromic film for carbon-ion beams

“…Radiochromic films are convenient tools widely used for QA measurements [1] . However, the film response of particle beams depends not only on the dose but also on the linear energy transfer (LET) [2] , [3] , [4] , [5] , [6] , [7] , [8] . Regarding the LET dependence known as LET quenching [9] , above 40% variations in film responses have been reported in the LET range of clinical carbon-ion beams [3] .…”