Sensitivity correction of fluorescent nuclear track detectors using alpha particles: Determining LET spectra of light ions with enhanced accuracy

Abstract

Background: Radiation fields encountered in proton therapy (PT) and ionbeam therapy (IBT) are characterized by a variable linear energy transfer (LET), which lead to a variation of relative biological effectiveness and also affect the response of certain dosimeters. Therefore, reliable tools to measure LET are advantageous to predict and correct LET effects. Fluorescent nuclear track detectors (FNTDs) are suitable to measure LET spectra within the range of interest for PT and IBT, but so far the accuracy and precision have been challenged by sensitivity variations between individual crystals. Purpose: To develop a novel methodology to correct changes in the fluorescent intensity due to sensitivity variations among FNTDs. This methodology is based on exposing FNTDs to alpha particles in order to derive a detector-specific correction factor. This will allow us to improve the accuracy and precision of LET spectra measurements with FNTDs. Methods: FNTDs were exposed to alpha particles. Afterward, the detectors were irradiated to monoenergetic protons, 4 He-, 12 C-, and 16 O-ions. At each step, the detectors were imaged with a confocal laser scanning microscope. The tracks were reconstructed and analyzed using in-house developed tools. Alpha-particle tracks were used to derive a detector-specific sensitivity correction factor (k s,i ). Proton, 4 He-, 12 C-, and 16 O-ion tracks were used to establish a traceable calibration curve that relates the fluorescence intensity with the LET in water (LET H 2 O ). FNTDs from a second batch were exposed and analyzed following the same procedures, to test if k s,i can be used to extend the applicability of the calibration curve to detectors from different batches. Finally, a set of blind tests was performed to assess the accuracy of the proposed methodology without user bias. Throughout all stages, the main sources of uncertainty were evaluated.Results: Based on a sample of 100 FNTDs, our findings show a high sensitivity heterogeneity between FNTDs, with k s,i having values between 0.57 and 2.55. The fitting quality of the calibration curve, characterized by the mean absolute percentage residuals and correlation coefficient, was improved when k s,i was considered. Results for detectors from the second batch show that, if the fluorescence signal is corrected by k s,i , the differences in the predicted LET H 2 O with respect to the reference set are reduced from 55%, 141%, 41%, and 186% to 4.2%, 6.5%, 5.0%, and 11.0%, for protons, 4 He-, 12 C-, and 16 O-ions, respectively.

Towards precise LET measurements based on energy deposition of therapeutic ions in Timepix3 detectors

“…more elaborated conversion methods), which is not within the scope of this work. The LET Si values calculated in the present work are in line with those reported in Muñoz et al (2023), since both investigations were carried out at the HIT facility. Building upon the aforementioned studies, our study has demonstrated a similar and even superior performance when considering the entire obtained LET Si range of 0.77 keV µm −1 to 93.16 keV µm −1 of four ion types with an absolute accuracy range of 0.8% to 6.6%.…”

“…In addition, passive detectors such as FNTDs are also employed for experimental purposes. Recently, FNTDs were used to determine the LET water values of therapeutic ions (Muñoz et al 2023). When conducting a coarse conversion of these LET water values to LET Si (Rosenfeld 2016) with the basic goal of comparing the measured range of track-averaged LETs for the monochromatic ion beams (free in air) that are therapeutically available at HIT, a range of 0.87 keV µm −1 to 93.90 keV µm −1 is obtained with an accuracy range of 0.3%-17%.…”

Assessment of fluence- and dose-averaged linear energy transfer with passive luminescence detectors in clinical proton beams

“…The fluorescence intensity is related to the LET through a calibration curve, as shown in figure 3. The reference LET values were estimated analytically from the residual energy at the detector surface, as detailed in Muñoz et al (2023). Hence, the calibration curve does not represent an average quantity, as it is associated to single particles from mono-energetic beams.…”

“…Hence, the calibration curve does not represent an average quantity, as it is associated to single particles from mono-energetic beams. For consistency, the calibration curve used in this work was determined from data obtained after re-scanning detectors exposedto mono-energetic protons, 4 He-and 12 C-ions in a previous work (Muñoz et al 2023), but following the scanning protocol used in this work. From the fluorescence intensity, the LET in water of each track was calculated through the calibration curve.…”

“…This curve was used to calculate the LET of each reconstructed track. Markers show data obtained after exposure to protons, 4 He-, and 12 C-ions(Muñoz et al 2023).…”