Correction of stopping power and LET quenching for radiophotoluminescent glass dosimetry in a therapeutic proton beam

Dosimetric Characteristics of Radiophotoluminescent Glass Dosimeters for Proton Beams

Purpose

The purpose of the study was to investigate the dosimetric characteristics of radiophotoluminescent glass dosimeters (RGDs) for pencil beam scanning proton therapy. The RGD's end-to-end testing of intensity-modulated proton therapy (IMPT) plans was also evaluated.

Materials and Methods

The dosimetric characteristics of the GD-302M type glass dosimeter were studied in terms of uniformity, short-term and long-term reproducibility, stability of the magazine position readout, dose linearity in the range from 0.2 to 20 Gy, energy response in 70-220 MeV, and fading effect. The reference conditions of the spot scanning beam from the Varian ProBeam Compact system were operation at 160 MeV, a 2 cm water-equivalent depth in a solid water phantom, a 10 cm × 10 cm field size at the isocenter, and 2 Gy dose delivery. End-to-end testing of IMPT plans for the head, abdomen, and pelvis was verified using the Alderson Rando phantom. The overall uncertainty analysis was confirmed in this study.

Results

The relative response of RGDs for the uniformity test was within 0.95-1.05. The percentages of the coefficients of variation for short-term and long-term reproducibility were 1.16% and 1.50%, respectively. The dose ACE glass dosimetry reader FGD-1000 showed a stable magazine position readout. The dose was found to be linear with R2 = 0.9988. The energy response relative to 160 MeV was approximately within 4.0%. The fading effect was within 2.4%. For the end-to-end test, the difference between the treatment plan and RGD measurement was within 1.0%. The overall uncertainty of the RGD measurement for the proton beam was 4.6%, which covered all energy ranges in this study.

Conclusion

The experimental study indicates that the RGDs have the potential to be used in the dosimetry of therapeutic proton beams, including end-to-end dosimetry.

Evaluation of radiophotoluminescent glass dosimeter response for therapeutic spot scanning proton beam: suggestion of linear energy transfer-based correction

A radiophotoluminescent glass dosimeter (RGD) is used for a postal audit of a photon beam because of its various excellent characteristics. However, it has not been used for scanning proton beams because its response characteristics have not been verified. In this study, the response of RGD to scanning protons was investigated to develop a dosimetry protocol using the linear energy transfer (LET)‐based correction factor. The responses of RGD to four maximum‐range‐energy‐pattern proton beams were verified by comparing it with ionization chamber (IC) dosimetry. The LET at each measurement depth was calculated via Monte Carlo (MC) simulation. The LET correction factor (kLETRGD) was the ratio between the uncorrected RGD dose (DrawRGD) and the IC dose at each measurement depth. kLETRGD can be represented as a function of LET using the following equation: kLETRGDLET=‐0.035LET+1.090. DrawRGD showed a linear under‐response with increasing LET, and the maximum dose difference between the IC dose and DrawRGD was 15.2% at an LET of 6.07 keV/μm. The LET‐based correction dose (DLETRGD) conformed within 3.6% of the IC dose. The mean dose difference (±SD) of DrawRGD and DLETRGD was –2.5 ± 6.9% and 0.0 ± 1.6%, respectively. To achieve accurate dose verification for scanning proton beams using RGD, we derived a linear regression equation based on LET. The results show that with appropriate LET correction, RGD can be used for dose verification of scanning proton beams.

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

PURPOSE

To investigate the feasibility of using the high Z_eff storage phosphor material BaFBrI:Eu²⁺ in conjunction with the low Z_eff storage phosphor material KCl:Eu²⁺ for simultaneous proton dose and linear energy transfer (LET) measurements by (a) measuring the fundamental optical and dosimetric properties of BaFBrI:Eu²⁺ , (b) evaluating its compatibility in being readout simultaneously with KCl:Eu²⁺ dosimeters, and (c) modeling and validating its LET dependence under elevated proton LET irradiation.

METHODS

A commercial BaFBrI:Eu²⁺ storage phosphor detector (Model ST-VI, Fujifilm) was characterized with energy dispersive x-ray spectroscopy (EDS) analysis to obtain its elemental composition. The dosimeters were irradiated using both a Mevion S250 proton therapy unit (at the center of a spread-out Bragg peak, SOBP) and a Varian Clinac iX linear accelerator with the latter being a low LET irradiation. The photostimulated luminescence (PSL) emission spectra, excitation spectra, and luminescent lifetimes of the detector were measured after proton and photon irradiations. Dosimetric properties including dose linearity, dose rate dependence, radiation hardness, temporal, and readout stabilities were studied using a laboratory optical reader after proton irradiations. In addition, its proton energy dependence was analytically modeled and experimentally validated by irradiating the detectors at various depths within the SOBP (Range: 15.0 g/cm² , Modulation: 10.0 g/cm² ).

RESULTS

The active detector composition for the high Z_eff storage phosphor detector was found to be BaFBr_0.85 I_0.15 :Eu²⁺ . The BaFBr_0.85 I_0.15 :Eu²⁺ material's excitation and emission spectra were in agreement under proton and photon irradiations, with peaks of 586 ± 1 nm and 400 ± 1 nm, respectively, with a full width at half maximum (FWHM) of 119 ± 3 nm and 30 ± 2 nm, respectively. As dosimeter response under photon irradiation is generally believed to be free from LET effect, these results suggest LET independence of charge storage center types resulted from ionizing radiations. There is sufficient spectral overlaps with KCl:Eu²⁺ dosimeters allowing both dosimeters to be readout under equivalent readout conditions, that is, 594 nm stimulation and 420 nm detection wavelengths. Its PSL characteristic lifetime was found to be less than 5 microseconds which would make it suitable for fast 2D readout post irradiation. Its 420 nm emission band intensity was found to be linear up to 10 Gy absolute proton dose under the same irradiation conditions, dose rate independent, stable in time and under multiple readouts, and with high radiation hardness under cumulative proton dose histories up to 200 Gy as tested in this study. BaFBr_0.85 I_0.15 :Eu²⁺ showed significant proton energy-dependent dose under-response in regions of high LET which could be modeled by stopping power ratio calculations with an accuracy of 3% in low LET regions and a distance-to-agreement (DTA) of 1 mm in high LET regions (>5 keV/μm).

CONCLUSION

We have proven the feasibility of dual-storage phosphor proton dosimetry for simultaneous proton dose and LET measurements. BaFBr_0.85 I_0.15 :Eu²⁺ has shown equally excellent dosimetry performance as its low Z_eff complement KCl:Eu²⁺ with distinctive LET dependence merely as a result of its higher Z_eff . These promising results pave the way for future studies involving simultaneous proton dose and LET measurements using this novel approach.

Dosimetric response of a glass dosimeter in proton beams: LET-dependence and correction factor

A radiophotoluminescent glass dosimeter (RGD) is widely used in postal audit system for photon beams in Japan. However, proton dosimetry in RGDs is scarcely used owing to a lack of clarity in their response to beam quality. In this study, we investigated RGD response to beam quality for establishing a suitable linear energy transfer (LET)-corrected dosimetry protocol in a therapeutic proton beam. The RGD response was compared with ionization chamber measurement for a 100-225 MeV passive proton beam. LET of the measurement points was calculated by the Monte Carlo method. An LET-correction factor, defined as a ratio between the non-corrected RGD dose and ionization chamber dose, of 1.226×(LET)^-0.171 was derived for the RGD response. The magnitude of the LET-dependence of RGD increased with LET; for an LET of 8.2 keV/μm, the RGD under-response was up to 16%. The coefficient of determination, mean difference ± SD of non-corrected RGD dose, residual range-corrected RGD dose, and LET-corrected RGD dose to the ionization chamber are 0.923, 3.7 ± 4.2%, -2.4 ± 7.5%, and 0.04 ± 2.1%, respectively. The LET-corrected RGD dose was within 5% of the corresponding ionization chamber dose at all energies until 200 MeV, where it was 5.3% lower than the ionization chamber dose. A corrected LET-dependence of RGD using a correction factor based on a power function of LET and precise dosimetric verification close to the maximum LET were realized here. We further confirmed establishment of an accurate postal audit under various irradiation conditions.