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

Abstract

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.

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

“…Because of the LET dependence of RGD, we observed a 15.2% under‐response compared with the IC dose. The LET dependence of the therapeutic proton beam was consistent with our previous study using a passive system 13 . We obtained from the linear fit between the RGD response and LET.…”

“…14 In our previous study, we established that LET changes depending on the irradiation conditions, specifically for low-energy proton beams, although R res is the same. 13 In this study, a scanning system indicated that the LET values varied by 10%-50% for the same R res . R res is a quality index primarily established for a passive system, and it is important to use LET for a spot scanning system.…”

“…Figure 4 shows the relationship between R res and LET obtained via MC simulation for proton beams with four maximum range energy patterns. The LET values for the 100–225 MeV passive proton beam have been reported to range from 10% to 30% for a similar R res 13 . In the scanning SOBP, the LET value suddenly increased for R res <20 mm and varied from 10% to 50% in the range of R res 40–80 mm.…”

“…(1) where Φ E (z) is the fluence of the proton beam with energy E at depth z, and LET (E) is the LET of the proton beam with energy E. Based on the MC calculations, we used the same parameters as in a previous report. 13 The physics lists G4mStandardPhysics_option4 and G4HadronPhysicsQGSP_BIC were used for simulating proton and nuclear interactions, respectively. The detection volume of the LET was 12 × 3 × 1.5 mm 3 , which corresponds to the actual RGD dimensions (diameter 1.5 mm, length 12 mm), although the lateral dimensions were expanded sufficiently to account for the uncertainty of MC calculation.…”

“…In our previous study, we used LET calculated through Monte Carlo (MC) simulation to correct the RGD response for the 100–225 MeV passive proton beam system 13 . A 15.7% dose reduction of RGD owing to LET was observed for passive proton beams, and the mean dose difference (±SD) was improved at 0.0 ±2.1% after LET‐based correction 13 . Previous studies have shown the response of RGD to proton beams with LET‐based corrections.…”