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 ( k LET RGD ) was the ratio between the uncorrected RGD dose ( D raw RGD ) and the IC dose at each measurement depth. k LET RGD can be represented as a function of LET using the following equation: k LET RGD LET = - 0.035 LET + 1.090 . D raw RGD showed a linear under-response with increasing LET, and the maximum dose difference between the IC dose and D raw RGD was 15.2% at an LET of 6.07 keV/µm. The LET-based correction dose ( D LET RGD ) conformed within 3.6% of the IC dose. The mean dose difference (±SD) of D raw RGD and D LET RGD 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.

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.

[Analysis of photonuclear activation effect of high-energy medical linear accelerators--measurement of spatial dose distribution and temporal decay curve, and assessment of personnel dosimetry].

The characteristics of activation after high-energy X-rays have been generated by medical linear accelerators were measured using an ionization chamber. Radiation doses increased with rising X-ray energy, based on 10 MV, 15 MV, and 18 MVX-ray measurements. When the total irradiation dose was changed, radiation dose increased with total irradiation dose. When the collimator opened, the radiation dose at a position 15 cm from the isocenter reached about the maximum, which was 2.2 times the dose at the isocenter. The radiation dose became about 0.3 times its level at a position 40 cm from the isocenter, in the outer irradiation field. The dose distribution in the treatment room became almost the same dose extending from the isocenter to 200 cm. Radiation dose decreased gradually while moving away from the target on the treatment beam axis. But it increased again as it approached the floor face. The occupational exposure dose, which was presumed from measurements of the radiation dose 50 cm from the isocenter, was about 0.9 mSv during a year, assuming 600 MU for 1 person, 8 people a day, and 245 days a year. Radiation dose changed with X-ray energy in the machine used, and it was a geometrical constituent in the treatment room. It is important to understand the characteristics of radiation generated by medical linear accelerators.