High Temperature Ratio Research Articles

Clinical application of ionization density dependence of the glow curve for measuring linear energy transfer in therapeutic proton beams

Linear energy transfer (LET) is related to the relative biological effect caused by particle therapy. LET is a candidate parameter for optimizing proton therapy treatment plans by considering the biological effects. Therefore, being able to measure LET in clinical sites has become crucial. The purpose of this study was to develop a feasible LET measurement system for clinical therapeutic proton therapy by using the ionization density dependence of the heating-induced fluorescence (LiF):Mg,Ti glow curve in the high-temperature region. The integrated thermoluminescent (TL) intensity over the specified high-temperature region after exposure to a proton beam was divided by that after exposure to a photon beam to obtain the high-temperature ratio (HTR). The relationship between the HTR and LET was determined. TL dosimeters (TLDs) were irradiated with the same dose at several depths of a water phantom using three types of radiation: a60Co gamma source at the National Measurement Laboratory of the Institute of Nuclear Energy Research, a 6-MV photon beam, and a 190-MeV proton beam at Chang Gung Memorial Hospital. The LET dependence on depth was simulated using a Monte Carlo code called TOPAS (version 3.0.1). The LET values ranged from 1 to 10 keV/μm in estimations using LiF:Mg,Ti for a proton beam. The LET measurement system was developed by comparing TL signals caused by proton beams with those caused by 60Co gamma rays or 6-MV X-rays. A system incorporating Linac X-rays would be most convenient for the majority of hospitals. Therefore, TLDs can not only be dosimeters but also LET detectors in proton therapeutic sites.

Absorbed dose estimation using LET dependence in glow curve of thermoluminescent phosphor Li3B7O12:Cu in therapeutic carbon beams

ABSTRACTA high temperature ratio (HTR) method has been proposed to correct the linear energy transfer (LET) dependence of thermoluminescent (TL) efficiency. To realize the use of the slab-type thermoluminescence detector (TLD) that based on the phosphor Li3B7O12:Cu for heavy charged particle beam, the HTR method has been considered. To improve the reproducibility of HTR, the slow heating rate method is introduced in this report and the coefficient variations of HTR decreased from 10%–20% to 8%. The relation between TL-efficiency, HTR, and LET for Li3B7O12:Cu was manifested and the TL-efficiency as a function of HTR was derived in an attempt to measure the absorbed dose without LET information. The feasibility of the HTR method in therapeutic carbon beams was evaluated by comparing the dose estimated by Li3B7O12:Cu and by an ionization chamber. The accuracy of dose estimation in carbon beams was improved by using the HTR method, but there is room for further improvement. The use of Li3B7O12:Cu in heavy charged particle beams can be materialized with further improvement of HTR sensitivity.

Dependence of the thermoluminescent high-temperature ratio (HTR) of LiF:Mg,Ti detectors on proton energy and dose

The high-temperature ratio (HTR) is a parameter quantifying changes of the shape of the high-temperature part of the LiF:Mg,Ti glow-curve after exposure to densely ionizing radiation. It was introduced in order to estimate the ‘effective LET’ of an unknown radiation field and to correct the decreased relative TL efficiency for high-Linear Energy Transfer (LET) radiation.In the present work the dependence of HTR on proton energy (14.5–58 MeV) and dose (0.5–30 Gy) was investigated. All measured HTR values were at the level of 1.2 or higher, therefore significantly different from the respective value for gamma-rays (HTR = 1), but HTR was found to be insensitive to changes of proton energy above 20 MeV. As a result the relationship between HTR and relative TL efficiency is not unequivocal. The HTR was found to be dependent on absorbed dose even for the lowest studied doses.

On the correctness of the thermoluminescent high-temperature ratio (HTR) method for estimating ionization density effects in mixed radiation fields

The high-temperature ratio (HTR) method which exploits changes in the LiF:Mg,Ti glow-curve due to high-LET radiation, has been used for several years to estimate LET in an unknown radiation field. As TL efficiency is known to decrease after doses of densely ionizing radiation, a LET estimate is used to correct the TLD-measured values of dose. The HTR method is purely empirical and its general correctness is questionable. The validity of the HTR method was investigated by theoretical simulation of various mixed radiation fields. The LET eff values estimated with the HTR method for mixed radiation fields were found in general to be incorrect, in some cases underestimating the true values of dose-averaged LET by an order of magnitude. The method produced correct estimates of average LET only in cases of almost mono-energetic fields (i.e. in non-mixed radiation conditions). The value of LET eff found by the HTR method may therefore be treated as a qualitative indicator of increased LET, but not as a quantitative estimator of average LET. However, HTR-based correction of the TLD-measured dose value (HTR-B method) was found to be quite reliable. In all cases studied, application of this technique improved the result. Most of the measured doses fell within 10% of the true values. A further empirical improvement to the method is proposed. One may therefore recommend the HTR-B method to correct for decreased TL efficiency in mixed high-LET fields.

Investigation of the ionisation density dependence of the glow curve characteristics of LIF:MG,TI (TLD-100).

The dependence of the shape of the glow curve of LiF:Mg,Ti (TLD-100) on ionisation density was investigated using irradiation with (90)Sr/(90)Y beta rays, 60 and 250 kVp X rays, various heavy-charged particles and 0.2 and 14 MeV neutrons. Special attention is focused on the properties of high-temperature thermoluminescence; specifically, the behaviour of the high-temperature ratio (HTR) of Peaks 7 and 8 as a function of batch and annealing protocol. The correlation of Peaks 7 and 8 with average linear-energy-transfer (LET) is also investigated. The HTR of Peak 7 is found to be independent of LET for values of LET approximately >30 keV microm(-1). The behaviour of the HTR of Peak 8 with LET is observed to be erratic, which suggests that applications using the HTR should separate the contributions of Peaks 7 and 8 using computerised glow curve deconvolution. The behaviour of the HTR following neutron irradiation is complex and not fully understood. The shape of composite Peak 5 is observed to be broader following high ionisation alpha particle irradiation, suggesting that the combined use of the HTR and the shape of Peak 5 could lead to improved ionisation density discrimination for particles of high LET.

Efficiency-corrected dose verification with thermoluminescence dosemeters in heavy-ion beams

One of the most essential difficulties in heavy-ion dosimetry by means of thermoluminescence dosemeters (TLDs)--often seen as a serious disadvantage of TLD utilisation--regards the changing TL-efficiency with increasing linear energy transfer (LET) of the particle. This behaviour leads to a falsification of absorbed dose that can be significant for many applications, e.g. in space or radiotherapeutic dosimetry. The high-temperature TL emission of LiF:Mg,Ti TL detectors can be exploited to obtain information about the LET of the heavy-ion radiation field under study. The high-temperature ratio (HTR) is used as a parameter to determine average LET. To correct the absorbed dose according to the TL-efficiency, the detailed dependence of HTR- and TL-efficiency on LET was recorded. These investigations were accomplished at the Heavy Ion Medical Accelerator (HIMAC) in Chiba, Japan, with a variety of high-energy ion beams (helium, carbon, neon, silicon and iron) ranging in LET from 2.2 to 393 keV microm(-1). The obtained relationships HTR vs. LET and TL-efficiency vs. LET were combined into a TL efficieny vs. HTR relationship. This enables correction of the absorbed dose (HTR-B method). The methodology is demonstrated by means of TLD 700 ((7)LiF:Mg,Ti) measurements in carbon beams of 290 and 400 MeV n(-1) available from HIMAC.

Austrian dose measurements onboard space station MIR and the International Space Station – overview and comparison

The Atominstitute of the Austrian Universities has conducted various space research missions in the last 12 years in cooperation with the Institute for Biomedical Problems in Moscow. They dealt with the exact determination of the radiation hazards for cosmonauts and the development of precise measurement devices. Special emphasis will be laid on the last experiment on space station MIR the goal of which was the determination of the depth distribution of absorbed dose and dose equivalent in a water filled Phantom. The first results from dose measurements onboard the International Space Station (ISS) will also be discussed. The spherical Phantom with a diameter of 35 cm was developed at the Institute for Biomedical Problems and had 4 channels where dosemeters can be exposed in different depths. The exposure period covered the timeframe from May 1997 to February 1999. Thermoluminescent dosemeters (TLDs) were exposed inside the Phantom, either parallel or perpendicular to the hull of the spacecraft. For the evaluation of the linear energy transfer (LET), the high temperature ratio (HTR) method was applied. Based on this method a mean quality factor and, subsequently, the dose equivalent is calculated according to the Q(LET ∞) relationship proposed in ICRP 26. An increased contribution of neutrons could be detected inside the Phantom. However the total dose equivalent did not increase over the depth of the Phantom. As the first Austrian measurements on the ISS dosemeter packages were exposed for 248 days, starting in February 2001 at six different locations onboard the ISS. The Austrian dosemeter sets for this first exposure on the ISS contained five different kinds of passive thermoluminescent dosemeters. First results showed a position dependent absorbed dose rate at the ISS.

On the use of LiF:Mg,Ti thermoluminescence dosemeters in space--a critical review.

The use of LiF:Mg,Ti thermoluminescence dosemeters (TLDs) in space radiation fields is reviewed. It is demonstrated in the context of modified track structure theory and microdosimetric track structure theory that there is no unique correlation between the relative thermoluminescence (TL) efficiency of heavy charged particles, neutrons of all energies and linear energy transfer (LET). Many experimental measurements dating back more than two decades also demonstrate the multivalued, non-universal, relationship between relative TL efficiency and LET. It is further demonstrated that the relative intensities of the dosimetric peaks and especially the high-temperature structure are dependent on a large number of variables, some controllable, some not. It is concluded that TL techniques employing the concept of LET (e.g. measurement of total dose, the high-temperature ratio (HTR) methods and other combinations of the relative TL efficiency of the various peaks used to estimate average Q or simulate Q-LET relationships) should be regarded as lacking a sound theoretical basis, highly prone to error and, as well, lack of reproducibility/universality due to the absence of a standardised experimental protocol essential to reliable experimental methodology.

Application of the high-temperature ratio method for evaluation of the depth distribution of dose equivalent in a water-filled phantom on board space station Mir.

A water-filled tissue equivalent phantom with a diameter of 35 cm was developed at the Institute for Biomedical Problems. Moscow. Russia. It contains four channels perpendicular to each other, where dosemeters can be exposed at different depths. Between May 1997 and February 1999 the phantom was installed at three different locations on board the Mir space station. Thermoluminescence dosemeters (TLDs) were exposed at various depths inside the phantom either parallel or perpendicular to the hull of the spacecraft. The high-temperature ratio (HTR) method was used for the evaluation of the TLDs. The method was developed at the Atominstitute of the Austrian Universities. Vienna, Austria, and has already been used for measurements in mixed radiation fields on earth and in space with great success. It uses the changes of peak height ratios in LiF:Mg,Ti glow curves in dependence on the linear energy transfer (LET), and therefore allows determination of an 'averaged' LET as well as measurement of the absorbed dose. A mean quality factor and, subsequently, the dose equivalent can be calculated according to the Q(LETinfinity) relationship proposed by the ICRP. The small size of the LiF dosemeters means that the HTR method can be used to determine the gradient of absorbed dose and dose equivalent inside the tissue equivalent body.

Correlation of the LET-dependent TL-response of LIF:Mg,Ti TL-dosemeters and gentoxic endpoints after proton irradiation

The high temperature ratio (HTR) method using LiF:Mg,Ti thermoluminescent dosemeters allows in addition to the measurement of absorbed dose, the evaluation of the biological effectiveness of radiation. In order to analyse the correlation of the measured parameter HTR and gentoxic endpoints irradiation of cell cultures and TLDs were performed in a 62 MeV proton beam. The gentoxic effects respectively the relative biological effectiveness RBE, show an excellent correlation with the HTR due to a similar dose and LET-dependent behaviour.

The LET Dependence of LiF:Mg,Ti Dosemeters and its Application for LET Measurements in Mixed Radiation Fields

Several types of standard TLDs were irradiated in various fields of high energy heavy ions and the TL emission in the high temperature region (peaks 6 and 7) was analysed in comparison with that of 60 Co radiation. The parameter determined is defined as the high temperature ratio (HTR). Irradiations were carried out in the JINR (Dubna) with fluoride ions in the energy range from 65 to 275 MeV.amu -1 and carbon ions with energies from 100 MeV.amu -1 up to 3650 MeV.amu -1 . Additionally, irradiations with protons (10 and 62 MeV) were performed. The results show the well known decrease of peak 5 efficiency with increasing LET of the absorbed radiation. The LET dependence of the parameter HTR and of the peak 5 efficiency in the range from 0.35 up to 180 keV.μm -1 are presented. Based on these calibrations, the HTR method allows the measurement of the average LET in mixed radiation fields and therefore the estimation of biologically relevant dose. Results from measurements on space station MIR are also presented.