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

OBJECTIVE: This work investigates the use of passive luminescence detectors to determine different types of averaged linear energy transfer (\overline{LET}) for the energies relevant to proton therapy. The experimental results are compared to reference values obtained from Monte Carlo simulations. APPROACH: Optically stimulated luminescence detectors (OSLDs), fluorescent nuclear track detectors (FNTDs), and two different groups of thermoluminescence detectors (TLDs) were irradiated at four different radiation qualities. For each irradiation, the fluence- (\overline{LET}f) and dose-averaged LET (\overline{LET}d) were determined. For both quantities, two sub-types of averages were calculated, either considering contributions from primary and secondary protons or from all protons and heavier charged particles. Both simulated and experimental data were used in combination with a phenomenological model to estimate the relative biological effectiveness (RBE). MAIN RESULTS: All types of \overline{LET} could be assessed with the detectors. The experimental determination of \overline{LET}fis in agreement with reference data obtained from simulations across all measurement techniques and types of averaging. On the other hand, \overline{LET}dcan present challenges as a radiation quality metric to describe the detector response in mixed particle fields. However, excluding secondaries heavier than protons from the \overline{LET}dcalculation, as their contribution to the luminescence is suppressed by ionization quenching, leads to equal accuracy between \overline{LET}fand \overline{LET}d. Assessment of RBE through the experimentally determined \overline{LET}dvalues agrees with independently acquired reference values, indicating that the investigated detectors can determine \overline{LET} with sufficient accuracy for proton therapy. SIGNIFICANCE: OSLDs, TLDs, and FNTDs can be used to determine \overline{LET} and RBE in proton therapy. With the capability to determine dose through ionization quenching corrections derived from \overline{LET}, OSLDs and TLDs can simultaneously ascertain dose, \overline{LET}, and RBE. This makes passive detectors appealing for measurements in phantoms, facilitating the validation of clinical treatment plans or experiments related to proton therapy.

More May Not be Better: Enhanced Spacecraft Shielding May Exacerbate Cognitive Decrements by Increasing Pion Exposures during Deep Space Exploration.

The pervasiveness of deep space radiation remains a confounding factor for the transit of humans through our solar system. Spacecraft shielding both protects astronauts but also contributes to absorbed dose through galactic cosmic ray interactions that produce secondary particles. The resultant biological effects drop to a minimum for aluminum shielding around 20 g/cm2 but increase with additional shielding. The present work evaluates for the first time, the impact of secondary pions on central nervous system functionality. The fractional pion dose emanating from thicker shielded spacecraft regions could contribute up to 10% of the total absorbed radiation dose. New results from the Paul Scherrer Institute have revealed that low dose exposures to 150 MeV positive and negative pions, akin to a Mars mission, result in significant, long-lasting cognitive impairments. These surprising findings emphasize the need to carefully evaluate shielding configurations to optimize safe exposure limits for astronauts during deep space travel.

A Monte Carlo study on the secondary neutron generation by oxygen ion beams for radiotherapy and its comparison to lighter ions.

Objective.To study the secondary neutrons generated by primary oxygen beams for cancer treatment and compare the results to those from primary protons, helium, and carbon ions. This information can provide useful insight into the positioning of neutron detectors in phantom for future experimental dose assessments.Approach.Mono-energetic oxygen beams and spread-out Bragg peaks were simulated using the Monte Carlo particle transport codesFLUktuierende KAskade, tool for particle simulation, and Monte Carlo N-Particle, with energies within the therapeutic range. The energy and angular distribution of the secondary neutrons were quantified.Main results.The secondary neutron spectra generated by primary oxygen beams present the same qualitative trend as for other primary ions. The energy distributions resemble continuous spectra with one peak in the thermal/epithermal region, and one other peak in the fast/relativistic region, with the most probable energy ranging from 94 up to 277 MeV and maximum energies exceeding 500 MeV. The angular distribution of the secondary neutrons is mainly downstream-directed for the fast/relativistic energies, whereas the thermal/epithermal neutrons present a more isotropic propagation. When comparing the four different primary ions, there is a significant increase in the most probable energy as well as the number of secondary neutrons per primary particle when increasing the mass of the primaries.Significance.Most previous studies have only presented results of secondary neutrons generated by primary proton beams. In this work, secondary neutrons generated by primary oxygen beams are presented, and the obtained energy and angular spectra are added as supplementary material. Furthermore, a comparison of the secondary neutron generation by the different primary ions is given, which can be used as the starting point for future studies on treatment plan comparison and secondary neutron dose optimisation. The distal penumbra after the maximum dose deposition appears to be a suitable location for in-phantom dose assessments.

Personal dosimetry at the Paul Scherrer Institute.

The Paul Scherrer Institute (PSI) is the largest research institute for natural and engineering sciences in Switzerland. PSI develops, builds and operates complex large research facilities. Every year, >2400 scientists from Switzerland and around the world come to PSI to use the facilities and to carry out experiments. Many areas at PSI are radiation protection areas. Depending on the radiation protection area, the work carried out and the time the users spend in these areas, they have to carry a personal dosemeter. PSI runs an individual monitoring service in compliance with the Swiss legislation on radiological protection and approved by the Swiss Federal Nuclear Safety Inspectorate. The service provides about 35 000 dosemeters per year for the internal and external customers consisting of whole-body dosemeters for photons and neutrons as well as extremity dosemeters. This paper gives an overview on the employed personal dosimetry techniques by the individual monitoring service of PSI, the number of distributed dosemeters for internal and external customers and statistics about the measured doses at PSI over 30 years.

The EURADOS CR-39 Quality task for the optimisation and harmonisation of personal neutron dosimetry with CR-39 (PADC).

CR-39 (PADC) nuclear track detectors are among the most widespread devices used for personal neutron dosimetry; however, some issues related to the variable material quality of the CR-39 polymer hinder the performance of CR-39-based dosemeters. For this reason, the Working Group 2 (WG2) of the European Radiation Dosimetry Group (EURADOS) has recently launched the CR-39 Quality task, a project aimed at improving and harmonising personal neutron dosimetry with CR-39 in Europe. Whitin this task, a close collaboration among researchers, individual monitoring services and dosemeter grade CR-39 manufacturers is achieved, thus facilitating the direct dialog between producer and consumer to reach an optimised material for personal neutron dosimetry applications.

Optically stimulated luminescence dosimeters for simultaneous measurement of point dose and dose-weighted LET in an adaptive proton therapy workflow.

Purpose: To demonstrate the suitability of optically stimulated luminescence detectors (OSLDs) for accurate simultaneous measurement of the absolute point dose and dose-weighted linear energy transfer (LETD) in an anthropomorphic phantom for experimental validation of daily adaptive proton therapy. Methods: A clinically realistic intensity-modulated proton therapy (IMPT) treatment plan was created based on a CT of an anthropomorphic head-and-neck phantom made of tissue-equivalent material. The IMPT plan was optimized with three fields to deliver a uniform dose to the target volume covering the OSLDs. Different scenarios representing inter-fractional anatomical changes were created by modifying the phantom. An online adaptive proton therapy workflow was used to recover the daily dose distribution and account for the applied geometry changes. To validate the adaptive workflow, measurements were performed by irradiating Al2O3:C OSLDs inside the phantom. In addition to the measurements, retrospective Monte Carlo simulations were performed to compare the absolute dose and dose-averaged LET (LETD) delivered to the OSLDs. Results: The online adaptive proton therapy workflow was shown to recover significant degradation in dose conformity resulting from large anatomical and positioning deviations from the reference plan. The Monte Carlo simulations were in close agreement with the OSLD measurements, with an average relative error of 1.4% for doses and 3.2% for LETD. The use of OSLDs for LET determination allowed for a correction for the ionization quenched response. Conclusion: The OSLDs appear to be an excellent detector for simultaneously assessing dose and LET distributions in proton irradiation of an anthropomorphic phantom. The OSLDs can be cut to almost any size and shape, making them ideal for in-phantom measurements to probe the radiation quality and dose in a predefined region of interest. Although we have presented the results obtained in the experimental validation of an adaptive proton therapy workflow, the same approach can be generalized and used for a variety of clinical innovations and workflow developments that require accurate assessment of point dose and/or average LET.

Assessment of secondary neutrons in particle therapy by Monte Carlo simulations.

Objective. The purpose of this study is to estimate the energy and angular distribution of secondary neutrons inside a phantom in hadron therapy, which will support decisions on detector choice and experimental setup design for in-phantom secondary neutron measurements.Approach. Dedicated Monte Carlo simulations were implemented, considering clinically relevant energies of protons, helium and carbon ions. Since scored quantities can vary from different radiation transport models, the codes FLUKA, TOPAS and MCNP were used. The geometry of an active scanning beam delivery system for heavy ion treatment was implemented, and simulations of pristine and spread-out Bragg peaks were carried out. Previous studies, focused on specific ion types or single energies, are qualitatively in agreement with the obtained results.Main results. The secondary neutrons energy distributions present a continuous spectrum with two peaks, one centred on the thermal/epithermal region, and one on the high-energy region, with the most probable energy ranging from 19 up to 240 MeV, depending on the ion type and its initial energy. The simulations show that the secondary neutron energies may exceed 400 MeV and, therefore, suitable neutron detectors for this energy range shall be needed. Additionally, the angular distribution of the low energy neutrons is quite isotropic, whereas the fast/relativistic neutrons are mainly scattered in the down-stream direction.Significance. It would be possible to minimize the influence of the heavy ions when measuring the neutron-generated recoil protons by selecting appropriate measurement positions within the phantom. Although there are discrepancies among the three Monte Carlo codes, the results agree qualitatively and in order of magnitude, being sufficient to support further investigations with the ultimate goal of mapping the secondary neutron doses both in- and out-of-field in hadrontherapy. The obtained secondary neutron spectra are available as supplementary material.

Triple channel analysis of Gafchromic EBT3 irradiated with clinical carbon-ion beams.

Self-developing radiochromic film is widely used in radiotherapy QA procedures. To compensate for typical film inhomogeneities, the triple channel analysis method is commonly used for photon-irradiated film. We investigated the applicability of this method for GafchromicTMEBT3 (Ashland) film irradiated with a clinically used carbon-ion beam. Calibration curves were taken from EBT3 film specimens irradiated with monoenergetic carbon-ion beams of different doses. Measurements of the lateral field shape and homogeneity were performed in the middle of a passively modulated spread-out Bragg peak and compared to simultaneous characterization by means of a 2D ionization chamber array. Additional measurements to investigate the applicability of EBT3 for quality assurance (QA) measurement in carbon-ion beams were performed. The triple-channel analysis reduced the relative standard deviation of the doses in a uniform carbon ion field by 30% (from 1.9% to 1.3%) and reduced the maximum deviation by almost a factor of 3 (from 28.6% to 9.8%), demonstrating the elimination of film artifacts. The corrected film signal showed considerably improved image quality and quantitative agreement with the ionization chamber data, thus providing a clear rationale for the usage of the triple channel analysis in carbon-beam QA.

Al2O3:C optically stimulated luminescence dosimeters (OSLDs) for ultra-high dose rate proton dosimetry.

The response of Al2O3:C optically stimulated luminescence detectors (OSLDs) was investigated in a 250 MeV pencil proton beam. The OSLD response was mapped for a wide range of average dose rates up to 9000 Gy s-1, corresponding to a ∼150 kGy s-1instantaneous dose rate in each pulse. Two setups for ultra-high dose rate (FLASH) experiments are presented, which enable OSLDs or biological samples to be irradiated in either water-filled vials or cylinders. The OSLDs were found to be dose rate independent for all dose rates, with an average deviation <1% relative to the nominal dose for average dose rates of (1-1000) Gy s-1when irradiated in the two setups. A third setup for irradiations in a 9000 Gy s-1pencil beam is presented, where OSLDs are distributed in a 3 × 4 grid. Calculations of the signal averaging of the beam over the OSLDs were in agreement with the measured response at 9000 Gy s-1. Furthermore, a new method was presented to extract the beam spot size of narrow pencil beams, which is in agreement within a standard deviation with results derived from radiochromic films. The Al2O3:C OSLDs were found applicable to support radiobiological experiments in proton beams at ultra-high dose rates.

THERMOLUMINESCENCE DOSIMETRY (TLD) IN MEDICINE: FIVE 'W'S AND ONE HOW.

Thermoluminescence dosimetry (TLD) has a long history of applications in medicine. However, despite its versatility and sensitivity its use is anecdotally diminishing, at least in part due to the complexity and work intensity of a quality TLD service. The present paper explores the role of TLD in medicine using a common inquiry methodology (5W1H) which systematically asks 'Who, What, When, Where, Why and How' to identify what role TLD could and should play in medical applications.

APPLICATIONS OF OPTICALLY STIMULATED LUMINESCENCE IN MEDICAL DOSIMETRY.

If the first decade of the new millennium saw the establishment of a more solid foundation for the use of the Optically Stimulated Luminescence (OSL) in medical dosimetry, the second decade saw the technique take root and become more widely used in clinical studies. Recent publications report not only characterization and feasibility studies of the OSL technique for various applications in radiotherapy and radiology, but also the practical use of OSL for postal audits, estimation of staff dose, in vivo dosimetry, dose verification and dose mapping studies. This review complements previous review papers and reports on the topic, providing a panorama of the new advances and applications in the last decade. Attention is also dedicated to potential future applications, such as LET dosimetry, 2D/3D dosimetry using OSL, dosimetry in magnetic resonance imaging-guided radiotherapy (MRIgRT) and dosimetry of extremely high dose rates (FLASH therapy).

AAPM TG 191: Clinical use of luminescent dosimeters: TLDs and OSLDs.

Thermoluminescent dosimeters (TLD) and optically stimulated luminescent dosimeters (OSLD) are practical, accurate, and precise tools for point dosimetry in medical physics applications. The charges of Task Group 191 were to detail the methodologies for practical and optimal luminescence dosimetry in a clinical setting. This includes: (a) to review the variety of TLD/OSLD materials available, including features and limitations of each; (b) to outline the optimal steps to achieve accurate and precise dosimetry with luminescent detectors and to evaluate the uncertainty induced when less rigorous procedures are used; (c) to develop consensus guidelines on the optimal use of luminescent dosimeters for clinical practice; and (d) to develop guidelines for special medically relevant uses of TLDs/OSLDs such as mixed photon/neutron field dosimetry, particle beam dosimetry, and skin dosimetry. While this report provides general guidelines for TLD and OSLD processes, the report provides specific details for TLD-100 and nanoDotTM dosimeters because of their prevalence in clinical practice.

MgO:Li,Ce,Sm as a high-sensitivity material for Optically Stimulated Luminescence dosimetry.

The goal of this work was to investigate the relevant dosimetric and luminescent properties of MgO:Li3%,Ce0.03%,Sm0.03%, a newly-developed, high sensitivity Optically Stimulated Luminescence (OSL) material of low effective atomic number (Zeff = 10.8) and potential interest for medical and personal dosimetry. We characterized the thermoluminescence (TL), OSL, radioluminescence (RL), and OSL emission spectrum of this new material and carried out a preliminary investigation on the OSL signal stability. MgO:Li,Ce,Sm has a main TL peak at ~180 °C (at a heating rate of 5 °C/s) associated with Ce(3+) and Sm(3+) emission. The results indicate that the infrared (870 nm) stimulated OSL from MgO:Li,Ce,Sm has suitable properties for dosimetry, including high sensitivity to ionizing radiation (20 times that of Al2O3:C, under the measurement conditions) and wide dynamic range (7 µGy-30 Gy). The OSL associated with Ce(3+) emission is correlated with a dominant, practically isolated peak at 180 °C. Fading of ~15% was observed in the first hour, probably due to shallow traps, followed by subsequent fading of 6-7% over the next 35 days. These properties, together with the characteristically fast luminescence from Ce(3+), make this material also a strong candidate for 2D OSL dose mapping.

Thermoluminescent microparticle thermal history sensors.

While there are innumerable devices that measure temperature, the nonvolatile measurement of thermal history is far more difficult, particularly for sensors embedded in extreme environments such as fires and explosions. In this review, an extensive analysis is given of one such technology: thermoluminescent microparticles. These are transparent dielectrics with a large distribution of trap states that can store charge carriers over very long periods of time. In their simplest form, the population of these traps is dictated by an Arrhenius expression, which is highly dependent on temperature. A particle with filled traps that is exposed to high temperatures over a short period of time will preferentially lose carriers in shallow traps. This depopulation leaves a signature on the particle luminescence, which can be used to determine the temperature and time of the thermal event. Particles are prepared-many months in advance of a test, if desired-by exposure to deep ultraviolet, X-ray, beta, or gamma radiation, which fills the traps with charge carriers. Luminescence can be extracted from one or more particles regardless of whether or not they are embedded in debris or other inert materials. Testing and analysis of the method is demonstrated using laboratory experiments with microheaters and high energy explosives in the field. It is shown that the thermoluminescent materials LiF:Mg,Ti, MgB4O7:Dy,Li, and CaSO4:Ce,Tb, among others, provide accurate measurements of temperature in the 200 to 500 °C range in a variety of high-explosive environments.

Time-resolved optically stimulated luminescence of Al2O3:C for ion beam therapy dosimetry.

The objective of this study was to characterize the time-resolved (TR) optically stimulated luminescence (OSL) from Al2O3:C detectors and investigate methodologies to improve the accuracy of these detectors in ion beam therapy dosimetry, addressing the reduction in relative response to high linear energy transfer (LET) particles common to solid-state detectors. Al2O3:C OSL detectors (OSLDs) were exposed to proton, (4)He, (12)C and (16)O beams in 22 particle/energy combinations and read using a custom-built TR-OSL reader. The OSL response rOSL, relative to (60)Co gamma dose to water, and the ratio between the UV and blue OSL emission bands of Al2O3:C (UV/blue ratio) were determined as a function of the LET. Monte-Carlo simulations with the multi-purpose interaction and transport code FLUKA were used to estimate the absorbed doses and particle energy spectra in the different irradiation conditions. The OSL responses rOSL varied from 0.980 (0.73 keV µm(-1)) to 0.288 (120.8 keV µm(-1)). The OSL UV/blue ratio varied by a factor of two in the investigated LET range, but the variation for (12)C beams was only 11%. OSLDs were also irradiated at different depths of carbon ion spread-out Bragg peaks (SOBPs), where it was shown that doses could be obtained with an accuracy of ± 2.0% at the entrance channel and within the SOBP using correction factors calculated based on the OSL responses obtained in this study. The UV/blue ratio did not allow accurate estimation of the dose-averaged LET for (12)C SOBPs, although the values obtained can be explained with the data obtained in this study and the additional information provided by the Monte-Carlo simulations. The results demonstrate that accurate OSLD dosimetry can be performed in ion beam therapy using appropriate corrections for the OSL response.

Pixel Bleeding Correction in Laser Scanning Luminescence Imaging Demonstrated Using Optically Stimulated Luminescence.

This paper describes and investigates the performance of an algorithm to correct for "pixel bleeding" caused by slow luminescence centers in laser scanning imaging (e.g., X-ray imaging using photostimulable phosphors and 2D dosimetry using optically stimulated luminescence). The algorithm is based on a deconvolution procedure that takes into account the lifetime of the slow luminescence center and is further constrained by the detection of fast and slow luminescence centers and combining rows scanned in opposite directions. The algorithm was tested using simulated data and demonstrated experimentally by applying it to image reconstruction of two types of Al2O3 X-ray detector films ( Al2O3:C and Al2O3 :C,Mg), whose use in 2D dosimetry in conjunction with laser-scanning readout has so far been prevented by slow luminescence centers (F-centers, 35 ms lifetime). We show that the algorithm allows the readout of Al2O3 film detectors 300-500 times faster than generally allowed considering the lifetime of the main luminescence centers. By relaxing the stringent requirements on the detector's luminescence lifetime, the algorithm opens the possibility of using new materials in 2D dosimetry as well as other laser scanning applications, such as X-ray imaging using storage phosphors and scanning confocal microscopy, although the effect of the noise introduced must be investigated for each specific application.

Survivability of immunoassay reagents exposed to the space radiation environment on board the ESA BIOPAN-6 platform as a prelude to performing immunoassays on Mars.

The Life Marker Chip (LMC) instrument is an immunoassay-based sensor that will attempt to detect signatures of life in the subsurface of Mars. The molecular reagents at the core of the LMC have no heritage of interplanetary mission use; therefore, the design of such an instrument must take into account a number of risk factors, including the radiation environment that will be encountered during a mission to Mars. To study the effects of space radiation on immunoassay reagents, primarily antibodies, a space study was performed on the European Space Agency's 2007 BIOPAN-6 low-Earth orbit (LEO) space exposure platform to complement a set of ground-based radiation studies. Two antibodies were used in the study, which were lyophilized and packaged in the intended LMC format and loaded into a custom-made sample holder unit that was mounted on the BIOPAN-6 platform. The BIOPAN mission went into LEO for 12 days, after which all samples were recovered and the antibody binding performance was measured via enzyme-linked immunosorbent assays (ELISA). The factors expected to affect antibody performance were the physical conditions of a space mission and the exposure to space conditions, primarily the radiation environment in LEO. Both antibodies survived inactivation by these factors, as concluded from the comparison between the flight samples and a number of shipping and storage controls. This work, in combination with the ground-based radiation tests on representative LMC antibodies, has helped to reduce the risk of using antibodies in a planetary exploration mission context.

Analytical modeling of relative luminescence efficiency of Al2O3:C optically stimulated luminescence detectors exposed to high-energy heavy charged particles.

The objective of this work is to test analytical models to calculate the luminescence efficiency of Al(2)O(3):C optically stimulated luminescence detectors (OSLDs) exposed to heavy charged particles with energies relevant to space dosimetry and particle therapy. We used the track structure model to obtain an analytical expression for the relative luminescence efficiency based on the average radial dose distribution produced by the heavy charged particle. We compared the relative luminescence efficiency calculated using seven different radial dose distribution models, including a modified model introduced in this work, with experimental data. The results obtained using the modified radial dose distribution function agreed within 20% with experimental data from Al(2)O(3):C OSLDs relative luminescence efficiency for particles with atomic number ranging from 1 to 54 and linear energy transfer in water from 0.2 up to 1368 keV µm(-1). In spite of the significant improvement over other radial dose distribution models, understanding of the underlying physical processes associated with these radial dose distribution models remain elusive and may represent a limitation of the track structure model.

Determination of multislice computed tomography dose index (CTDI) using optically stimulated luminescence technology.

PURPOSE: The extensive use of multislice computed tomography (MSCT) and the associated increase in patient dose calls for an accurate dose evaluation technique. Optically stimulated luminescence (OSL) dosimetry provides a potential solution to the arising concerns over patient dose. This study was intended to evaluate the feasibility and accuracy of OSL dosimeter systems in the diagnostic CT x-ray beam energy range. METHODS: MSCT dose profiles were measured by irradiating OSL strips placed inside the extended PMMA head and body phantoms at different scan conditions by varying kVp settings (100, 120, and 140 kVp) and collimated beam widths (5, 10, 20, and 40 mm). All scans in this study were performed using a GE Lightspeed VCT scanner in axial mode. The exposed strips were then read out using a custom-made OSL strip reader and corrected with field-specific conversion factors. Based on the corrected OSL dose profile, the CTDI(450-OSL) and CTDI(l00-OSL) were evaluated. CTDI(100-IC) was also obtained using a 100 mm long pencil ionization chamber for accuracy verification. CTDI(100-efficiency) can be further evaluated by calculating the ratio of CTDI(100-OSL) and CTDI(450-OSL), which was compared to results from previous studies as well. RESULTS: The OSL detectors were found to have good sensitivity and dose response over a wide range of diagnostic CT x-ray beam energy viz. the primary beam and the scatter tail section of the dose profile. The differences between CTDI100 values obtained using the OSL strips and those obtained with 100 mm long pencil ionization chamber were < +/- 5% for all scan conditions, indicating good accuracy of the OSL system. It was also found that the CTDI(100-efficiency) did not significantly change as the beam width increased and tube voltage changed. The average CTDI(100-efficiency) at the center of the head and body phantoms were 72.6% and 56.2%, respectively. The corresponding values for the periphery of the head and body phantoms were 85.0% and 81.7%. These results agreed very well with previous results from the literature using other detection techniques or Monte Carlo simulations. CONCLUSIONS: The LED-based OSL system can be an accurate alternative device for CT dose evaluations. CTDI100 measurement with the use of a 100 mm pencil ionization chamber substantially underestimates the CTDIinfinity value even with 5 mm collimated beam width. The established complete set of CTDI(100-efficiency) correction factors for various scan parameters allows for accurately estimating CTDIinfinity with the current use of pencil chamber and dose phantoms. Combined with the simple calibration, it gives this work great potential to be used not only in routine clinical quality assurance checks but also as a promising tool for patient organ dose assessment.

Determination of average LET of therapeutic proton beams using Al2O3:C optically stimulated luminescence (OSL) detectors.

In this work we present a methodology and proof of concept to experimentally determine average linear energy transfer (LET) of therapeutic proton beams using the optically stimulated luminescence (OSL) of small Al(2)O(3):C detectors. Our methodology is based on the fact that the shape of the OSL decay curve of Al(2)O(3):C detectors depends on the LET of the radiation field. Thus, one can use the shape of the OSL decay curves to establish an LET calibration curve, which in turn permits measurements of LET. We performed irradiations at the M D Anderson Cancer Center Proton Therapy Center, Houston (PTCH), with passive scattering beams. We determined the average LET of the passive scattering beams using a validated Monte Carlo model of the PTCH passive scattering nozzle and correlated them with the shape of the OSL decay curve to obtain an LET calibration curve. Using this calibration curve and OSL measurements, we determined the averaged LET at various water-equivalent depths for therapeutic spread-out Bragg peaks and compared the results with averaged LETs determined using the Monte Carlo simulations. Agreement between measured and simulated fluence-averaged LET was within 24% for low energy spread-out Bragg peak (SOBP) fields and within 14% for high energy SOBP fields. Agreement between measured and simulated dose-averaged LET was within 12% for low energy SOBP fields and within 47% for high energy SOBP fields. The data presented in this work demonstrated the correlation between the OSL decay curve shapes and the average LET of the radiation fields, providing proof of concept of the feasibility of using OSL from Al(2)O(3):C detectors to measure average LET of therapeutic proton beams.