Response of current dosemeters to new operational quantities in rotational geometry.

Recently, the ICRU released Report 95, where new operational quantities for external radiation exposure are defined. The new quantities are defined in close relation to the protection quantities. This change affects the practice use of dosemeters. That is why the instruments must be adapted to the measurement of new quantities before their implementing as legally binding. The discrepancies depend on radiation spectra-particle type, energy of particles and direction of incidence. To analyse the performance of currently used instruments, irradiations in photon and neutron fields of various energies were performed for personal and area dosemeters. In this work, the response of photon and neutron personal dosemeters in conditions of rotational geometry is presented. The difference between the responses with respect to the new or old operational quantity was not large, which corresponded to the similar reference values for this irradiation geometry. The mutual ratio depended on the specific radiation quality and geometry. The behaviour of different types of dosemeters varied too.

EURADOS project on the impact of the proposed ICRU operational dose quantities.

Following the publication of the joint The International Commissions on Radiation Units and Measurements (ICRU) and on Radiological Protection (ICRP) report on new operational quantities for radiation protection, the European Dosimetry Group (EURADOS) have carried out an initial evaluation. The EURADOS report analyses the impact that the new quantities will have on: radiation protection practice; calibration and reference fields; European and national regulation; international standards and, especially, dosemeter and instrument design. The task group included experienced scientists drawn from across the various EURADOS working groups.

RESPONSE OF CURRENT AREA DOSEMETERS TO NEW OPERATIONAL QUANTITIES.

New operational quantities for external radiation exposure presented by the ICRU brought changes, which can affect the use of current dosemeters. Dosemeters calibrated in the old quantities should be tested, and the results obtained should be analyzed with respect to the new quantities. The difference between the old and new quantities depends primarily on photon energy. This work concerns instruments for area monitoring, which were exposed to reference photon and neutron spectra of mean energies from 65 to 1250 keV and from 0.02 to 10 MeV, respectively. As expected, it was revealed that the current photon dosemeters overestimate the new quantity ambient dose. For the measurements within the energy interval chosen, it seems to be acceptable to implement a correction factor to optimize the response. For the purposes of measurements of photons of lower energies, further research would be needed. To adapt the response of the neutron dosemeter tested, recalibration or redesign can be considered, but no fundamental changes seem to be necessary for the investigated spectra.

OSL SENSITIVITY OF QUARTZ EXTRACTED FROM FIRED BRICKS FOR RETROSPECTIVE DOSIMETRY.

Optically stimulated luminescence of quartz extracted from fired materials has been used for retrospective dosimetry for about 40 years. Quartz sensitivity to thermoluminescence and optically stimulated luminescence can vary over few orders of magnitude. These discrepancies in sensitivity for the quartz originating from different parts of the world are still not yet fully understood. We have extracted quartz from bricks produced by three different companies in the Czech Republic. Quartz was extracted from fired bricks, as well as from the bricks before firing. Significant changes in quartz sensitivity were observed after firing process. Sensitivity can either increase or decrease, depending mainly on the maximum temperature achieved during firing process. For some samples, the sensitivity increased significantly after firing (7 times). Sensitivity also increased after repeated irradiation exposures. On the other hand, there were also samples for which the sensitivity did not increase after repeated cycles of irradiation.

Retrospective OSL Dosimetry With Common Pharmaceuticals and Food Supplements.

Several common pharmaceuticals such as ibuprofen, paracetamol, aspirin, oral contraceptives, drugs for the prevention of motion sickness and food supplements such as table vitamins and minerals have been studied for the purposes of retrospective dosimetry using optically stimulated luminescence (OSL). The essence is that the tablets with these drug substances contain additive crystalline materials which, after irradiation and stimulation, may exhibit luminescence. For most of the pharmaceuticals and food supplements, a radiation-induced dose-dependent OSL signal was detected. Subsequently, basic dosimetric characteristics of the materials were studied, specifically sensitivity changes during repeated OSL readings, dose response, zero-dose, minimum detectable dose (MDD) and fading. The most radiation sensitive materials were food supplements with Mg providing zero-dose and MDD values at the level of several mGy. For Mg supplements, considerable sensitivity changes in OSL signal were observed. Despite this, they could be corrected using a Single-Aliquot Regenerative-dose (SAR) protocol. The OSL signals of the other materials were relatively weak but they were well reproducible and exhibited linear dose response. The MDD values were variable among the materials and ranged from 0.1 to several Gy. However, for some of the pharmaceuticals, a very high and variable zero-dose of more than 3 Gy was observed that would rule out the possibility of dose reconstruction for triage purposes. The OSL signal exhibited a significant fading rate for most of the materials. The measurements for dose reconstruction should be performed as soon as possible after irradiation, i.e. within a maximum of a few days.

Retrospective physical dosimetry in the Czech Republic: an overview of already established methods and recent research.

PURPOSE: The threat of serious radiation exposures to members of the public from radiological incidents and nuclear events has led to intensive study of a number of emergency dosimetry techniques for purposes of triage. As such, a national laboratory of retrospective dosimetry was established in our institute. The purpose of this work is to provide a summary of the well-established and already implemented retrospective physical dosimetry techniques based on thermoluminescence (TL), optically stimulated luminescence (OSL) and neutron activation including their specifics. Moreover, we present some new results of the experimental work, in which we compared dosimetry potential of various dental repair materials and human teeth. MATERIALS AND METHODS: At first, an overview of already established retrospective physical retrospective methods including their main features was compiled. As regards recent research, an experimental comparative study was performed under defined conditions. The materials used were aliquots prepared from both pure and repaired teeth and aliquots of unused dental ceramics of known type. Following irradiation, we compared TL and OSL curves of the materials. We also compared dosimetry characteristics of OSL signal as reproducibility, dose dependence and fading. RESULTS: After irradiation, the teeth aliquots of dental enamel and dentin exhibited very low OSL and TL signals compared with aliquots containing some dental repair materials or aliquots of pure dental ceramics. With a few exceptions, the OSL signal of dental enamel and dentin aliquots irradiated to 2 Gy was hardly distinguishable from OSL signal corresponding to unirradiated aliquots. In contrast, aliquots of teeth containing some dental repair material and aliquots of pure dental ceramics provided a well reproducible OSL signal exhibiting linear dose response. All the materials tested exhibited a significant fading of the OSL signal. The loss of OSL signal during the first 24 hours after irradiation was from 20 to 99% of its original value obtained immediately after the irradiation. CONCLUSIONS: The already established physical methods of retrospective dosimetry use a spectrum of verified materials and techniques for dose assessment in the aftermath of serious radiological incidents and nuclear events. In the comparative study, we found that the dosimetry potential of teeth in natural state is much worse compared to teeth repaired with dental ceramics or dental cement fillings. Teeth restored with dental repair materials exhibited relatively favorable dosimetry characteristics. However, they can be usable for a dose reconstruction only on condition that the main practical problems connected with fading and optical bleaching were solved.

Comparison of luminescence detectors of Al2O3:C, CaF2:Mn, LiF:Mg,Cu,P and quartz with respect to retrospective dose reconstructions using bricks.

In the aftermath of an orphan radiation source find, a complex retrospective dose reconstruction can be required to estimate doses of persons who were staying in the vicinity. In retrospective dose reconstructions based on luminescence measurements of quartz extracted from bricks, high sensitivity thermoluminescence detectors (TLD) can be used as an ancillary tool for dose distribution measurements or natural radiation background measurement. We investigated the potential and limits of Al2O3:C, CaF2:Mn and LiF:Mg,Cu,P detectors for such applications. We measured depth-dose profiles in bricks using quartz and the TLDs. We factored in important dosimetry characteristics such as dose response, energy response and detection threshold. The work included Monte Carlo simulations. Depth-dose profiles and radiation spectra inside of the bricks were calculated for purposes of comparison and interpretation. The measurements and calculations were performed for two different photon spectra with mean energies of 662 and 118 keV. As regards comparison of the measured and Monte Carlo calculated depth-dose profiles, the best agreement was found for LiF:Mg,Cu,P. Quartz, Al2O3:C and CaF2:Mn tend to overestimate dose for lower photon energies and greater depths in bricks. The overestimation was the most marked for CaF2:Mn. For measurements related to quartz, especially for natural radiation background dose measurement, the most suitable TLDs are Al2O3:C and LiF:Mg,Cu,P. CaF2:Mn is the least useful material.

RESPONSE OF CURRENT PHOTON PERSONAL DOSEMETERS TO NEW OPERATIONAL QUANTITIES.

The ICRU proposed new operational quantities, which are defined in close relation to effective dose and absorbed dose. Most of the current personal dosemeters were designed to measure personal dose equivalents. Because of differences between the new and old quantities, the existing dosemeters may not be ideal for measurements of the new quantities, personal dose, Hp, and absorbed dose in local skin, Dp local skin. For photon radiation sources, we investigated relative responses of the current personal dosemeters with respect to the measurement of the new quantities. Introduction of the new quantities into practice will require some changes in practical radiation protection. A recalibration of the current dosemeters will be essential for Hp measurements in higher photon energy region (>50 keV). For lower photon energies, a redesign of specific evaluation algorithms will be necessary. For purposes of Dp local skin measurements the dosemeters do not require any fundamental changes.

POSTAL TLD AUDIT OF HETEROGENEITY CORRECTIONS IN RADIOTHERAPY IN THE CZECH REPUBLIC.

In the Czech Republic, a more advanced version of postal audit in radiotherapy (RT) is available. It covers dose measurements with thermoluminescent dosemeters (TLD) in more complex conditions of irradiation, when dose distribution is affected by heterogeneities in the irradiated volume. Relative deviation between doses measured with TLDs and doses stated by RT centre should not exceed 3%. During 2015-2017, all Czech RT centres equipped with modern linear accelerators were subjected to this more advanced TLD audit. A total of  70% of participants complied with the limit of 3% in the first round of this audit.

RETROSPECTIVE DOSE RECONSTRUCTION WITH MOBILE PHONES AND CHIP CARDS.

Mobile phones and common chip cards are very widespread items that almost everyone owns. They contain some radiation-sensitive materials that can be used for dosimetry based on stimulated luminescence. We investigated and compared reproducibility, dose response and fading of luminescence signal for the particular materials. Subsequently, we performed an experiment of a dose reconstruction using mobile phones and chip cards, which were fixed to a slab phantom and irradiated by a 137Cs radiation source in our laboratory. Doses obtained were compared with reference values. The materials investigated can be used for dosimetry in cases of serious radiation accidents or malevolent acts with radioactive materials, when it is extremely important to identify as quickly as possible individuals who received high-radiation doses.

NEUTRON DOSE ASSESSMENT USING SAMPLES OF HUMAN BLOOD AND HAIR.

The unique feature of nuclear accidents with neutron exposure is the induced radioactivity in body tissues. For dosimetry purposes, the most important stable isotopes occurring in human body, which can be activated by neutrons, are 23 Na and 32 S. The respective activation reactions are as follows:23Na(n,γ)24Na and32S(n,p)32P. While sodium occurs in human blood, sulfur is present in human hair. In order to verify the practical feasibility of this dosimetry technique in conditions of our laboratory, samples of human blood and hair were irradiated in a channel of a training reactor VR-1.24Na activity was measured by gamma-ray spectrometry.32P activity in hair was measured by means of a proportional counter. Based on neutron-spectrum calculation, relationships between neutron dose and induced activity were derived for both blood and hair.

Testing the methodology for a dosimetric end-to-end audit of IMRT/VMAT: results of IAEA multicentre and national studies.

Introduction: Within an International Atomic Energy Agency (IAEA) co-ordinated research project (CRP), a remote end-to-end dosimetric quality audit for intensity modulated radiation therapy (IMRT)/ volumetric arc therapy (VMAT) was developed to verify the radiotherapy chain including imaging, treatment planning and dose delivery. The methodology as well as the results obtained in a multicentre pilot study and national trial runs conducted in close cooperation with dosimetry audit networks (DANs) of IAEA Member States are presented.Material and methods: A solid polystyrene phantom containing a dosimetry insert with an irregular solid water planning target volume (PTV) and organ at risk (OAR) was designed for this audit. The insert can be preloaded with radiochromic film and four thermoluminescent dosimeters (TLDs). For the audit, radiotherapy centres were asked to scan the phantom, contour the structures, create an IMRT/VMAT treatment plan and irradiate the phantom. The dose prescription was to deliver 4 Gy to the PTV in two fractions and to limit the OAR dose to a maximum of 2.8 Gy. The TLD measured doses and film measured dose distributions were compared with the TPS calculations.Results: Sixteen hospitals from 13 countries and 64 hospitals from 6 countries participated in the multicenter pilot study and in the national runs, respectively. The TLD results for the PTV were all within ±5% acceptance limit for the multicentre pilot study, whereas for national runs, 17 participants failed to meet this criterion. All measured doses in the OAR were below the treatment planning constraint. The film analysis identified seven plans in national runs below the 90% passing rate gamma criteria.Conclusion: The results proved that the methodology of the IMRT/VMAT dosimetric end-to-end audit was feasible for its intended purpose, i.e., the phantom design and materials were suitable; the phantom was easy to use and it was robust enough for shipment. Most importantly the audit methodology was capable of identifying suboptimal IMRT/VMAT delivery.

A multinational audit of small field output factors calculated by treatment planning systems used in radiotherapy.

BACKGROUND AND PURPOSE: An audit methodology for verifying the implementation of output factors (OFs) of small fields in treatment planning systems (TPSs) used in radiotherapy was developed and tested through a multinational research group and performed on a national level in five different countries. MATERIALS AND METHODS: Centres participating in this study were asked to provide OFs calculated by their TPSs for 10 × 10 cm2, 6 × 6 cm2, 4 × 4 cm2, 3 × 3 cm2 and 2 × 2 cm2 field sizes using an SSD of 100 cm. The ratio of these calculated OFs to reference OFs was analysed. The action limit was ±3% for the 2 × 2 cm2 field and ±2% for all other fields. RESULTS: OFs for more than 200 different beams were collected in total. On average, the OFs for small fields calculated by TPSs were generally larger than measured reference data. These deviations increased with decreasing field size. On a national level, 30% and 31% of the calculated OFs of the 2 × 2 cm2 field exceeded the action limit of 3% for nominal beam energies of 6 MV and for nominal beam energies higher than 6 MV, respectively. CONCLUSION: Modern TPS beam models generally overestimate the OFs for small fields. The verification of calculated small field OFs is a vital step and should be included when commissioning a TPS. The methodology outlined in this study can be used to identify potential discrepancies in clinical beam models.

DOSIMETRY WITH SALT IN MIXED RADIATION FIELDS OF PHOTONS AND NEUTRONS.

Salt (NaCl) represents a radiation sensitive material with a considerable potential for dosimetry in mixed radiation fields of photons and neutrons. In consequence of a gamma radiation exposure, it exhibits a strong luminescence signal following stimulation with blue light. Optically stimulated luminescence (OSL) technique can be used for measurement. Photon dose reconstruction can be done using so-called Single-Aliquot Regenerative-dose protocol. However, a part of OSL signal is caused by NaCl self-irradiation resulting from neutron reactions. This NaCl neutron sensitivity is comparable with neutron sensitivity of TLD-700. Neutron dose can be determined based on neutron activation of NaCl via reaction 23Na(n,γ)24Na. A relation between neutron dose and activity of 24Na can be derived. The total dose is thus determined based on the combination of results of OSL measurements and gamma activity measurements. Practical feasibility of this approach was experimentally verified for salt samples irradiated in a channel of a training reactor.

Testing the methodology for dosimetry audit of heterogeneity corrections and small MLC-shaped fields: Results of IAEA multi-center studies.

UNLABELLED: The International Atomic Energy Agency (IAEA) has a long tradition of supporting development of methodologies for national networks providing quality audits in radiotherapy. A series of co-ordinated research projects (CRPs) has been conducted by the IAEA since 1995 assisting national external audit groups developing national audit programs. The CRP 'Development of Quality Audits for Radiotherapy Dosimetry for Complex Treatment Techniques' was conducted in 2009-2012 as an extension of previously developed audit programs. MATERIAL AND METHODS: The CRP work described in this paper focused on developing and testing two steps of dosimetry audit: verification of heterogeneity corrections, and treatment planning system (TPS) modeling of small MLC fields, which are important for the initial stages of complex radiation treatments, such as IMRT. The project involved development of a new solid slab phantom with heterogeneities containing special measurement inserts for thermoluminescent dosimeters (TLD) and radiochromic films. The phantom and the audit methodology has been developed at the IAEA and tested in multi-center studies involving the CRP participants. RESULTS: The results of multi-center testing of methodology for two steps of dosimetry audit show that the design of audit procedures is adequate and the methodology is feasible for meeting the audit objectives. A total of 97% TLD results in heterogeneity situations obtained in the study were within 3% and all results within 5% agreement with the TPS predicted doses. In contrast, only 64% small beam profiles were within 3 mm agreement between the TPS calculated and film measured doses. Film dosimetry results have highlighted some limitations in TPS modeling of small beam profiles in the direction of MLC leave movements. DISCUSSION: Through multi-center testing, any challenges or difficulties in the proposed audit methodology were identified, and the methodology improved. Using the experience of these studies, the participants could incorporate the auditing procedures in their national programs.

Fast neutron dosemeter using pixelated detector Timepix.

A Timepix detector covered with polyethylene convertors of different thicknesses is presented as a fast neutron real-time dosemeter. The application of different weighting factors in connection with the position of a signal in a Timepix detector enables one to obtain an energy-dependent signal equal to neutron dose equivalents. A simulation of a Timepix detector covered with polyethylene convertors using monoenergetic neutrons is presented. The experimental set-up of a dosemeter was also produced. The first results of detector response using different fast neutron sources are presented.

Retrospective dosimetry with alumina substrate from electronic components.

Alumina substrate can be found in electronic components used in portable electronic devices. The material is radiation sensitive and can be applied in dosimetry using thermally or optically stimulated luminescence. Electronic portable devices such as mobile phones, USB flash discs, mp3 players, etc., which are worn close to the body, can represent  personal dosemeters for members of the general public in situations of large-scale radiation accidents or malevolent acts with radioactive materials. This study investigated dosimetric properties of alumina substrates and aspects of using mobile phones as personal dosemeters. The alumina substrates exhibited favourable dosimetry characteristics. However, anomalous fading had to be properly corrected in order to achieve sufficient precision in dose estimate. Trial dose reconstruction performed by means of two mobile phones proved that mobile phones can be used for reconstruction of personal doses.

NaCl as a retrospective and accident dosemeter.

NaCl is a cheap and widely available material. This study investigated the potential of NaCl in the form of a household salt as a retrospective and accident dosemeter using optically stimulated luminescence (OSL). Samples of the investigated household salt were stimulated using blue light of linearly modulated power. Attention was concentrated on sensitivity, dose dependence of the OSL signal, fading, optimisation of the read-out procedure and application of analytical protocols that do not require a specific calibration. A potential of NaCl as a complementary dosemeter within emergency preparedness was considered. The behaviour of the OSL signal observed was found to be favourable for dosimetry.

Testing of a thermoluminescent personal dosimeter at interim storages for spent nuclear fuel.

The National Radiation Protection Institute in Prague is about to use a commercially available multi-element whole-body thermoluminescence dosimeter for its radiation personnel. During the phase of practical verification of the method, the dosimeter was tested, among others, in mixed neutron-photon fields in the area of interim storages for spent nuclear fuel of the Czech nuclear power plant Dukovany. The testing was made with the intention to get information about accuracy of H(p)(10) determination for cases when no site-specific calibration is available or neutron spectra are not known. The measured photon and neutron personal dose equivalents were evaluated and discussed in relation to results obtained by survey meters and neutron spectra measured before.