Accuracy Of Dosimeters Research Articles

Beta dose rate estimation of soil samples with CW-OSL technique using LiCaAlF6:Eu,Y phosphor for retrospective dosimetry

Retrospective dosimetry technique makes use of commonly available materials such as soil, brick, electronic components etc. for dose estimation. The measurement of internal component of annual dose is necessary for estimating the background dose received by the sample. This dose is then subtracted from the cumulative dose received by the sample in order to estimate the accident dose received by the sample retrospectively. In most of the cases it is sufficient to determine only the beta dose for the internal component of the annual dose. For the annual beta dose in soil/pottery thermoluminescence (TL) technique is widely used. However, in recent years Optically Stimulated Luminescence (OSL) dosimetry is gaining importance due to its various advantages over TL dosimetry. LiCaAlF6:Eu,Y (LCAF) is one of the promising OSL phosphor having remarkable dosimetric characteristics and sensitivity. By varying the stimulation parameters, the minimum detectable dose (MDD) of LCAF could be achieved in the range of 0.1–0.2 μGy. Leveraging this advancement, LCAF based dosimeters were successfully utilized for assessing the beta dose originating from various soil samples. The obtained results exhibit notable concurrence with Monte Carlo-based computations, validating the accuracy and reliability of LCAF dosimeters.

Biocompatibility and radiosensitivity of a fiber optical-based dosimeter: biological applications

This study introduces a cutting-edge fiber-optic dosimetry (FOD) sensor designed for measuring radiation in biological settings. The accuracy and precision of dosimeters for small animals, particularly prolonged exposure to nonuniform radiation fields, are always challenging. A state-of-the-art in-vivo dosimeter utilizing glass-encapsulated Thermoluminescence cylindrical detector (TLD) was introduced. The FODs are implanted into the rat during a prolonged irradiation scenario involving 137Cs where the rat has the freedom to move within a heterogeneous radiation domain. The implantation surgery was verified with X-ray computed tomography (CT) in addition to biochemical and pathological tests to assess the biocompatibility of FOD in vivo. A versatile FOD is designed for industrial and medical fields, which demand accurate and resilient radiation dosimeters. The dose measurements are associated with precise two-dimensional (2D) radiation distribution imaging. Three cylindrical FODs and three standards TLD_100 for each rat were tested. The measurements of peak irradiation before and after exposure reveal greater stability and superior sensitivity when compared to standard thermo-luminescence detectors in an in-vivo animal test. To the best of our knowledge, FOD testing on live animals is presented for the first time in this paper. Regarding the safety and biocompatibility of FOD, no morphological signs with any kind of inflammation or sensitivity toward the FOD material have been remarked. Moreover, with the current FOD, there is no oedema between the epidermal, dermal, and subdermal sections at the site of implantation. The results also show the stable levels of white blood cells (lymphocytes, granulocytes, MID) as blood inflammatory markers before surgery and at the time of extraction of the implanted dosimeters, thus confirming the biocompatibility for each optical fiber cylinder dosimeter. As a result, the new dosimeters have excellent biocompatibility in living tissues and have 100% accurate reusability intensity of the delivered radiation doses compared to TLD_100 which demonstrated a 45% reduction in its intensity accuracy.

Electron streaming dose measurements and calculations on a 1.5 T MR-Linac.

To evaluate the accuracy of different dosimeters and the treatment planning system (TPS) for assessing the skin dose due to the electron streaming effect (ESE) on a 1.5 T magnetic resonance (MR)-linac. Skin dose due to the ESE on an MR-linac (Unity, Elekta) was investigated using a solid water phantom rotated 45° in the x-y plane (IEC61217) and centered at the isocenter. The phantom was irradiated with 1 × 1, 3 × 3, 5 × 5, 10 × 10, and 22 × 22cm2 fields, gantry at 90°. Out-of-field doses (OFDs) deposited by electron streams generated at the entry and exit surface of the angled phantom were measured on the surface of solid water slabs placed±20.0cm from the isocenter along the x-direction. A high-resolution MOSkin™ detector served as a benchmark due to its shallower depth of measurement that matches the International Commission on Radiological Protection (ICRP) recommended depth for skin dose assessment (0.07mm). MOSkin™ doses were compared to EBT3 film, OSLDs, a diamond detector, and the TPS where the experimental setup was modeled using two separate calculation parameters settings: a 0.1cm dose grid with 0.2% statistical uncertainty (0.1cm, 0.2%) and a 0.2cm dose grid with 3.0% statistical uncertainty (0.2cm, 3.0%). OSLD, film, the 0.1cm, 0.2%, and 0.2cm, 3.0% TPS ESE doses, underestimated skin doses measured by the MOSkin™ by as much as -75.3%, -7.0%, -24.7%, and -41.9%, respectively. Film results were most similar to MOSkin™ skin dose measurements. These results show that electron streams can deposit significant doses outside the primary field and that dosimeter choice and TPS calculation settings greatly influence the reported readings. Due to the steep dose gradient of the ESE, EBT3 film remains the choice for accurate skin dose assessment in this challenging environment.

Extension of the RADTriage Colorimetric Dosimeter to Low-Dose Gamma-Ray Exposure Using Scanning Densitometry.

There is a need for an instantly indicating, easy-to-read, and inexpensive ionizing radiation dosimeter for first responders and members of the general public. One commercially available option is the RADTriage50 TM colorimetric dosimeter. However, existing literature has not adequately addressed the accuracy of RADTriage50 dosimeters at low doses of ionizing radiation (<50 mSv) or the need for methods to quantitatively read the RADTriage50 dosimeters after they are exposed. In this paper, we use digital scanning methods to read the RADTriage50 dosimeters. The performance of the dosimeters was evaluated by irradiation with a gamma irradiator traceable to national standards. Experiments covered a range of deep dose equivalents (50 mSv to 2,000 mSv) within the manufacturer's specified range (50 mSv to 4,000 mSv) and also below 50 mSv to determine if the digital scanning densitometry method allowed for a quantitative readout with a greater dynamic range. We also conducted tests using different gamma energies, 137 Cs (662 keV) and 60 Co (1.17 and 1.33 MeV), and different dose rates to evaluate the dependency of the RADTriage50 dosimeters on these parameters. Modeling of our measurements suggests that the dose-response of the RADTriage50 dosimeter is linear at low doses with strong non-linearity beginning at ~750 mSv and the dosimeter response appearing to plateau at ~2,000 mSv, although additional measurements at doses beyond 2,000 mSv are needed to confirm this finding. We also found that the RadTriage50 dosimeter response varied with gamma energy, but not with dose rate.

Silver-Neodymium Codoped Lithium Aluminum Metaphosphate Glasses for Radio-Photoluminescence Dosimeter.

Commercial radio-photoluminescence (RPL) glass dosimeters generally use Ag single-doped phosphate glass as a single-wavelength sensor. Now, a novel type of Ag–Nd-codoped phosphate glass has been developed, which can be applied to dual-wavelength or multi-wavelength RPL sensors, and can thus improve the accuracy and stability of RPL dosimeters. An anhydrous 99.5 (0.7LiPO3–0.3Al (PO3)3) −0.25Ag2O–0.25Nd2O3 glass was prepared and irradiated at different doses, and then the absorption, fluorescence, infrared transmission spectra, as well as fluorescence lifetimes were tested and analyzed. The results show that there is an energy transfer between the Ag defect center and Nd3+ ions, and the transfer efficiency using 380 nm excitation is greater than that using 310 nm excitation. Aside from the 650 nm fluorescence of the Ag defect center, strong 882 nm and 1054 nm fluorescences of Nd ions are exhibited. It is possible that these fluorescences would allow the developed Ag–Nd-codoped phosphate glass to be applied to new RPL glass sensors and dosimeters.

Effects of high ambient temperature on the accuracy of thermoluminescent dosimeters for environmental monitoring

Thermoluminescence dosimeters (TLDs) are widely used for both personal and environmental dosimetry. TLDs should have high accuracy under different conditions. The TL signal can drop over time because of fading (loss of signal due to thermally induced recombination of trapped charriers), thus leading to underestimation of the irradiation dose. The Saudi climate is extremely hot for most of the year, which could significantly affect TLD measurements. Therefore, the effect of ambient temperature, storage time and irradiation dose were investigated both in laboratory controlled-temperature conditions and field experiments, for two commercial dosimeters: Harshaw (TLD-100H™) and RADCARD (MCP-N™), which are used for environmental monitoring. The irradiated TLDs were exposed to a range of ambient temperatures (25 °C–65 °C) then stored for 30, 60, and 90 days. A signal fading due to increasing ambient temperature and storage time was generally observed. MCP-N shows good stability and is less responsive to increasing ambient temperature compared to TLD-100H. TLD-100H is less affected by storage time compared to MCP-N. Irradiation doses play a role in TL signal fading, and TLDs irradiated with 5 mSv have a higher rate of loss compared to those irradiated with 2 mSv in all TLD types. The obtained results permitted to conclude that all TLD types used in this study suffer from TL signal fading, and its degree varies between TLD types.

Determination of effective source to surface distance and cutout factor in small fields in electron beam radiotherapy: A comparison of different dosimeters

Abstract Objective: The main purpose of this study is to calculate the effective source to surface distance (SSDeff) of small and large electron fields in 10, 15, and 18 MeV energies, and to investigate the effect of SSD on the cutout factor for electron beams a linear accelerator. The accuracy of different dosimeters is also evaluated. Materials and methods: In the current study, Elekta Precise linear accelerator was used in electron beam energies of 10, 15, and 18 MeV. The measurements were performed in a PTW water phantom (model MP3-M). A Semiflex and Advanced Markus ionization chambers and a Diode E detector were used for dosimetry. SSDeff in 100, 105, 110, 115, and 120 cm SSDs for 1.5 × 1.5 cm2 to 5 × 5 cm2 (small fields) and 6 × 6 cm2 to 20 × 20 cm2 (large fields) field sizes were obtained. The cutout factor was measured for the small fields. Results: SSDeff in small fields is highly dependent on energy and field size and increases with increasing electron beam energy and field size. For large electron fields, with some exceptions for the 20 × 20 cm2 field, this quantity also increases with energy. The SSDeff was increased with increasing beam energy and field size for all three detectors. Conclusion: The SSDeff varies significantly for different field sizes or cutouts. It is recommended that SSDeff be determined for each electron beam size or cutout. Selecting an appropriate dosimetry system can have an effect in determining cutout factor.

MOSFET dosimeter characterization in MR‐guided radiation therapy (MRgRT) Linac

PurposeWith the increasing use of MR‐guided radiation therapy (MRgRT), it becomes important to understand and explore accuracy of medical dosimeters in the presence of magnetic field. The purpose of this work is to characterize metal‐oxide‐semiconductor field‐effect transistors (MOSFETs) in MRgRT systems at 0.345 T magnetic field strength.MethodsA MOSFET dosimetry system, developed by Best Medical Canada for in‐vivo patient dosimetry, was used to study various commissioning tests performed on a MRgRT system, MRIdian® Linac. We characterized the MOSFET dosimeter with different cable lengths by determining its calibration factor, monitor unit linearity, angular dependence, field size dependence, percentage depth dose (PDD) variation, output factor change, and intensity modulated radiation therapy quality assurance (IMRT QA) verification for several plans. MOSFET results were analyzed and compared with commissioning data and Monte Carlo calculations.ResultsMOSFET measurements were not found to be affected by the presence of 0.345 T magnetic field. Calibration factors were similar for different cable length dosimeters either placed at the parallel or perpendicular direction to the magnetic field, with variations of less than 2%. The detector showed good linearity (R2 = 0.999) for 100–600 MUs range. Output factor measurements were consistent with ionization chamber data within 2.2%. MOSFET PDD measurements were found to be within 1% for 1–15 cm depth range in comparison to ionization chamber. MOSFET normalized angular response matched thermoluminescent detector (TLD) response within 5.5%. The IMRT QA verification data for the MRgRT linac showed that the percentage difference between ionization chamber and MOSFET was 0.91%, 2.05%, and 2.63%, respectively for liver, spine, and mediastinum.ConclusionMOSFET dosimeters are not affected by the 0.345 T magnetic field in MRgRT system. They showed physics parameters and performance comparable to TLD and ionization chamber; thus, they constitute an alternative to TLD for real‐time in‐vivo dosimetry in MRgRT procedures.

Relative response of dosimeters to variations in scattered X-ray energy spectra encountered in interventional radiology

PurposeThe new lower eye lens dose limit is of relevance in interventional radiology, where higher dose procedures result in increased scattered radiation to staff. The eye lens dose may be monitored using the directional dose equivalent at 3 mm depth, Hp(3), or through Hp(10) or Hp(0.07) measurements and using conversion factors. However, there are a considerable range of factors which contribute to measurement uncertainties, one of which is the incident photon energy. This study investigated the energy spectra of scattered radiation in interventional radiology, and the dosimetry accuracy of dosimeter types, evaluating their energy dependence. MethodsScatter X-ray energy spectra were recorded under varied conditions in a fluoroscopy imaging suite. Dosimetry accuracy of eye dosimeters, including TLDs (100 s, 100Hs), Landauer Hp(3), John Caunt ED3 and Electronic Personal dosimeters (EPDs) were compared to air kerma measurements across a range of tube voltages. ResultsThe variation of energy spectra with changing phantom thickness, spectrometer angulation and filtration are presented. The 100 and 100H TLDs, and EPDs showed a consistent air kerma response (within 10%) with changes in energy. The real-time silicon diode detectors showed a variable over response of between 10 and 25% across the energies investigated while Landauers dedicated Hp(3) eye dosimeters showed considerable variation between dosimeters for similar conditions, a 17% variation at 50 kVp. ConclusionThe work aimed to validate the scattered energy spectra typically encountered in interventional radiology and to further determine the accuracy of eye dosimeters in relation to energy response variations.

Comparison of gamma-ray dosimeters in a field study in the Chernobyl exclusion zone

ABSTRACT Five different models of dosimeters were compared in the Chernobyl Inner Exclusion Zone by measuring gamma radiation in 12 locations. We used an instrument made by Mirion Technologies, Inc., as the reference, since that instrument had an NIST-traceable calibration. Two models of dosimeters gave radiation values similar to the Mirion at all levels of radiation encountered. Two other models gave similar values to the Mirion instrument at low radiation levels but not at higher radiation levels encountered. These results offer a caution regarding the accuracy of inexpensive commercially available radiation instruments. Implications: Comparison of gamma ray dosimeters in the Chernobyl Exclusion Zone showed varying agreement with an NIST-calibrated instrument, as well as varying linearity of response to ambient radiation. These results suggest caution regarding accuracy of inexpensive dosimeters. However, dosimeters used that were manufactured in Ukraine exhibited good agreement with the reference instrument.

SU‐E‐T‐643: Pure Alanine Dosimeter for Verification Dosimetry in IMRT

Purpose:The objective of this study was evaluation of accuracy of pure alanine dosimeters measuring intensity‐modulated radiation therapy (IMRT) dose distributions in a thorax phantom.Methods:Alanine dosimeters were prepared in the form of 110 mg pure L‐α‐alanine powder filled into clear tissue‐equivalent polymethylmethacrylate (PMMA) plastic tubes with the dimensions 25 mm length, 3 mm inner diameter, and 1 mm wall thickness. A dose‐response calibration curve was established for the alanine by placing the dosimeters at 1.5 cm depth in a 30×30×30 cm3 solid water phantom and then irradiating on a linac with 6 MV photon beam at 10×10 cm2 field size to doses ranging from 1 to 5 Gy. Electron paramagnetic resonance (EPR) spectroscopy was used to determine the absorbed dose in alanine. An IMRT treatment plan was designed for a commercial heterogeneous CIRS thorax phantom and the dose values were calculated at three different points located in tissue, lung, and bone equivalent materials. A set of dose measurements was carried out to compare measured and calculated dose values by placing the alanine dosimeters at those selected locations inside the thorax phantom and delivering the IMRT to the phantom.Results:The alanine dose measurements and the IMRT plan dose calculations were found to be in agreement within ±2%. Specifically, the deviations were −0.5%, 1.3%, and −1.7% for tissue, lung, and bone; respectively. The slightly large deviations observed for lung and bone may be attributed to tissue inhomogeneity, steep dose gradients in these regions, and uncontrollable changes in spectrometer conditions.Conclusion:The results described herein confirmed that pure alanine dosimeter was suitable for in‐phantom dosimetry of IMRT beams because of its high sensitivity and acceptable accuracy. This makes the dosimeter a promising option for quality control of the therapeutic beams, complementing the commonly used ionization chambers, TLDs, and films.

Do saccharide doped PAGAT dosimeters increase accuracy?

To improve the dosimetric accuracy of normoxic polyacrylamide gelatin (PAGAT) gel dosimeters, the addition of saccharides (glucose and sucrose) has been suggested. An increase in R2-response sensitivity upon irradiation will result in smaller uncertainties in the derived dose if all other uncertainties are conserved. However, temperature variations during the magnetic resonance scanning of polymer gels result in one of the highest contributions to dosimetric uncertainties. The purpose of this project was to study the dose sensitivity against the temperature sensitivity. The overall dose uncertainty of PAGAT gel dosimeters with different concentrations of saccharides (0, 10 and 20%) was investigated. For high concentrations of glucose or sucrose, a clear improvement of the dose sensitivity was observed. For doses up to 6 Gy, the overall dose uncertainty was reduced up to 0.3 Gy for all saccharide loaded gels compared to PAGAT gel. Higher concentrations of glucose and sucrose deteriorate the accuracy of PAGAT dosimeters for doses above 9 Gy.

TU‐F‐BRE‐08: Significant Variations in Measured Small Cone Output Factor for FFF Beams

Purpose:To evaluate the measurement accuracy of several dosimeters for small cone output factors in two SRS/SBRT dedicated systems with Flattening Filter Free (FFF) beams: a Varian TrueBeam STx (TB) and an Accuray CyberKnife VSI (CK). Output factors (OFs) were measured for both machines and for CK, compared against a Monte Carlo model.Methods:Dose measurements were taken using three different FFF beams (TB 6XFFF, TB 10XFFF, and CK 6XFFF). Three commonly used types of dosimeters were examined in this work: a micro‐ion chamber (Exradin A16), two shielded diodes (PTW TN60008 and PTW TN60017), and radiochromic film (Gafchromic EBT2). Measured OFs from these dosimeters were compared with each other and OFs measured with an Exradin W1 scintillator. Monte Carlo determined correction factors for the CK beam for the micro‐ion chamber and diodes were applied to the respective OF measurements and compared against scintillator measured OFs corrected for volume averaging.Results:OFs measured for the smallest fields using the micro‐ion chamber, diodes, scintillator, and film varied substantially (with up to a 16% difference between dosimeters). Micro‐ion chamber and film OF measurements were up to 9% and 10%, respectively, lower than scintillator measurements for the smallest fields. OF measurements by diode were up to 6% greater than scintillator measurements for the smallest fields. With correction factors, the micro‐ion chamber and diode measured OFs showed good agreement with scintillator measured OFs for the CK 6XFFF beam (within 3% and 1.5%, respectively).Conclusion:Uncorrected small field OFs vary significantly with dosimeter. The accuracy of scintillator measurements for small field OFs may be greater than the other dosimeters studied in this work (when uncorrected). Measurements involving EBT2 film may Result in lower accuracy for smaller fields (less than 10mm). Care should be taken in the choice of the dosimeter used for small field OF measurements.

Commissioning of optically stimulated luminescence dosimeters for use in radiotherapy

Abstract The Australian Clinical Dosimetry Service (ACDS) is a government initiative to provide all radiation therapy centers in Australia with access to free and independent dosimetric audit services. Based out of the Australian Radiation Protection and Nuclear Safety Agency, the ACDS commenced audit operations in 2010. Aim The objective of this work was to commission Optically Stimulated Luminescence Dosimeters (OSLDs) as a replacement for Thermoluminescence Dosimeters (TLD) for use in the ACDS's remote auditing program, with a view to extending their application to higher level dosimetry in anthropomorphic phantoms. Materials and Method The In-Light nanoDots™ OSLD system from Landauer Inc. was commissioned using both the Australian National Linear Accelerator, an Elekta Synergy II, and an Eldorado Co-60 unit. The linearity, signal depletion, fading, energy dependence, reproducibility and resetting of nanoDot™ dosimeters were determined for clinical photon and electron beams. Results NanoDots™ OSLDs demonstrate; supralinearity dependent on the overall accumulated dose. Little signal depletion per readout (0.03% per read). Reproducible fading, characterised by an initial transient signal decay over a period of 16 min post irradiation, followed by a predictable logarithmic decay resulting in signal decrease of 3% over several months. Little dependence on energy or modality over the clinical range. Optical resetting tests show negligible change in sensitivity post reset for up to five cycles. Conclusion Modern OSLDs such as the nanoDots™ dosimeters provide a viable alternative to the established TLD technology with comparable accuracy and greater efficiency. The efficiency, reusability and accuracy of nanoDot™ dosimeters makes them an ideal candidate for the large-scale dosimetry operations currently undertaken by the ACDS.

Accurate calibration of a polymer gel dosimeter with a plastic scintillation detector

Three dimensional dose polymer gel dosimetry measurements provide unique information on sophisticated dose distributions. In this study, the authors propose a novel method to improve the accuracy of polymer gel dosimeters by inserting a plastic scintillation detector (PSD) to provide a dose reference. PSD dosimeters were calibrated using chromatic deconvolution and then inserted into polyacrylanide gel (PAG) dosimeters. The gel and the PSDs were immersed into water and irradiated with 6 MV wedge filtered beams to obtain a wide range of dose variation. Calibration vials containing the same gel were also irradiated to generate a standard calibration curve. The distribution of magnetic nuclear transverse relaxation rate (R2) values of the gel was determined with a multislice multiecho MRI sequence at 1.5 T. Another calibration curve was obtained by assigning the R2 values in the gel surrounding the scintillators to the dose determined by the PSDs. A reference calibration point from a PSD located in a low dose gradient area served to correct the standard calibration method yielding three novel calibration methods. The results were compared with EBT2 GAFCHROMIC film measurements acquired in the same condition and with the Pinnacle3 treatment planning dose calculations, The mean absolute error of the standard calibration method ranged from 6.1 to 12.4%. The corresponding gamma index (3%/3 mm distance to agreement) criterion was satisfied for only 56% of the pixels in the middle slice of the gel compared to Pinnacle3 dose calculations and to EBT2 film measurements in the center part of the field. Calibration methods using a PSD reduced the mean absolute error to less than 4%; this value was under 2.6% for one of the three methods. In that case, 98% of the pixels satisfied the gamma index criterion. The accuracy of PAG dosimeters may be highly improved using one reference dose point measurement using a plastic scintillation detector. The best calibration procedure corrected the slope of the calibration curve derived from the calibration vials to match the R2 value around a PSD calibration, while keeping the R2 value at 0 Gy constant.

TH‐C‐BRB‐03: On the Suitability of Using NanoDot for Measuring Dose Distributions Resulting from KV‐CBCT Acquisitions

Purpose: There is a growing need to determine radiation exposure and dose distributions resulting from an image‐guidance procedure. Although Monte Carlo simulations are capable to calculate the dose distributions experimental measurements remain the gold standards to validate the accuracy of calculations. This study investigated the suitability of nanoDot dosimeters for measuring accumulative dose at several locations simultaneously to determine 3D dose distributions resulting from a kilovoltage cone‐beam CT (kV‐CBCT) scan. Method and Materials: The nanoDot dosimeters read with a microStar reader by Landauer were used in the study. The system can be calibrated to measure dose from kilovoltage to megavoltage energy beams. The manufacturer provides a set of calibration dosimeters for 80 kVp x‐rays. The accuracy of the nanoDot dosimeters for absorbed dose measurements for kV‐CBCT scans was validated by using an ionization chamber in which the air‐kerma calibration factors were traceable to National Standards Laboratory. The relative dose distributions measured with nanoDot were compared with Monte Carlo calculated dose distributions in a PLASTIC WATER® LR phantom. Results: Seven nanoDot dosimeters were placed at different locations to determine dose distributions resulting from each kV‐CBCT scan protocol. There are five different kV‐CBCT scan protocols in the Varian OBI 1.4 system including Head Low‐dose Thorax Pelvis and Pelvis Spot Light with two different Bow‐tie filters. Both absolute and relative dose distributions were investigated. The dose values measured from 35 points generally agree with that of Monte Carlo calculations within 2–4%. Conclusion: The nanoDot dosimeters have been shown to be very suitable detectors for measuring 3D dose distributions resulting from kV‐CBCT scans due to its small size the ease of use good reproducibility and manageable photon energy dependence.Conflict of Interest: The microStar reader used in the research was provided by Landauer Inc. Glenwood IL

Reproducibility study of normoxic polyacrylamide gel (nPAG) dosimeters

In this study, the overall accuracy of normoxic polyacrylamide gel (nPAG) dosimeters is considered. Different badges of nPAG are fabricated, poured in containers of glass and Barex" and irradiated with a 6MV square photon beam. The polymer gel dosimeters were read out using MRI. The overall reproducibility and accuracy of nPAG gel dosimeters was determined by comparison with depth-dose profiles acquired with a Pin Point ionization chamber. Additionally, the effect of the container wall on the depth-dose profile and the effect of temperature changes before and after irradiation on the R2-dose response have been investigated. The average standard deviation and maximum deviation between 8 gel-measured depth-dose profiles and a depth-dose profile measured with an ionization chamber amounted to 0.543 Gy (2.5%) and 2.579 Gy (11.7%) respectively.

The influence of cooling rate on the accuracy of normoxic polymer gel dosimeters

Polymer gel dosimeters offer a wide range of applications in the three-dimensional verification of complex radiation dose distributions such as in intensity-modulated radiotherapy (IMRT). With the release of polymer gel dosimeters that can be fabricated in normal atmospheric (‘normoxic’) conditions, the gel manufacturing process has been significantly simplified. Gel dosimeters are calibrated by use of a series of calibration vials irradiated with known doses or by use of a calibration phantom with a known dose distribution. The overall accuracy of the polymer gel dosimeters is determined by different dosimetric properties. In this study, we show the influence of the temperature history during storage of the gel dosimeter on the dose response curve for two gel dosimeters using the monomers acrylamide/N,N′-methylene-bis-acrylamide (nPAG) and methacrylic acid (nMAG) respectively and bis[tetrakis(hydroxymethyl)phosphonium]sulphate (THP) as antioxidant in both gel dosimeters. This study reveals that differences in temperature history after fabrication of normoxic polymer gel dosimeters may compromise the dosimetric accuracy. It was found that the acrylamide based gel dosimeter (nPAG) is less dependent on the post-manufacture temperature history than the methacrylic acid based gel dosimeter (nMAG). The importance of an equal temperature history for the gel dosimeter and calibration vials is emphasized by this study. A reproducibility study has also been performed on the nPAG gel dosimeter when additional efforts are made to control the temperature changes upon cooling.

Essential characteristics of polymer gel dosimeters

For a physical measurement instrument different requirements have to be fulfilled such as its insensitivity to uncontrollable environmental parameters and its stability. On the other hand, in order to meet an assigned accuracy, all measurement instruments should be operated in an approved manner. Polymer gel dosimeters are unique in their kind as they are able to integrate the dose in three dimensions and can be shaped in a humanoid form. In this paper, we focus on different characteristics that determine the accuracy of polymer gel dosimeters from the point-of-view of their use as 3D dosimeters in radiotherapy. It is shown that the accuracy is highly dependent on the composition of polymer gel. The comparison of the radiological characteristics may help in understanding the underlying mechanisms of the polymer gel dosimeters and in optimizing the chemical composition in terms of both dose and spatial accuracy.

A Passive, Direct Reading, Length-of-Stain Dosimeter for Ammonia

A line of passive, length-of-stain dosimeters has been developed for ammonia, amines, carbon dioxide, carbon monoxide, chlorine, hydrogen cyanide, hydrogen chloride, hydrogen sulfide, nitrogen dioxide and sulfur dioxide. These were described in a previous publication. This paper deals with the calibration and accuracy testing of the ammonia (NH3) dosimeter. These dosimeters operate according to Fick's Law of Diffusion. For the NH3 dosimeter, this law can be modified to the form: 15S+S2=KCt, where S=stain length (mm), K is a constant, C=concentration (ppm) and t=time (hours). By experimentally determining K for each lot of tubes, the concentration corresponding to the stain length can readily be caluclated for a given exposure time within the applicable concentration range. The range of the NH3 dosimeter is from about 1 to 280 ppm. The accuracy of these NH3 dosimeters has been validated internally by using a technique described by DuPont. Accuracies of four recent lots were: ±16.3,19.7, 23.4,14.8%. The accuracy of Lot 5 was independently checked by the Pittsburgh Testing Laboratories. In summary, a simple, cost-effective, direct reading, length-of-stain personal (or area) dosimeter for ammonia has been developed. This dosimeter meets the NIOSH guidelines for accuracy of ±25% for this type of device.