Ronaldo Minniti

The use of gel dosimetry to measure the 3D dose distribution of a 90Sr/90Y intravascular brachytherapy seed

Absorbed dose distributions in 3D imparted by a single (90)Sr/(90)Y beta particle seed source of the type used for intravascular brachytherapy were investigated. A polymer gel dosimetry medium was used as a dosemeter and phantom, while a special high-resolution laser CT scanner with a spatial resolution of 100 microm in all dimensions was used to quantify the data. We have measured the radial dose function, g(L)(r), observing that g(L)(r) increases to a maximum value and then decreases as the distance from the seed increases. This is in good agreement with previous data obtained with radiochromic film and thermoluminescent dosemeters (TLDs), even if the TLDs underestimate the dose at distances very close to the seed. Contrary to the measurements, g(L)(r) calculated through Monte Carlo simulations and reported previously steadily decreases without a local maximum as a function of the distance from the seed. At distances less than 1.5 mm, differences of more than 20% are observed between the measurements and the Monte Carlo calculations. This difference could be due to a possible underestimation of the energy absorbed into the seed core and encapsulation in the Monte Carlo simulation, as a consequence of the unknown precise chemical composition of the core and its respective density for this seed. The results suggest that g(L)(r) can be measured very close to the seed with a relative uncertainty of about 1% to 2%. The dose distribution is isotropic only at distances greater than or equal to 2 mm from the seed and is almost symmetric, independent of the depth. This study indicates that polymer gel coupled with the special small format laser CT scanner are valid and accurate methods for measuring the dose distribution at distances close to an intravascular brachytherapy seed.

Characteristics of a new polymer gel for high-dose gradient dosimetry using a micro optical CT scanner.

The properties of a new polymer gel with two sensitivities, made specifically for high-dose-gradient dosimetry, were investigated. The measurements were performed at NIST using a 1cmx1cm calibrated (60)Co field, and a 1cm active diameter (90)Sr/(90)Y beta particle source. A high-resolution laser CT scanner was used to quantify the response. The results show that the high-sensitivity gel responds linearly to the absorbed dose for doses from 0.5 up to 15Gy, while the low-sensitivity one is linear up to 225Gy. For both radiation types, the gel response remains stable in time up to a month after the irradiation. The response of the gel was found to have no dose rate dependence for dose rates ranging from 3.7 to 15mGy/s. Within the measurement uncertainty, the gel response is more sensitive for beta particles than high energy photons.

Exposure of self-assembled monolayers to highly charged ions and metastable atoms

The doses of neutral metastable argon atoms (Ar*) and highly charged xenon ions (HCIs) required to damage self-assembled monolayers (SAMs) of alkanethiolates on gold are compared in a set of experiments carried out concurrently. The extent of damage to the SAM is determined by developing the samples in a gold etching solution, then measuring the decrease in reflectivity of the gold; ≈105 Ar* are required to cause the same amount of damage as 1 HCI, as measured by this assay. We have also demonstrated HCI micropatterning of a surface using a physical mask, suggesting the application of this system in lithography.

Influence of phantom materials on the energy dependence of LiF:Mg,Ti thermoluminescent dosimeters exposed to 20-300 kV narrow x-ray spectra, 137Cs and 60Co photons.

LiF:Mg,Ti, are widely used to estimate absorbed-dose received by patients during diagnostic or medical treatment. Conveniently, measurements are usually made in plastic phantoms. However, experimental conditions vary from one group to another and consequently, a lack of consensus data exists for the energy dependence of thermoluminescent (TL) response. This work investigated the energy dependence of TLD-100 TL-response and the effect of irradiating the dosimeters in different phantom materials for a broad range of energy photons in an attempt to understand the parameters that affect the discrepancies reported by various research groups. TLD-100s were exposed to 20-300 kV narrow x-ray spectra, (137)Cs and (60)Co photons. Measurements were performed in air, PMMA, wt1, polystyrene and TLDS as surrounding material. Total air-kerma values delivered were between 50 and 150 mGy for x-rays and 50 mGy for (137)Cs and (60)Co beams; each dosimeter was irradiated individually. Relative response, R, defined as the TL-response per air-kerma and relative efficiency, RE, described as the TL-response per absorbed-dose (obtained through Monte Carlo (MC) and analytically) were used to describe the TL-response. Both R and RE are normalized to the responses in a (60)Co beam. The results indicate that the use of different phantom materials affects the TL-response and this response varies with energy and material type. MC simulations reproduced qualitatively the experimental data: a) R increases, reaches a maximum at ~25 keV and decreases; b) RE decreases, down to a minimum at ~60 keV, increases to a maximum at ~150 keV and after decreases. Independent of the phantom materials, RE strongly depends on how the absorbed dose is evaluated and the discrepancies between RE evaluated analytically and by MC simulation are around 4% and 18%, dependent on the photon energy. The comparison between our results and that reported in the literature suggests that the discrepancy observed between different research groups appears to be most likely related to supralinearity effect, phantom materials, difference on the energy-spectra and geometry conditions during each experiment rather than parameters such as heating-rate or annealing procedure, which was supported by MC simulation. From the results obtained in this work and the strict analysis performed, we can conclude that for clinical applications of TLD-100, special attention must be taken when published data are used to convert TL calibration curve from (60)Co to low-energy photons. Otherwise, this can lead to incorrect results when later used to measure absorbed dose in human tissue.

CT head‐scan dosimetry in an anthropomorphic phantom and associated measurement of ACR accreditation‐phantom imaging metrics under clinically representative scan conditions

PURPOSE To measure radiation absorbed dose and its distribution in an anthropomorphic head phantom under clinically representative scan conditions in three widely used computed tomography (CT) scanners, and to relate those dose values to metrics such as high-contrast resolution, noise, and contrast-to-noise ratio (CNR) in the American College of Radiology CT accreditation phantom. METHODS By inserting optically stimulated luminescence dosimeters (OSLDs) in the head of an anthropomorphic phantom specially developed for CT dosimetry (University of Florida, Gainesville), we measured dose with three commonly used scanners (GE Discovery CT750 HD, Siemens Definition, Philips Brilliance 64) at two different clinical sites (Walter Reed National Military Medical Center, National Institutes of Health). The scanners were set to operate with the same data-acquisition and image-reconstruction protocols as used clinically for typical head scans, respective of the practices of each facility for each scanner. We also analyzed images of the ACR CT accreditation phantom with the corresponding protocols. While the Siemens Definition and the Philips Brilliance protocols utilized only conventional, filtered back-projection (FBP) image-reconstruction methods, the GE Discovery also employed its particular version of an adaptive statistical iterative reconstruction (ASIR) algorithm that can be blended in desired proportions with the FBP algorithm. We did an objective image-metrics analysis evaluating the modulation transfer function (MTF), noise power spectrum (NPS), and CNR for images reconstructed with FBP. For images reconstructed with ASIR, we only analyzed the CNR, since MTF and NPS results are expected to depend on the object for iterative reconstruction algorithms. RESULTS The OSLD measurements showed that the Siemens Definition and the Philips Brilliance scanners (located at two different clinical facilities) yield average absorbed doses in tissue of 42.6 and 43.1 mGy, respectively. The GE Discovery delivers about the same amount of dose (43.7 mGy) when run under similar operating and image-reconstruction conditions, i.e., without tube current modulation and ASIR. The image-metrics analysis likewise showed that the MTF, NPS, and CNR associated with the reconstructed images are mutually comparable when the three scanners are run with similar settings, and differences can be attributed to different edge-enhancement properties of the applied reconstruction filters. Moreover, when the GE scanner was operated with the facilitys scanner settings for routine head exams, which apply 50% ASIR and use only approximately half of the 100%-FBP dose, the CNR of the images showed no significant change. Even though the CNR alone is not sufficient to characterize the image quality and justify any dose reduction claims, it can be useful as a constancy test metric. CONCLUSIONS This work presents a straightforward method to connect direct measurements of CT dose with objective image metrics such as high-contrast resolution, noise, and CNR. It demonstrates that OSLD measurements in an anthropomorphic head phantom allow a realistic and locally precise estimation of magnitude and spatial distribution of dose in tissue delivered during a typical CT head scan. Additional objective analysis of the images of the ACR accreditation phantom can be used to relate the measured doses to high contrast resolution, noise, and CNR.

Measurement of the absorbed dose distribution near an 192Ir intravascular brachytherapy seed using a high-spatial-resolution gel dosimetry system

The absorbed dose distribution at sub-millimeter distances from the Best single (192)Ir intravascular brachytherapy seed was measured using a high-spatial-resolution gel dosimetry system. Two gel phantoms from the same batch were used; one for the seed irradiation and one for calibration. Since the response of this gel is energy independent for photons between 20 and 1250 keV, the gel was calibrated using a narrowly collimated (60)Co gamma-ray beam (cross-sectional area ~1 cm(2)). A small format laser computed tomography scanner was used to acquire the data. The measurements were carried out with a spatial resolution of 100 µm in all dimensions. The seed was calibrated at NIST in terms of air-kerma strength. The absorbed dose rate as well as the radial dose function, g(L)(r), was measured for radial distances between 0.6 and 12.6 mm from the seed center. The dose rate constant was measured, yielding a value of Λ = (1.122 ± 0.032) cGy h(-1) U(-1), which agrees with published data within the measurement uncertainty. For distances between 0.6 and 1.5 mm, g(L)(r) decreases from a maximum value of 1.06 down to 1.00; between 1.5 and 6.7 mm, an enhancement is clearly observed with a maximum value around 1.24 and beyond 6.7 mm, g(L)(r) has an approximately constant value around 1.0, which suggests that this seed can be considered as a point source only at distances larger than 6.7 mm. This latter observation agrees with data for the same seed reported previously using Gafchromic film MD-55-2. Additionally, published Monte Carlo (MC) calculations have predicted the observed behavior of the radial dose function resulting from the absorbed dose contributions of beta particles and electrons emitted by the (192)Ir seed. Nonetheless, in the enhancement region, MC underestimates the dose by approximately 20%. This work suggests that beta particles and electrons emitted from the seed make a significant contribution to the total absorbed dose delivered at distances near the seed center (less than 6 mm) and therefore cannot be neglected, given the dimensions of blood vessel walls.

Comparison of the air kerma standards for 137Cs and 60Co gamma-ray beams between the IAEA and the NIST

A comparison of the air kerma standards for 137Cs and 60Co gamma-ray beams was performed between the NIST and the IAEA. Two reference class ionization chambers owned by the IAEA were used as part of this comparison and were calibrated at each facility. The calibration coefficients, NK, were determined for both chambers and in both gamma-ray beams. The measurements were performed at the IAEA and NIST facilities starting in the fall of 2009 and were completed in 2010. The comparison ratio of the calibration coefficients for each chamber, NK,IAEA/NK,NIST, between the IAEA and NIST was 0.999 and 0.997 for the 137Cs and 60Co gamma-ray beams respectively. The relative standard uncertainty for each of these ratios is 0.5%. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by SIM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Measurements and standards for bulk-explosives detection

Recent years have seen a dramatic expansion in the application of radiation and isotopes to security screening. This has been driven primarily by increased incidents involving improvised explosive devices as well as their ease of assembly and leveraged disruption of transportation and commerce. With global expenditures for security-screening systems in the hundreds of billions of dollars, there is a pressing need to develop, apply, and harmonize standards for x-ray and gamma-ray screening systems used to detect explosives and other contraband. The National Institute of Standards and Technology has been facilitating the development of standard measurement tools that can be used to gauge the technical performance (imaging quality) and radiation safety of systems used to screen luggage, persons, vehicles, cargo, and left-behind objects. After a review of this new suite of national standard test methods, test objects, and radiation-measurement protocols, we highlight some of the technical trends that are enhancing the revision of baseline standards. Finally we advocate a more intentional use of technical-performance standards by security stakeholders and outline the advantages this would accrue.

Report on EUROMET.RI(I)-K1 and EUROMET.RI(I)-K4 (EUROMET project no. 813): Comparison of air kerma and absorbed dose to water measurements of 60Co radiation beams for radiotherapy

The results of an unprecedented international effort involving 26 countries are reported. The EUROMET.RI(I)-K1 and EUROMET.RI(I)-K4 key comparisons were conducted with the goal of supporting the relevant calibration and measurement capabilities (CMC) planned for publication by the participant laboratories. The measured quantities were the air kerma (Kair) and the absorbed dose to water (Dw) in 60Co radiotherapy beams. The comparison was conducted by the pilot laboratory MKEH (Hungary), in a star-shaped arrangement from January 2005 to December 2008. The calibration coefficients of four transfer ionization chambers were measured using two electrometers. The largest deviation between any two calibration coefficients for the four chambers in terms of air kerma and absorbed dose to water was 2.7% and 3.3% respectively. An analysis of the participant uncertainty budgets enabled the calculation of degrees of equivalence (DoE), in terms of the deviations of the results and their associated uncertainties. As a result of this EUROMET project 813 comparison, the BIPM key comparison database (KCDB) will include eleven new Kair and fourteen new Dw DoE values of European secondary standard dosimetry laboratories (SSDLs), and the KCDB will be updated with the new DoE values of the other participant laboratories. The pair-wise degrees of equivalence of participants were also calculated. In addition to assessing calibration techniques and uncertainty calculations of the participants, these comparisons enabled the experimental determinations of NDw/NKair ratios in the 60Co gamma radiation beam for the four radiotherapy transfer chambers. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCRI Section I, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

TU‐FF‐A2‐04: New High‐Resolution Method to Measure the 3D Dose Distribution Around Brachytherapy Seeds Using BANG3‐Pro Gel Dosimetry

Purpose: In this work, we propose a new method to measure absorbed dose distribution in 3D imparted by low‐energy photon sources used for cancer treatment using high‐resolution geldosimetry.Method and Materials: BANG3‐Pro (12% Gelatin, 12% proprietary high‐viscosity‐component, 6% Methacrylic Acid and 70% Water) gel, which is a muscle, tissue and water‐equivalent polymergel, originally known as BANG (bis, acrylamide, nitrogen and gelatin) gel has been used as a dosimeter and phantom. We have studied the properties of the gel under different irradiation conditions and measured the dose distribution around three different brachytherapy seeds: 192 Ir , 125 I or 103 Pd , each one with a particular internal geometry. For the seed irradiations, a 200 μm wall Barex tube is embedded inside the gel phantom. To calibrate the gel, a small 60 Co gamma ray field of 1 × 1 cm2 has been used as a reference radiation field. The gel response was quantified using a special high‐resolution DRYOCTOPUS laser CT scanner recently developed by MGS Research Inc. with a resolution of 100 μm in all dimensions for brachytherapy seeds and 330 μm for 60 Co irradiations.Results: The response of the gel to gamma rays is almost linear up to 15 Gy; for higher doses, the response decreases as a function of the time between irradiation and readout (scanning). At low doses, the gel is reproducible and stable. Based on these results and due to the high dose rate gradient at short distances from the sources, the dose distributions were measured using accumulative time exposures in order to evaluate the dose with acceptable accuracy very close to the seed. The spatial uncertainty was about 100 μm. Conclusion: To our knowledge, this study is the first high‐resolution gel measurement of the dose distribution in 3D for brachytherapy seeds at distances under 5 mm from the seeds.