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.

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.

Key comparison BIPM.RI(I)-K3 of the air-kerma standards of the NIST, USA and the BIPM in medium-energy x-rays.

A key comparison has been made between the air-kerma standards of the NIST, USA and the BIPM in the medium-energy x-ray range. The results show the standards to be in agreement at the level of the standard uncertainty of the comparison of 3.8 parts in 103, except at 250 kV where the difference is 1.5 times the standard uncertainty. The results are analysed and presented in terms of degrees of equivalence, suitable for entry in the BIPM key comparison database.

Validation testing of ANSI/IEEE n42.49 standard requirements for personal emergency radiation detectors.

Various radiation detectors including electronic personal emergency radiation detectors (PERDs), radiochromic film cards and thermoluminescent dosimeters (TLDs) were used to validate a subset of the radiological test requirements listed in the American National Standards Institute/The Institute of Electrical and Electronic Engineers (ANSI/IEEE) N42.49 standard. The subset of tests included the following: comparing the readout of the detectors with the value given at the National Institute of Standards and Technology (NIST); testing of the alarm settings (when applicable) in air-kerma (or exposure) and air-kerma rate (or exposure rate) mode; and investigating the effect of testing the detectors mounted on a phantom and free in air. The purpose of this work was not to test the performance of the sample of detectors used. Instead, the detectors were used to validate the requirements of the written standard being developed. For this purpose, the performance and response of these instruments were recorded when placed in (137)Cs, and x-ray beams at different air-kerma rates and test conditions. The measurements described in this report were performed at the NIST x-ray and gamma-ray radiation calibration facilities. The data in this report provide a benchmark in support of the development of the ANSI/IEEE N42.49 standard.

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.

Zygomycosis in Italy: a survey of FIMUA-ECMM (Federazione Italiana di Micopatologia Umana ed Animale and European Confederation of Medical Mycology).

The aims of the study were to analyze the clinical and epidemiological characteristics and treatments for patients who developed zygomycosis enrolled in Italy during the European Confederation of Medical Mycology of medical mycology survey. This prospective multicenter study was performed between 2004 and 2007 at 49 italian Departments. 60 cases of zygomycosis were enrolled: the median age was 59.5 years (range 1-87), with a prevalence of males (70%). The majority of cases were immunocompromised patients (42 cases, 70%), mainly hematological malignancies (37). Among non-immunocompromised (18 cases, 30%), the main category was represented by patients with penetrating trauma (7/18, 39%). The most common sites of infection were sinus (35%) with/without CNS involvement, lung alone (25%), skin (20%), but in 11 cases (18%) dissemination was observed. According to EORTC criteria, the diagnosis of zygomycosis was proven in 46 patients (77%) and in most of them it was made in vivo (40/46 patients, 87%); in the remaining 14 cases (23%) the diagnosis was probable. 51 patients received antifungal therapy and in 30 of them surgical debridement was also performed. The most commonly used antifungal drug was liposomal amphotericin B (L-AmB), administered in 44 patients: 36 of these patients (82%) responded to therapy. Altogether an attributable mortality rate of 32% (19/60) was registered, which was reduced to 18% in patients treated with L-AmB (8/44). Zygomycosis is a rare and aggressive filamentous fungal infection, still associated with a high mortality rate. This study indicates an inversion of this trend, with a better prognosis and significantly lower mortality than that reported in the literature. It is possible that new extensive, aggressive diagnostic and therapeutic procedures, such as the use of L-AmB and surgery, have improved the prognosis of these patients.

Epidemic multidrug-resistant Acinetobacter baumannii related to European clonal types I and II in Rome (Italy).

The molecular epidemiology and the genetic basis of antibiotic resistance in 88 multidrug-resistant (MDR) Acinetobacter baumannii strains isolated during 18 months from infected patients in seven intensive care units (ICUs) in Rome were investigated. Random amplified polymorphic DNA and macrorestriction analysis identified two predominant clonal types, genetically related to the European epidemic clones I (type 2) and II (type 1), accounting for 98.9% of A. baumannii ICU isolates. Type 1 was isolated from all ICUs under survey. Class 1 integrons of 2.2 and 2.5 kb were detected in type 1 and type 2 isolates, respectively. The integron structures were similar to those previously determined for epidemic A. baumannii strains from various European countries, and suggestive of integron rearrangement/exchange among isolates related to the European epidemic clones I and II. Carbapenem resistance was associated with the presence of the bla(OXA-58) gene in type 1 isolates. The results indicate that the A. baumannii type 1 clone has a high potential of spreading among hospitals.

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.

The air-kerma rate constant: application to air-kerma measurements for homeland security.

Air-kerma rate measurements from 57Co, 60Co, and 137Cs radioactive sources were performed. These measurements were motivated by the development of new sources at the National Institute of Standards and Technology (NIST) for radiological testing of equipment for homeland security applications. The testing of radiation detection equipment relies on knowing the values of the air-kerma rate for the radioactive sources at a fixed distance from the source. The air-kerma rate can be measured or alternatively estimated by using published values of the air-kerma rate constant. Although there are a large number of published values of the air-kerma rate constant for radionuclide sources based on theoretical calculations, strong disagreement is observed throughout the literature. Furthermore, most of the published values have no uncertainties assigned, and therefore their use for testing radiological equipment is limited. In this work we report experimentally-measured values of the air-kerma rate for three radionuclides with well defined source geometries and activities. The results are compared to estimates based on published values of the air-kerma rate constant. Such values are easily found in the literature from the last three decades and are used commonly by the scientific community.

Calibration of a 137Cs gamma-ray beam irradiator using large size chambers.

A (137)Cs gamma-ray beam irradiator has been calibrated in terms of air kerma using large size chambers. The available air-kerma rates range between 1.8 microGy/h (0.2 mR/h) and 5.3 mGy/h (0.6R/h). Large-volume chambers were used to characterize the source in terms of the radiation quantity air kerma (and exposure). Two types of chambers with significantly different characteristics and energy responses were used. This work shows that very good agreement can be obtained between the measurements performed with such different types of chambers. An agreement of 0.3% is observed between chambers even for the lowest air-kerma rates measured.

Technical aspects of the Naval Dosimetry Center quality assurance programme.

The purpose of this paper is to describe the technical aspects of the Naval Dosimetry Center (NDC) quality programme. The Navy has been formally monitoring personnel for occupational exposure to ionising radiation since at least 1946. The current system, the DT-702/PD, is the Harshaw 8840 holder and 8841 card. New card and holder checks are performed to verify that the correct LiF elements and holder filters are in the correct location and are of the correct composition. Element correction coefficient (ECC) magnitude and repeatability are also verified. Several quality assurance parameters are checked by a specially designed shipping machine. Calibration cards are used to calibrate each reader and quality control cards are inserted throughout a group of field cards to verify reader operation during the read process. The success of the programme is measured by annual proficiency tests administered by the National Voluntary Laboratory Accreditation Programme and Pacific Northwest National Laboratories.

The US radiation dosimetry standards for 60Co therapy level beams, and the transfer to the AAPM accredited dosimetry calibration laboratories.

This work reports the transfer of the primary standard for air kerma from the National Institute of Standards and Technology (NIST) to the secondary laboratories accredited by the American Association of Physics in Medicine (AAPM). This transfer, performed in August of 2003, was motivated by the recent revision of the NIST air-kerma standards for 60Co gamma-ray beams implemented on July 1, 2003. The revision involved a complete recharacterization of the two NIST therapy-level 60Co gamma-ray beam facilities, resulting in new values for the air-kerma rates disseminated by the NIST. Some of the experimental aspects of the determination of the new air-kerma rates are briefly summarized here; the theoretical aspects have been described in detail by Seltzer and Bergstrom ["Changes in the U.S. primary standards for the air-kerma from gamma-ray beams," J. Res. Natl. Inst. Stand. Technol. 108, 359-381 (2003)]. The standard was transferred to reference-class chambers submitted by each of the AAPM Accredited Dosimetry Calibration Laboratories (ADCLs). These secondary-standard instruments were then used to characterize the 60Co gamma-ray beams at the ADCLs. The values of the response (calibration coefficient) of the ADCL secondary-standard ionization chambers are reported and compared to values obtained prior to the change in the NIST air-kerma standards announced on July 1, 2003. The relative change is about 1.1% for all of these chambers, and this value agrees well with the expected change in chambers calibrated at the NIST or at any secondary-standard laboratory traceable to the new NIST standard.

Test of radiation detectors used in homeland security applications.

This work was performed as part of the National Institute of Standards and Technology (NIST) program to support the development of the new American National Standards Institute (ANSI) standards N42.32-2003 and N42.33-2003 for hand-held detectors, and personal electronic dosimeters, as well as to support the Office of Law Enforcement Standards (OLES) and the Department of Homeland Security (DHS) in testing these types of detectors for their use by first responders. These instruments are required to operate over a photon energy range of 60 keV to 1.33 MeV and over a wide range of air-kerma rates. The performance and response of various radiation detectors, purchased by the NIST, was recorded when placed in 60Co, 137Cs, and x-ray beams at different air-kerma rates. The measurements described in this report were performed at the NIST x-ray and gamma-ray radiation calibration facilities. The instruments' response (exposure or dose rate readings) shows strong energy dependence but almost no dependence to different air-kerma rates. The data here reported provide a benchmark in support of current protocols that are being developed for radiation detection instrumentation used in homeland security applications. A future plan is to test these devices, plus other commercially available detectors, against ANSI standards N42.32-2003 and N42.33-2003.

The attribution of affect in pain clinic patients: a psychophysiological study of the conversion process.

In a study to examine the relationship between the conversion process and physiological correlates of emotional arousal, three key elements of conversion were assessed separately in patients experiencing chronic pain for which no adequate somatic cause could be demonstrated. Thirty-seven patients referred to a pain clinic were categorized as members of either high, intermediate, or low conversion groups on the basis of their scores on the Disease Conviction, Affective Disturbance and Denial scales of the Illness Behaviour Questionnaire (IBQ). All patients scored in the high range on Disease Conviction. High conversion patients acknowledge little dysphoric affect and denied life problems apart from physical illness. Intermediate conversion patients also denied life problems other than somatic, but acknowledged high levels of dysphoria. Low conversion patients reported dysphoria and acknowledged life problems which they did not attribute to physical illness. The prediction that the high conversion group would show lower levels of resting skin conductance than the low conversion group was confirmed. Patients in the intermediate conversion group resembled those in the high conversion group in that their resting skin conductance was significantly lower than that observed in members of the low conversion group. These findings are consistent with those of previous studies of conversion disorders. They point to the importance in these conditions of the interaction between dysphoria and the cause to which it is attributed by the patient.

The Illness Behavior Assessment Schedule: reliability and validity.

A patient's illness behavior represents an important component of any clinical presentation, and may in itself be pathological. Illness behavior and abnormal illness behavior are not easily evaluated, and considerable disagreement exists as to the criteria for making diagnoses such as "conversion reaction," "hypochondriacal reaction," etc. This paper describes the background, reliability and validity of the Illness Behavior Assessment Schedule (IBAS) which has been developed as a basis for a systematic and standardized approach to illness behavior.