The 54th Annual Meeting of the Health Physics Society
July 12-16, 2009
Minneapolis, MN

Single Session

WAM-C - External Dosimetry Room: L100 D/E   8:30 AM - Noon

Chair(s): Chris Passmore, Peter Caracappa

WAM-C.1   08:30   The new VARSKIN 4 Photon Dosimetry Model of the Skin M C Ryan, Oregon State University ; C L Lodwick*, Oregon State University; D M Hamby, Oregon State University Abstract: Improvements to the current photon dose model have been developed for implementation in an updated VARSKIN 4. The VARSKIN code is a U.S. Nuclear Regulatory Commission (NRC) product used to assess radiation dose to the skin following skin contamination or skin exposure. Upgrades to VARSKIN 3 include an enhanced photon dosimetry model that is based on Monte Carlo simulations of hot-particle contamination. The relationship between kerma and dose was obtained from simulations to develop a correction factor accounting for the lack of charged particle equilibrium at shallow depths, thus providing a more accurate prediction of photon dose. The photon model is implemented such that mathematical formulations, rather than look-up tables, drive the estimation of dose. Various integration schemes for dose averaging were investigated to provide efficient convergence of the solution. The enhanced photon dosimetry model also incorporates parameters of energy, attenuation, dose-averaging area, air gap, protective clothing thickness, as well as simple volumetric sources. With the addition of these parameters, current deficiencies have also been addressed such as creating the capability to calculate dose while accounting for attenuation and correcting the assumption of using the same effective-Z for all materials. An overview of the enhanced photon dose model is presented along with a comparison of results obtained from Monte Carlo, VARSKIN 3, and the new VARSKIN 4.

WAM-C.2   08:45  Measurement of x-ray spectra at NIST as part of a program to establish facility specific air-kerma to dose equivalent conversion coefficients C. G. Soares*, National Institute of Standards and Technology ; C. M. O'Brien, National Institute of Standards and Technology; R. Minniti, National Institute of Standards and Technology Abstract: The air-kerma to dose equivalent conversion coefficients used in standards such as ANSI N13.11 were determined in 1994 and are based on spectrum averages of calculated monoenergetic conversion coefficients using published measured x-ray spectra. The resulting spectrum averages are themselves averaged to obtain the conversion coefficient for a particular beam quality. It has been long recognized that for low energy beam qualities (<30 keV average energy) there can be significant variations from facility to facility in the conversion coefficient from air kerma to personal dose equivalent at 10 mm depth, Hp(10)/Ka. As part of a program to quantify this effect, spectral measurements of all the NIST x-ray calibration beams were begun in 2007. Measurements included use of a commercially available Compton scatter spectrometry system. Spectral measurements of the NIST beams were last performed in 1983 and since then many additions to the list of NIST beam qualities have been made, including the introduction of mammography beam qualities and addition of the ISO 4037 beam qualities. Also, a commercially available secondary standard ionization chamber to measure Hp(10) has been obtained. The aim of the measurement program is to calculate the conversion coefficients from the newly measured and unfolded NIST spectra for the low-energy NIST beams and compare these calculations to direct measurements of the conversion coefficients using the Hp(10) secondary standard chamber. In this way, it is hoped to establish a framework to transfer this measurement method to secondary laboratories so that facility specific air-kerma to dose equivalent conversion coefficients can be established. In this paper, progress on the spectral measurements and their unfolding and analysis is described, and initial measurements with the Hp(10) secondary standard chamber are discussed.

WAM-C.3   09:00  Study of the Linearity, Accuracy, and Precision of Pocket Ionization Chambers R. J. Bergen*, University of Michigan, Ann Arbor ; J. A. Harvey, University of Michigan, Ann Arbor; K. J. Kearfott, University of Michigan, Ann Arbor Abstract: Passive pocket ionization chambers are useful when working in areas where radiation could be present in significant amounts and inexpensive immediate feedback about radiation exposure is needed. They allow for real-time self-monitoring of an individual’s exposure. This experiment was designed to test the linearity, accuracy, and precision of older versions of these radiation detectors that are readily obtainable. Initial tests used 20 United States Civil Defense (CD) V-138 5.16×10-5 C-kg-1 maximum scale pencil dosimeters. These were exposed five times each to a range of exposures: 6.45×10-6 C-kg-1, 1.29×10-5 C-kg-1, 3.87×10-5 C-kg-1, and 4.52×10-5 C-kg-1. They were also exposed ten times each to 2.58×10-5 C-kg-1, the midpoint of their scale. The experiments were conducted with a 3.3 × 1011 Bq 137Cs source with exposure rates characterized using a National Institute of Standards and Technology traceable, calibrated ion chamber to ensure accuracy of the delivered exposure. The dosimeters were suspended on a 40 × 40 × 15 cm3 polymethyl methacrylate phantom. The data were charted, graphed, and a linear fit applied. The accuracy of the dosimeter readings decreased as the exposure level increased. Individual detectors showed excellent repeatability from trial to trial, but there were more significant variations in responses among detectors for a given irradiation. The average percent standard deviation of each dosimeter over five trials was 11.4% at 6.45×10-6 C-kg-1, and 1.40% at 4.52×10-5 C-kg-1. However, the average percent standard deviation of all dosimeters in each trial was 21.7% at 6.45×10-6 C-kg-1 and 6.48% at 4.52×10-5 C-kg-1. This is roughly twice as high and four and a half times as high respectively. Percent error from known exposure values decreased as exposure increased, from 25.6% at 6.45×10-6 C-kg-1 to 8.92% at 4.52×10-5 C-kg-1.

WAM-C.4   09:15  Measurements of radiation detectors made on a tissue equivalent phantom and free in air R. Minniti*, NIST ; L. S. Pibida, NIST; C. G. Soares, NIST Abstract: ANSI N42.49 is a new standard that is being developed to specify technical performance requirements for Personal Emergency Radiation Instruments "PERDs" for Homeland Security and other radiological emergency applications. Such instruments are used to monitor exposure and/or exposure rate to photon radiation. Radiation measurements were performed at the National Institute of Standards and Technology (NIST) to help in the development and validation of this new standard. Since PERDs that meet all ANSI N42.49 requirements do not exist, similar existing types of radiation detectors including electronic dosimeters, radiochromic film cards and thermoluminescent dosimeters were used. To assess the potential need of using phantoms during testing of PERDs, tests were performed in the NIST Cs-137 gamma ray beam calibration facilities. The tests included exposing various types of detectors to a well known air kerma (measured free in air). Each detector was exposed in two different configurations: free in air and mounted on a polymethyl methacralate (PMMA) phantom. The latter configuration simulates the case of the detector being worn on a person's body. The phantom is a rectangular slab of 30 cm x 30 cm x 15 cm as specified in the ANSI N13.11 standard. For both configurations, the detector was placed at the same distance from the radiation source. For the exposure made on the phantom, the detectors were placed right against the PMMA slab. The detector responses made in the presence of the phantom are larger than those made in air. The differences in the responses between the two configurations vary for each type of detector used and are as large as 10 %. The measurements presented here allow one to assess, among other things, the contribution to the measured dose from radiation backscattered from the phantom.

WAM-C.5   09:30  OSL Albedo Neutron Dosimeter Chris Passmore*, Landauer, Inc. ; Dr. Craig Yoder, Landauer, Inc. Abstract: A new OSL dosimeter material which is sensitivity to thermal neutrons using Al2O3:C material has been developed. Al2O3:C has high sensitivity to photons but the neutron capture cross section of Al2O3:C is very small and thus interactions between neutrons and the material are non-existent. The neutron interaction was produced by coating the Al2O3:C material with Li2CO3. The 6Li isotope which is about 7.5% of the natural lithium in Li2CO3, possesses a large thermal neutron cross section. By using 6Li, a charge can be imparted into the aluminum oxide from the neutron interactions with the new material. In this presentation, we will discuss the characteristics of the new material and its application as an albedo neutron dosimeter. Dosimeter response matrix and comparison to original (neutron insensitive) OSL material characteristics will be discussed.

WAM-C.6   09:45  Monte Carlo Modeling of Workers Walking on Contaminated Ground for Accurate Environmental Dosimetry B Han*, Rensselaer Polytechnic Institute ; JY Zhang, Rensselaer Polytechnic Institute; YH Na, Rensselaer Polytechnic Institute; PF Caracappa, Rensselaer Polytechnic Institute; XG Xu, Rensselaer Polytechnic Institute Abstract: One of the challenges in radiation protection dosimetry is the reconstruction of exposures that involve postures not represented in standard external-beam irradiation geometries. Examples of such postures include walking, sitting, bending over, lying, and raised arms. Since the 1960s, nearly 100 computational phantoms have been reported in the literature for studies involving ionizing and non-ionizing radiation. However, the majority of these phantoms were designed to represent an upright standing individual with arms on the sides of the body. This paper describes efforts to develop phantoms that have desirable postures for environmental exposure dosimetry. The modeling is based on a pair of mesh-based computational phantoms, RPI Adult Male and RPI Adult Female, recently developed to represent the ICRP-89 50th-percentile adult males and adult females. Geometric deformation algorithms using the meshes allow the positions of the legs and arms to be adjusted. These walking phantoms were implemented in the MCNPX code to simulate the entire walking cycle and to calculate the organ doses from planar sources of Cs-137 and Co-60. The contamination concentration of 30 kBq/m2 was considered to simulate the deposition on the soil of nuclear fallout or the result of a dirty bomb. Organ dose results show a pronounced difference for doses to several organs. Comparing to the standing phantom, the doses received by the lungs, the gonads, the liver, the thyroid, the brain, the kidneys and the spleen of the walking phantom increased 6.6%, 5.1%, 21.5%, 26.9%, 54.5% and 94.7% respectively for the assumptions made in this study. It is found that the walking postures cause several organs, especially the kidneys and the spleen, to receive more radiation because the legs now provide much less shielding for the source on the ground. The doses received by the gonads of the walking phantom increased only 5.1% because the legs are open in both postures. It is concluded that those specific postures can and should be considered in organ dose calculations in certain environmental dosimetry to improve the accuracy. *The walking phantom was developed with grant supports from National Cancer Institute (R01CA116743) and National Library of Medicine (R01LM009362). Mr. Bin Han was a recipient of the Robert Gardner Fellowship from the Health Physics Society.

WAM-C.7   10:00  The Impact of the ICRP-103 Recommendations: A Dosimetric Study of External Photon and Neutron Beams* Peter F. Caracappa*, Rensselaer Polytechnic Institute ; Juying Zhang, Rensselaer Polytechnic Institute; X. George Xu, Rensselaer Polytechnic Institute Abstract: In its Report # 103, the ICRP has recently revised the radiation and tissue weighting factors previously recommended in Report #60 for the calculation of the effective dose. In this study, organ doses have been calculated with a pair of computational phantoms, RPI Adult Male (RPI-AM) and RPI Adult Female (RPI-AF), which represent the ICRP-89 50th-percentile adult males and adult females. The MCNPX Monte Carlo code was used to simulate photons in the energy range of 0.01 MeV to 10 MeV and neutrons in the range of 0.001 eV to 10 GeV under six standard external exposure geometries: anterior-posterior, posterior-anterior, left lateral and right lateral parallel beams, 360-degree rotational, and isotropic sources. For photons less than 500 keV, the effective doses under ICRP-103 are greater than those under ICRP-60 by as much as 22% for the left and right lateral directions, and by 12% for AP. For the same low photon energies, however, the effective dose for PA is up to 10% lower. For photon energies greater than 500 keV, the difference between the two sets of recommendations is less than 5% in all geometries. For neutron exposures, ICRP-103 and ICRP-60 compare closely in all geometries for energies below 0.01 MeV and greater than 100 MeV. In the intermediate neutron energy range, the ICRP-103 recommendations result in an effective dose that is greater by up to 15% for all geometries, except for the PA geometry which is lower by up to 5%. In both photon and neutron cases, the lower effective dose under ICRP-103 recommendations in the PA geometry is due to the increased weight given to the male gonads which are heavily shielded by the rest of the body in the PA geometry. This finding is also applicable to other phantoms. Comparisons are also made with Effective Dose Equivalents derived from the ICRP-26 recommendations that are currently adopted by the U.S. NRC. * The work was supported in part by a grant from the National Cancer Institute (R01CA116743)

WAM-C.8   10:45  Organ Doses from External Proton Beams Calculated from A Pair of ICRP-89 50th-Percentile Adult Phantoms* Juying Zhang, Rensselaer Polytechnic Institute ; Yong Hum Na, Rensselaer Polytechnic Institute; Bin Han*, Rensselaer Polytechnic Institute; Peter F. Caracappa, Rensselaer Polytechnic Institute; X. George Xu, Rensselaer Polytechnic Institute Abstract: A new set of fluence-to-organ-absorbed-dose conversion coefficients has been calculated for external monoenergetic proton beams from 10 MeV to 10 GeV using a pair of average adult male and female phantoms, the RPI Adult Male (RPI-AM) and Adult Female (RPI-AF) phantoms, developed using a novel surface-geometry modeling method derived from the mesh data library from AnatomiumTM 3D to represent the ICRP-89 50th-percentile adult males and adult females. Doses to organs and tissues from external proton sources using of the RPI-AM and RPI-AF phantoms were calculated to provide data in radiation protection dosimetry involving exposures in deep space and high-energy accelerator environments. Organ dose calculations were performed using the MCNPX Monte Carlo code under six different irradiation geometries: anterior-posterior, posterior-anterior, left-lateral, right-lateral, isotropic and rotational. The absorbed organ doses and the effective dose calculated under ICRP Publication 103 recommendation from the RPI-AM and RPI-AF phantoms are presented. The reported dose dataset is recommended as a reference for radiation protection of the average adult male and female population against external proton exposures. * The project was supported in part by grants from National Cancer Institute (R01CA116743) and National Library of Medicine (R01LM009362). Mr. Bin Han was supported by the Van Auken Research Fellowship from Rensselaer Polytechnic Institute as well as the Robert Gardner Fellowship from the Health Physics Society.

WAM-C.9   11:00  Organ Doses from External Neutron Beams for A Pair of ICRP-89 50th-percentile Adult Phantoms* Juying Zhang, Rensselaer Polytechnic Institute ; Yong Hum Na, Rensselaer Polytechnic Institute; Bin Han, Rensselaer Polytechnic Institute; Peter F. Caracappa*, Rensselaer Polytechnic Institute; X. George Xu, Rensselaer Polytechnic Institute Abstract: A new set of fluence-to-organ-absorbed-dose conversion coefficients has been calculated for external monoenergetic neutron beams from 0.001 eV to 10 GeV using a pair of mesh-based computational phantoms, RPI Adult Male (RPI-AM) and RPI Adult Female (RPI-AF) that were recently developed to represent the ICRP-89 50th-percentile adult males and adult females. The MCNPX Monte Carlo code was used to simulate six standard external neutron exposure geometries: anterior-posterior, posterior-anterior, left lateral and right lateral parallel beams, 360-degree rotational, and isotropic sources. Conversion coefficients from this study are compared with those from ICRP Publication 74 and reported for the VIP-Man phantom. Effective Dose results were also calculated based on recommendations of the ICRP Publication 103. RPI-AM and RPI-AF phantoms were constructed using a novel mesh modeling method that afforded a large degree of flexibility and anatomical realism. More than 100 organs and tissues in each of the phantoms are defined using the ICRP-89 50th-percentile values. The comparison of these results with previously reported data shows about 10% relative error for radio-sensitive organs in most neutron energy range. This paper also discusses the potential for this set of data to be adopted as a reference for radiation protection against external neutron exposures. * The project was supported in part by grants from National Cancer Institute (R01CA116743) and National Library of Medicine (R01LM009362). Mr. Bin Han was supported by the Van Auken Research Fellowship from Rensselaer Polytechnic Institute as well as the Robert Gardner Fellowship from the Health Physics Society.

WAM-C.10   11:15  Dose Response Modeling for Critical Organs In Intensity Modulation Radiation Therapy (IMRT) Treatments ANIL PYAKURYAL*, Northwestern Memorial Hospital/ University of Illinois at Chicago Abstract: The purpose of this study was to explore dose response polynomial models(POLYMODELS) using cumulative dose volume histograms(cDVH) for various critical-organs(ORGANS) based on sequential IMRT boost (SqIB) treatments on twenty head and neck (HN) cancer patients (N=20) . Four different IMRT plans (PTV1, PTV2, PTV3 and COMPOSITE) were designed at prescription doses(PD) of 39-46 Gy,12-15 Gy,12-24 Gy and 64-75 Gy for each patients respectively. The patients were treated at 6 MV X-ray photons.A DVH analysis software called HART (S.Jang et al.,2008,Med Phys 35, p.2812) was utilized to extract DVH statistics and to perform polynomial simulations of the cDVH curves for 26 structures(n=26).Fano-Factor(FF) index was used to scale the tissue inhomogeneity of the ORGANS using POLYMODELS in HART.The impacts of PD on tolerance dose (TD50) of the ORGANS were also analyzed using POLYMODELS for ten HN patients(N=10). FF indices were equal to unity for all POLYMODELS of structures(N=20;n=26) in all plans. POLYMODELS satisfied relatively higher order polynomials for 11 to 16 ORGANS (order>30) and lower order polynomials for 7 to 15 ORGANS (order<30) respectively.Gross Tumor Volume (GTV; FF=1)was used to normalize the relative tissue inhomogeneity of the ORGANS.The POLYMODELS were used to extract the fractional volumes of the ORGANS at TD50 and PD.Risk analysis of the treatments revealed that a largest volume (79%;N=10;n=10) of oropharynx became irradiated at PD. However,larynx was found to be the most vulnerable structure with 98%(N=10;n=10)volume coverage at TD50 (37Gy) and brainstem was the least affected structure at TD50(55 Gy). Polynomial technique can be used for evaluation of the relative inhomogeneity of the ORGANS.The ORGANS with higher inhomogeneity(FF>1) followed relatively higher order POLYMODELS (order>10).Such techniques can be used to create precise dose response models for various critical organs that can be efficiently applied for various studies in radiation therapy.

WAM-C.11   11:30  Neutron and Gamma Measurements within a Mixed Field at the University of Massachusetts Lowell Research Reactor M.C. Talmadge*, UMass Lowell ; G.H.R. Kegel, UMass Lowell; L. Bobek, UMass Lowell Abstract: An experimental procedure used to characterize the neutron and gamma components of a radiation field adjacent to 1 mega-watt research reactor is presented. The process is guided by a standardized approach that uses silicon bipolar transistors as neutron sensors and Calcium Fluoride TLDs as neutron-insensitive gamma dosimeters. The particular transistor used exhibits degradation in electrical current gain that varies linearly with fast neutron exposure as a result of atomic displacements within the device’s Silicon. This property is exploited in order to determine a hypothetical monoenergetic fluence of 1 MeV neutrons that would result in the same level of damage in silicon. For calibration of individual transistors a Van de Graff Accelerator provides a well known fast neutron exposure using a unique target-activation technique. Gamma dose rate measurements that were made within the reactor's mixed radiation field using Calcium Fluoride thermoluminescent material is also discussed, with emphasis on technical challenges encountered, such as TLD activation.

WAM-C.12   11:45  Radio Frequency (RF) Field Strength Fluctuation Due to Digital Conversion of Television Signals: A Pilot Study PB Lane*, Colorado State University ; TE Johnson Abstract: On June 12, 2009 all television stations in the United States will no longer broadcast on analog airwaves and will only broadcast in a digital format. Currently, most stations broadcast both analog and digital signals. We are focusing on a television-transmitting site located on top of Lookout Mountain in Golden, Colorado, which is approximately 10 miles west of the Denver metropolitan area. This site is unique since there are homes located at and above the elevation of the towers and some homes are even located within 100 yards of the towers. There is public concern that the digital transition will result in a significant increase in RF exposure to homes located at these higher elevations. Measurements were taken during daylight hours at 21 locations where highest exposures were expected using an electromagnetic radiation meter. RF field strength measurements were taken at the same locations before and after the June 12th deadline.

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