VARSKIN 4: Computer Code System to Assess Skin Dose from Skin Contamination, Version 4.0.0.
Center Department of Nuclear Engineering and Radiation Health Physics, Oregon State University, Corvallis, OR, and the U.S. Nuclear Regulatory Commission.
C++; x86 processors (C00781PCX86000).
VARSKIN 4 code is designed to operate in both Windows® and MacIntosh® environments and is expected to be significantly easier to learn and use than its predecessors. PC and MAC users will unzip different executable files, but the functionality is identical. Five different predefined source configurations are available in VARSKIN 4 to allow simulations of point, disk, cylinder, sphere, and slab sources. Improvements to VARSKIN 4 include an enhanced photon dosimetry model, as well as models to account for air gap and cover materials for photon dosimetry. Although the user can choose any dose-averaging area, the default area for skin dose calculations in VARSKIN 4 is 10 square centimeters, to conform to regulatory requirements pursuant to Title 10 of the Code of Federal Regulations, Section 20.1201(c). Data entry is condensed to a single screen, and the user does not need to enter the data in any particular order. A variety of unit options are provided, including both British and International System (SI) units, and the source strength can be entered in units of total activity or distributed in units of activity per unit area or activity per unit volume. The output page and the user’s ability to add radionuclides to the library are greatly simplified. A library file contains data on photons, internal conversion electrons, and Auger electrons. VARSKIN 4 allows the user to eliminate radionuclides that are not of interest and thus build a customized library. Finally, an extensive, context-sensitive help file is made available to provide guidance and to offer new users a tutorial in the use of VARSKIN.
VARSKIN 4 calculates beta dose using the same method as in VARSKIN Mod 2 and VARSKIN 3. In general, VARSKIN 4 performs a five-dimensional integration of the source volume and the target area. With the exception of the slab model, the integration is simplified significantly because the dose is symmetric for a circular target area centered under the source.
The VARSKIN 4 backscatter correction model also accounts for material located between the source and the skin depth of interest. For sources with a thickness of less than 5 percent of the X99 distance, the BCF changes when the source is not in contact with the skin. The BCF does not change with depth for sources with a thickness greater than 5 percent of the X99 distance in the source material.
Cover materials and air gaps can be modeled using VARSKIN 4. The models use the concept of effective path length to determine the beta energy lost in either a cover material or air before it enters the skin. The path length is not the true path traversed by the beta particle; it is merely a mathematical convenience introduced to provide a measure of the energy lost in each layer. To prevent unintended applications of VARSKIN 4, the air gap is limited to a maximum of 5 cm.
The volume-averaged dose model allows the calculation of dose averaged over a given tissue volume. Any two planes of irradiated skin can be assigned to bound the skin volume. For sources in contact with the skin, the maximum penetration depth for beta particles is equal to the X99 distance. Doses averaged over the dose-averaging area are calculated at 10 skin depths between two limits set by the user, and a cubic spline (a third-order piecewise polynomial curve fit) is fit to this depth-dose distribution. When the user specifies the skin depths corresponding to the volume of interest, VARSKIN 4 integrates the depth dose function over the region of interest to obtain the volume-averaged dose.
The offset particle model allows calculation of skin dose averaged over areas that are not directly beneath the contaminant. This model was developed to determine dose to a single averaging area resulting from multiple hot particles. The offset particle model is available only for the point geometry. It requires only one input variable, the distance of the offset. For multiple particle irradiations, the dose from each particle must be calculated separately, with the user running VARSKIN 4 once for each particle. The offset particle model does not calculate the maximum dose to skin from several particles (Section 6.2 outlines the iterative process for determining the maximum dose to the dose-averaging area); rather, the user must manually add doses from each of the sources to a common dose-averaging disk at depth.
The photon dose model implemented by VARSKIN 4 is new and is an improvement to the basic photon model used in VARSKIN 3. The photon model uses a point kernel method that considers the buildup of CPE, transient CPE, photon attenuation, and off-axis scatter. The photon dose model has many of the basic assumptions carried in the beta dosimetry model, namely that the source can be a point, disk, cylinder, sphere, or slab and that dose is calculated to an averaging disk immediately beneath the surface of skin at a depth specified by the user. Photon dose is calculated for a specific skin averaging area, also specified by the user.
In VARSKIN, radioactive progeny are not included with the parent radionuclide and must be entered explicitly, i.e., selecting 137Cs gives you only that nuclide, 137mBa is not included unless it is specifically called. The user is additionally cautioned to consider the half-life of the progeny when selecting the appropriate dose contribution (decay-corrected or not) from daughter products.
VARSKIN has been shown to be reliable for particulate sources that have dimensions smaller than eight times the X99 distance of the radionuclide in tissue. The X99 distance is essentially 99 percent of the range of beta particles in tissue emitted by nuclides in the source term. When the physical size of the source approaches this value, VARSKIN may give unreliable results. If the source dimensions selected are too large, VARSKIN 4 prompts the user with a warning of the potential for inaccurate results.
VARSKIN has not been tested extensively for dose-averaging areas other than 1 and 10 cm2. However, because of the nature of the calculations performed by VARSKIN, there is no reason to believe that doses to areas less than or greater than 10 cm2 will result in errors. A quick and limited study of dose results as a function of averaging disk area shows that the code appears to be stable and linear in this regard from 0.01 to 100 cm2.
Dose calculations involving air gaps greater than 5 cm have not been tested and are, therefore, not allowed. It is likely that erroneous results may be obtained for large air gaps because the code does not account for multiple scattering events in air. These events may result in the dose being delivered to an area greater than that determined using VARSKIN and can lead to inaccurate results. VARSKIN is limited such that calculations for air gaps greater than 5 cm are not possible and a warning message is displayed.
Run times for VARSKIN 4 vary from a few seconds to several minutes. The code is interactive.
VARSKIN 4 was tested at RSICC on an Intel Xeon processor running Windows 7 and Mac OS 10.5.8
a: Included in documentation:
D. M. Hamby, et al., VARSKIN 4: A Computer Code for Skin Contamination Dosimetry, NUREG/CR-6918/Rev. 1 (June 2011) U.S. Nuclear Regulatory Commission, Washington, D.C.
b: Background information:
F. H. Attix, Introduction to Radiological Physics and Radiation Dosimetry, New York, NY; John Wiley & Sons, 1986.
M. J. Berger, “Distribution of Absorbed Dose Around Point Sources of Electrons and Beta Particles in Water and Other Media,” Medical Internal Radiation Dose Committee, Pamphlet No. 7, Journal of Nuclear Medicine, Vol. 12, Supplement No. 5, pp. 5–22, 1971.
M. J. Berger, J. S. Coursey, M. A. Zucker, and J. Change, “Stopping-Power and Range Tables for Electrons, Protons, and Helium Ions,” ESTAR, PSTAR, and ASTAR databases, National Institute of Standards and Technology. Washington, DC. <http://physics.nist.gov/PhysRefData/Star/Text/contents.html> (February 14, 2006).
W. G. Cross, N. O. Freedman, and P. Y. Wong, “Tables of Beta-Ray Dose Distributions in Water,” AECL 10521, CA9200298, Chalk River Laboratories, Dosimetric Research Branch, Chalk River, Ontario, Canada, 1992.
D. Delacroix, “Beta Particle and Electron Absorbed Dose Calculations for Skin Surface Contaminations,” DCES/SPR/SRI/86–656, Commissariat ŕ l’Energie Atomique Paris, France, 1986 (in French).
J. S. Durham, VARSKIN 3: A Computer Code for Assessing Skin Dose from Skin Contamination. NUREG/CR-6918 (2006), U.S. Nuclear Regulatory Commission, Washington, DC.
J. S. Durham, VARSKIN Mod 2 and SADDE Mod 2: Computer Codes for Assessing Skin Dose from Skin Contamination, NUREG/CR-5873, PNL-7913, U.S. Nuclear Regulatory Commission, Washington, DC, 1992.
J. S. Durham, and M. W. Lantz, “Determination of Gamma Dose Rates and Charged Particle Equilibrium from Hot Particles,” Radiation Protection Management, Vol. 8, No. 3, pp. 35–41, 1991.
International Commission on Radiation Units and Measurements, Tissue Substitutes in Radiation Dosimetry and Measurement, ICRU Report 44, International Commission on Radiation Units and Measurements, Bethesda, MD, 1989.
International Commission on Radiological Protection, The Biological Basis for Dose Limitation in the Skin, Publication 59, Pergamon Press, Oxford, England, 1991.
International Commission on Radiological Protection, Radionuclide Transformations, Publication 38, Pergamon Press, Oxford, England, 1983.
H. E. Johns, and J. R. Cunningham, The Physics of Radiology, 4th Ed., Charles C. Thomas, Springfield, IL, 1983.
D. C. Kocher, and K. F. Eckerman, “Electron Dose-Rate Conversion Factors for External Exposure of the Skin from Uniformly Deposited Activity on the Body Surface,” Health Physics, Vol. 53, pp. 135–141 (1987).
M. W. Lantz, and M. W. Lambert, “Charged Particle Equilibrium Corrections for the Gamma Component of Hot Particle Skin Doses,” Radiation Protection Management, Vol. 7, No. 5, pp. 38–48 (1990).
Los Alamos National Laboratory, X-5 Monte Carlo Team, MCNP—A General Monte Carlo N-Particle Transport Code, Version 5, LA-CP-03-0245, Los Alamos National Laboratory, Los Alamos, NM, 2003.
J. Piechowski, “Dosimetry and Therapy of Skin Contaminations,” CEA–R–5441. Commissariat ŕ l’Energie Atomique, Paris, France, 1988 (in French). .
Radiation Safety Information Computational Center, “MCNP4c2, Coupled Neutron, Electron Gamma 3-D Time-Dependent Monte Carlo Transport Calculations,” CCC–701, Radiation Safety Information Computational Center, Oak Ridge National Laboratory, Oak Ridge, TN 2001.
Radiation Safety Information Computational Center. “NUCDECAY: Nuclear Decay Data for Radiation Dosimetry Calculations for ICRP and MIRD.” CCC-701. Oak Ridge, TN: RSICC. 1995.
W. D. Reece, S. D. Miller, and J. S. Durham, SADDE (Scaled Absorbed Dose Distribution Evaluator), A Code to Generate Input for VARSKIN, NUREG/CR–5276. U.S. Nuclear Regulatory Commission, Washington, D.C., 1989.
F. Rohloff and M. Heinzelmann, “Calculation of Dose Rates for Skin Contamination by Beta Radiation,” Radiation Protection Dosimetry, Vol. 14, pp. 279–287, 1986.
S. Sherbini, J. DeCicco, A. T. Gray, and R. Struckmeyer, “Verification of the Varskin Beta Skin Dose Calculation Computer Code,” Health Physics, Vol. 94, pp. 527–538, 2008.
R. J. Traub, W. D. Reece, R. I. Scherpelz, and L. A. Sigalla, Dose Calculation for Contamination of the Skin Using the Computer Code VARSKIN NUREG/CR-4418, U.S. Nuclear Regulatory Commission, Washington, D.C., 1987.
U.S. Nuclear Regulatory Commission, News Release No. 02-039, http://www.nrc.gov/reading-rm/doc-collections/news/2002/02-039.html, April 2, 2002. Last Accessed March 2006.
The package is transmitted on a CD which includes the referenced document cited in 10a. Windows and Mac OS executables, built-in data libraries and on-line help. QA information is included in manual.
December 1987, revised February 1993, March 1995, October 2004, November 2004 and November 2006, February 2009, May 2009, June 2011
KEYWORDS: BETA-RAY; KERNEL; RADIOACTIVITY; ENERGY DEPOSITION; CONTAMINATION