Exposure standardization of iodine-125 seeds used for brachytherapy.
Accepted: May 1, 1984
A method for calibrating iodine-125 seeds in terms of exposure has been established. The standard free-air ionization ionization: see ion.
Process by which electrically neutral atoms or molecules are converted to electrically charged atoms or molecules (ions) by the removal or addition of negatively charged electrons. chamber, used for measuring soft x rays, was chosen for the measurements. Arrays of four to six seeds were used to enhance the ionization-current-to-background-current ratio. Seeds from an array were measured individually in a re-entrant (programming) re-entrant - Used to describe code which can have multiple simultaneous, interleaved, or nested invocations which will not interfere with each other. This is important for parallel processing, recursive functions or subroutines, and interrupt handling. chamber. The quotient quotient - The number obtained by dividing one number (the "numerator") by another (the "denominator"). If both numbers are rational then the result will also be rational. of the exposure rate for the array by the sum or the ionization currents in the re-entrant chamber is the calibration calibration /cal·i·bra·tion/ (kal?i-bra´shun) determination of the accuracy of an instrument, usually by measurement of its variation from a standard, to ascertain necessary correction factors. factor for the re-entrant chamber. Calibration factors were established for three types of iodine-125 seeds. The overall uncertainty for the seed exposure calibrations is less than 6%.
Key words: calibration; exposure rate; free-air chamber; iodine-125 seed; re-entrant chamber; standards.
The radionuclide radionuclide /ra·dio·nu·clide/ (-noo´klid) a nuclide that disintegrates with the emission of corpuscular or electromagnetic radiations.
n. iodine-125, encapsulated encapsulated Localized Oncology adjective Confined to a specific area, surrounded by a thin layer of fibrous tissue; encapsulation generally refers to a tumor confined to a specific area, surrounded by a capsule. See Islet encapsulation. in titanium titanium (tītā`nēəm, tĭ–) [from Titan], metallic chemical element; symbol Ti; at. no. 22; at. wt. 47.88; m.p. 1,675°C;; b.p. 3,260°C;; sp. gr. 4.54 at 20°C;; valence +2, +3, or +4. seeds, is used for brachytherapy brachytherapy /brachy·ther·a·py/ (-ther´ah-pe) treatment with ionizing radiation whose source is applied to the surface of the body or within the body a short distance from the area being treated. . Presently the seeds are characterized char·ac·ter·ize
tr.v. character·ized, character·iz·ing, character·iz·es
1. To describe the qualities or peculiarities of: characterized the warden as ruthless.
2. by stating a range for the activities of a group of seeds, with the stated activity computed from measurements made external to the seeds. In order to achieve traceability to NBS (National Bureau of Standards) See NIST.
NBS - National Bureau of Standards: part of the US Department of Commerce, now NIST. exposure standards, a manufacturer requested that NBS establish an iodine-125 calibration service for the seeds. Calibration data in terms of exposure for this radionuclide would then be consistent with calibrations for the radionuclides cobalt-60, cesium-137, and iridium-192. This procedure has also been requested by medical physicists Below is a list of famous physicists. Many of these from the 20th and 21st centuries are found on the list of recipients of the Nobel Prize in physics. A
Three types of iodine-125 seeds were supplied for measurement. One type incorporates a gold marker which separates two resin spheres on which the radionuclide is adsorbed. The dimensions of the seed and of the spheres are shown in figure 1. The spheres are free to move in the seed as much as 1.5 mm with the result that the distribution of radiation near the end welds can change considerably.
A second type of source, also shown in figure 1, is a seed with the same dimensions as the gold-marker type, but with the radionuclide adsorbed on a silver wire 3.0 mm in length. The movement of the wire in the seed is restricted to 0.5 mm.
The third type of source, with higher activity than in the other two types, is one in which the gold marker sphere is replaced by another iodine-125-coated resin ball.
Mobility of the active material in the seeds does not appear to be an important factor in the primary standardization standardization
In industry, the development and application of standards that make it possible to manufacture a large volume of interchangeable parts. Standardization may focus on engineering standards, such as properties of materials, fits and tolerances, and drafting of the seeds, where good geometry measurements are made with a standard free-air ionization chamber, but movement of the active material affects the reproducibility of measurements in [pi] geometry, as in the re-entrant ionization chamber used as a laboratory standard.
Standard Free-Air Chamber:
Measurement Conditions and Corrections
The instrument most nearly suitable for exposure measurements of radiation from iodine-125 is the Ritz Ritz
elegant and luxurious hotel opened in Paris in 1898 by César Ritz; hence, ‘ritzy, putting on the ritz.’ [Fr. Hist.: Wentworth, 429]
See : Luxury  free-air ionization chamber (FAC FAC - Functional Array Calculator. An APL-like language, but purely functional and lazy. It allows infinite arrays.
["FAC: A Functional APL Language", H.-C. Tu and A.J. Perlis, IEEE Trans Soft Eng 3(1):36-45 (Jan 1986)]. ). This is the standard chamber used for all NBS instrument calibrations for x rays in the 20-kV to 100-kV region. Important factors in the use of the FAC for the iodine-l25 measurements are the defined air volume of the chamber (about 5.5 [cm.sup.3]) and the mean background current (about 1.6 fA). With these constraints CONSTRAINTS - A language for solving constraints using value inference.
["CONSTRAINTS: A Language for Expressing Almost-Hierarchical Descriptions", G.J. Sussman et al, Artif Intell 14(1):1-39 (Aug 1980)]. , an iodine iodine (ī`ədīn, –dĭn) [Gr.,=violet], nonmetallic chemical element; symbol I; at. no. 53; at. wt. 126.9045; m.p. 113.5°C;; b.p. 184.35°C;; sp. gr. 4.93 at 20°C;; valence −1, +1, +3, +5, or +7. source of at least 400 MBq (12 mCi) would be required to provide measurement conditions of minimal acceptability, i.e., background corrections no greater than 10% of the readings and a source-to-chamber distance of no less than 0.25 m. The limit on improvement of the measurement conditions is governed by the available activity per seed and the physical size of the source relative to the FAG volume-defining aperture An orifice. It often refers to an opening in which light is allowed to pass in optical systems such as cameras and lasers. See f-stop and numerical aperture. .
The important FAG dimensions, and corrections relevant to exposure measurements of iodine-125, are given in table 1. The photon-scattering and electron-loss corrections are those developed for measurements of 30-kV x rays filtered by 0.5 mm Al. There is no significant change in those corrections for x-ray energies down to 20 kV. The air attenuation Loss of signal power in a transmission.
The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities. correction given is based on measurements using the iodine-125 radiation. The magnitude of the air attenuation correction is in general agreement with corrections for x rays in the same energy region but is greater than the value calculated using mass attenuation coefficients The attenuation coefficient, is a basic quantity used in calculations of the penetration of materials by quantum particles. Linear Attenuation Coefficient
The Linear attenuation coefficient, also called the narrow beam attenuation coefficient (3) weighted for the source spectrum. The linear attenuation coefficient for air at room conditions based on the measurements is 1.5 X [10.sup.-3] [cm.sup.-1]. The calculated coefficient coefficient /co·ef·fi·cient/ (ko?ah-fish´int)
1. an expression of the change or effect produced by variation in certain factors, or of the ratio between two different quantities.
2. is 0.4 X [10.sup.-3] [cm.sup.-1]. In all subsequent computations, the measured coefficient was chosen to correct for air attenuation in the FAG and for attenuation between the sources and the defining plane o f the FAG aperture.
Source Arrays and Positioning Device
The maximum activity per seed (about 1.3 GBq, 36 mCi) is provided by the no-marker type seed (see table 2). To reduce the importance of background currents on the FAC measurements, the seeds were measured in groups.
Seeds of the same type were mounted on transparent tape and supported on a frame inside a thick-walled aluminum cylinder. They were affixed af·fix
tr.v. af·fixed, af·fix·ing, af·fix·es
1. To secure to something; attach: affix a label to a package.
2. to the tape in a uniform array and the frame was positioned in the aluminum cylinder such that the array was in line with two diametrically di·a·met·ri·cal also di·a·met·ric
1. Of, relating to, or along a diameter.
2. Exactly opposite; contrary.
di opposite holes in the cylinder wall. The frame can be rotated rotated
turned around; pivoted.
see rotated tibia. while the center of the array is maintained at the center of rotation center of rotation,
n a point or line around which all other points in a body move. . The source array when in a vertical position is seen by the FAC as a source with dimensions 4.5 mm high and 0.8n mm wide, where n is the number of sources in the array. The number of seeds of each type ranged from four to six. The diameter of the FAG aperture is 10 mm, so these array dimensions do not present source-chamber alignment problems.
Two series of the three types of sources were measured on separate occasions. Information regarding the various groups is provided in table 2.
Source Array Exposure Measurements
Although the source positioning device allows rotation of the source arrays, the main thrust of the FAG exposure .measurements was to establish a mean exposure rate from an array of sources for the direction perpendicular to the plane of the array. This approach was taken since the eventual calibration of an individual seed will be given as a mean exposure rate at unit distance for the direction perpendicular to the long axis long axis
A line parallel to an object lengthwise, as in the body the imaginary line that runs vertically through the head down to the space between the feet. of the seed. To determine the mean exposure rate for a group of seeds, their positions and orientations were randomized ran·dom·ize
tr.v. ran·dom·ized, ran·dom·iz·ing, ran·dom·iz·es
To make random in arrangement, especially in order to control the variables in an experiment. on the transparent tape to make different arrays. The results of the first series of measurements for the gold-marker seeds are given in table 3. (2) The mean for the three arrays at 0.50 m differs from the mean for the three arrays at 0.25 m by 1.5%, but the standard deviation In statistics, the average amount a number varies from the average number in a series of numbers.
(statistics) standard deviation - (SD) A measure of the range of values in a set of numbers. of the mean for the 0.50 m data is 0.9%. Since different arrays were used (with one exception) for measurements at the two distances, the mean value for the exposure rate, for this group of seed s, is taken as the mean of the measurements for all the arrays. The mean for the arrays of gold-marker type sources in the first series is 2.336 [micro]R [m.sup.2]/s on 1980 Dec 6.5. It is unimportant un·im·por·tant
Not important; petty.
unim·portance n. that array number 3 is included twice in computing computing - computer the mean exposure rate.
All decay corrections for this work were computed using a half-life of 58.9 days for iodine-125. This half-life was determined when measurements of five seeds, taken 145 days apart and corrected using a half-life of 60.14 days (5), were all found to be inconsistent with earlier measurements. The average difference was 3.6%, with a standard deviation of the mean of 0.2%. The long-term stability The long-term stability of an oscillator, the degree of uniformity of frequency over time, when the frequency is measured under identical environmental conditions, such as supply voltage, load, and temperature. for the measurement system used was shown, by 10 radium-226 check-source measurements, to have a standard deviation of the mean of 0.3%. Correcting the apparent activity and exposure data for decay, the exposure rate at unit distance per unit of apparent activity is 41.3 nR [m.sup.2]/"mCi"s.
Calibration of a Re-Entrant Chamber for Routine Individual Source Measurements
A spherical spher·i·cal
Having the shape of or approximating a sphere; globular. aluminum re-entrant ionization chamber  was chosen as the means for transferring the FAC exposure data, for the mean exposure rate from an array, to exposure data for individual seeds. The original brass tube in the chamber was replaced with an aluminum tube with walls 0.64 mm thick. The new tube has an inside diameter Inside diameter is the diameter of the addendum circle of an internal gear.1
1. ANSI/AGMA 1012-G05, "Gear Nomenclature, Definition of Terms with Symbols". slightly greater than the length of a seed and has a flat bottom. When the seed is dropped into the tube, it will lie horizontally and be constrained con·strain
tr.v. con·strained, con·strain·ing, con·strains
1. To compel by physical, moral, or circumstantial force; oblige: felt constrained to object. See Synonyms at force.
2. to take a position near the center.
The calibration of the re-entrant chamber consists in determining the quotient of the FAC exposure data, for an array of seeds, by the sum of the ionization currents produced in the chamber by individual seeds. The result is a calibration factor for the chamber in terms of exposure at unit distance per unit charge.
The mean exposures measured by the FAC are representative of exposure data for random seed orientations in the arrays. It is therefore necessary to randomize ran·dom·ize
tr.v. ran·dom·ized, ran·dom·iz·ing, ran·dom·iz·es
To make random in arrangement, especially in order to control the variables in an experiment. the seeds in the re-entrant tube to determine a mean ionization current for each seed. The variability of the currents with seed position is shown in table 4. The standard deviation of the mean for seed number 4 is a factor of 10 greater than the same statistic statistic,
n a value or number that describes a series of quantitative observations or measures; a value calculated from a sample.
a numerical value calculated from a number of observations in order to summarize them. for measurements of an undisturbed un·dis·turbed
Not disturbed; calm.
1. quiet and peaceful: an undisturbed village
The re-entrant chamber current measurements must be corrected for recombination recombination, process of "shuffling" of genes by which new combinations can be generated. In recombination through sexual reproduction, the offspring's complete set of genes differs from that of either parent, being rather a combination of genes from both parents. . Tests show the correction factor is about 1.004 for currents of about 100 pA and unity for currents of about 0.8 pA. Interpolation interpolation
In mathematics, estimation of a value between two known data points. A simple example is calculating the mean (see mean, median, and mode) of two population counts made 10 years apart to estimate the population in the fifth year. for currents of about 20 pA (table 4) indicates the recombination correction is 1.001.
With the exposure data and re-entrant chamber currents corrected to the same date, the re-entrant chamber calibration factor for the first series of gold-marker type seeds is 3.842 kR [m.sup.2]/C.
Iodine-125 Spectrum and Exposure Calculations
It is of interest to test the consistency of the iodine-125 exposure measurements with exposure data determined from other physical quantities. Such a check can be accomplished by comparison of exposure measurements with exposure data calculated from measurements of the iodine-125 spectrum, after attenuation in the seed wall, and decay data.
Iodine-125 decays by electron capture Electron capture
The process in which an atom or ion passing through a material medium either loses or gains one or more orbital electrons. In the passage of charged particles (defined here as nuclei having more or less than Z atomic electrons, where , producing tellurium tellurium (tĕlr`ēəm) [Lat.,=earth], semimetallic chemical element; symbol Te; at. no. 52; at. wt. 127.60; m.p. 450°C;; b.p. 990°C;; sp. gr. 6. x-rays, in addition to 0.0667 gamma rays Gamma rays
Electromagnetic radiation emitted from excited atomic nuclei as an integral part of the process whereby the nucleus rearranges itself into a state of lower excitation (that is, energy content). per transition. Recent nuclear-decay data (5) are given in table 5, where energy-absorption data (3) are also listed. The equation used to compute To perform mathematical operations or general computer processing. For an explanation of "The 3 C's," or how the computer processes data, see computer. the exposure rate at unit distance from a point source of unit activity in vacuum, is as follows:
[[GAMMA].sub.0] = X[L.sub.2]/A = 1/4[pi]W/e [summation summation n. the final argument of an attorney at the close of a trial in which he/she attempts to convince the judge and/or jury of the virtues of the client's case. (See: closing argument) over (i)] [(PE [[micro].sub.en]/[rho].sub.i]
where X is the exposure rate at a distance L from a point source in vacuum
A is the activity of the source
W/e is the mean energy expended ex·pend
tr.v. ex·pend·ed, ex·pend·ing, ex·pends
1. To lay out; spend: expending tax revenues on government operations. See Synonyms at spend.
2. per unit charge in air (33.7 J/C J/C Just Curious
J/C Just Checking
J/C Just Chilling )
P is the fraction of the nuclear transitions giving rise to photons of the associated energy
E is the photon energy
[[micro].sub.en]/[rho] is the mass energy-absorption coefficient for air for photons of the associated energy.
The exposure rate at unit distance per unit activity computed using the above equation and the data in table 5 is 42.2 nR [m.sup.2]/mCi s (after conversion to special units). The weighted mean uncertainty, for the value of P in table 5, is 1.8%, while the uncertainty given for the [u.sub.en]/P data is 2% in this energy region. Obviously values [GAMMA].sub.0] of will result if different data for P, E, and [u.sub.en]/P are used. A commonly used value is 40.3 nR [m.sup.2]/mCi s (6).
Silver-Wire and No-Marker Type Seeds
The measurement procedures established for the gold-marker seeds were also used for the silver-wire and no-marker type seeds. Six arrays were measured for each type. The exposure data for the silver-wire type seeds are shown in table 6. These data are calculated from measurements at 0.25 m from the arrays. Table 7 gives the re-entrant chamber current measurements for each of the five silver-wire type seeds. The mean currents listed are the result of three independent sets of measurements, with each set the result of measurements for many randomized seed positions. With a correction for recombination of 1.004, the sum of the source currents is 562.4 pA. Correcting for decay to a consistent date and dividing the FAC exposure data by the sum of the re-entrant chamber currents gives a calibration factor of 4.467 kR [m.sub.2]/C for the first series of silver-wire type seeds.
The quotient of the exposure data, by the apparent activity given in table 2 for the same date is 40.64 nR [m.sub.2]/"mCi"s.
Exposure data for the first series no-marker type seeds are given in table 8. Six arrays of the four seeds, in randomized positions on the tape, were measured. The re-entrant chamber data are given in table 9 where the currents are corrected for recombination (1.004). The sum of these currents is 1.364 nA, and the quotient of the exposure data and re-entrant chamber data is 3.7 16 kR [m.sub.2]/C.
With corrections for decay, the quotient of the exposure data by the apparent activity given in table 2 is 38.30 nR [m.sub.2]/"mCi"s.
Second Series, Au-Marker, Ag-Wire and No-Marker ' Source Measurements
Completely independent sets of measurements for each of the three types of iodine-125 seeds were carried Out to check the validity of the initial re-entrant chamber calibration factors. It was discovered that some scattered Scattered
Used for listed equity securities. Unconcentrated buy or sell interest. radiation from the cylindrical cyl·in·dri·cal
Of, relating to, or having the shape of a cylinder, especially of a circular cylinder. aluminum source container was being measured by the FAC and that a correction of 0.993 to the exposure data was required. At this time, the half-life of 58.9 days was introduced and recombination corrections for the re-entrant chamber were measured and used in the calculations. (All these improvements are incorporated in the data given for the first series of measurements.) For the second series of measurements, a lead plate 3 mm thick, and with an aperture matching the aperture in the aluminum cylinder, was used to eliminate the effect of scattered radiation on the FAC exposure measurements.
The measurement procedures established in the first series were for arrays of sources exclusively. In the second series, by using improved equipment it was possible to perform FAG measurements for single seeds as well. The data show that the FAG array measurements are not larger than the sum of the data for the individual seeds in the array. The data for the gold-marker seeds are given in table 10. The mean exposure rate, for the group of seeds, used to compute the re-entrant chamber calibration factor is the mean for all the measurements, i.e., the sums of the individual source measurements are averaged with the group measurements since there appears to be no significant statistical difference.
The results of many re-entrant chamber measurements of the gold-marker seeds are given in table 11.
The re-entrant chamber calibration factor is the quotient of the mean of the exposure data from table 10 by the sum of the re-entrant chamber currents corrected for recombination, i.e., 3.872 kR [m.sup.2]/C.
The data for the second series of measurements using the silver-wire sources are given in tables 12 and 13. The calibration factor for the re-entrant chamber computed from these data is the quotient of the mean for all FAG measurements by the sum of the mean re-entrant chamber currents corrected for recombination, or 4.400 kR [m.sup.2]/C.
The data for the second series of measurements for the no-marker type sources are given in tables 14 and 15. There is almost 2% difference between the single source sum data and the array data for exposure measurements. Since no significant difference for the two types of measurements was observed for the gold-marker and silver-wire sources, the large difference is believed to be random, and all data are included in the average for the exposure rate at unit distance. The quotient of the mean for the exposure data by the sum of the re-entrant chamber currents for the sources, corrected for recombination, is 3.701 kR [m.sup.2]/C.
Summary of Calibration Factor Data for Re-Entrant Chamber
The calibration factors for the re-entrant chamber are summarized in table 16. The mean difference between the two independent sets of determinations of the calibration factors, referenced to their means, is 0.9%
Comparison of Iodine-125 Source Calibrations: Exposure and Activity
A direct comparison of the exposure measurements and exposure data calculated from spectral spectral /spec·tral/ (spek´tral) pertaining to a spectrum; performed by means of a spectrum.
Of, relating to, or produced by a spectrum. measurements was made possible through the cooperation of the NBS Radioactivity radioactivity, spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications. Group. Two sources, one a gold-marker type and one a silver-wire type, were calibrated cal·i·brate
tr.v. cal·i·brat·ed, cal·i·brat·ing, cal·i·brates
1. To check, adjust, or determine by comparison with a standard (the graduations of a quantitative measuring instrument): after decaying to activities within the range of radioactivity measurement equipment. The data are provided in tables 17 and 18 where n is the number of photons per unit time. The two sources were then calibrated for exposure using the re-entrant chamber and the calibration factors appropriate to their types. A comparison of the results is shown in table 19. It should be noted that although little self-scattered radiation was observed in the radioactivity measurement it is not counted, whereas the FAC with no discrimination will measure self-scattered radiation.
A problem associated with the use of activity as a measure for iodine-125 seeds is illustrated by the different values for this quantity which can be calculated from the spectrum for the gold-marker source (table 17) and decay data (table 5). Using A =n/P and the data for the 35.5-ke V gamma ray gamma ray
Penetrating very short-wavelength electromagnetic radiation, similar to an X-ray but of higher energy, that is emitted spontaneously by some radioactive substances (see gamma decay; radioactivity). , the computed activity is 2.5 MBq (69 [micro]Ci). If the data for the Te K[beta] 31-ke V x ray are used, the activity is 2.9 MBq (78 [micro]]Ci); if the Te K[alpha] data are used, the activity is 2.7 MBq (72 [micro]Ci). As a result, calculated gamma-ray exposure constants will differ depending on the reference radiation.
Directional In one direction. Contrast with omnidirectional. Dependence of Radiation from Iodine-125 Source Arrays
The calibration procedure for the iodine-125 seeds is designed to provide data for radiation emitted perpendicular to the seed axis. Since the seeds are implanted im·plant
v. im·plant·ed, im·plant·ing, im·plants
1. To set in firmly, as into the ground: implant fence posts.
2. in tissue, the variation of exposure with direction from the source is of interest. Measurement of this characteristic was carried Out by rotating ro·tate
v. ro·tat·ed, ro·tat·ing, ro·tates
1. To turn around on an axis or center.
2. the source arrays through 180[degrees] by means of the device already described. The exposure rate from a source array was found to be nearly a linear function of the sine of the azimuthal az·i·muth
1. The horizontal angular distance from a reference direction, usually the northern point of the horizon, to the point where a vertical circle through a celestial body intersects the horizon, usually measured clockwise. angle of the array (at 0 degrees, the source axes axes
[L., Gr.] plural of axis. The straight lines which intersect at right angles and on which graphs are drawn. Usually the horizontal axis is the x-axis and the vertical one the y-axis. Called also axes of reference. are parallel to the axis of the the diameter of the sphere which is perpendicular to the plane of the circle.
See also: Axis FAC). Normalized exposure data and [X.sub.n] =f(sin [theta Theta
A measure of the rate of decline in the value of an option due to the passage of time. Theta can also be referred to as the time decay on the value of an option. If everything is held constant, then the option will lose value as time moves closer to the maturity of the option. ]) are shown in figures 2 and 3. The data were normalized to unity at [theta]=[pi]/2. The constants for the equations result from a least-squares fit to the data of a first order polynomial polynomial, mathematical expression which is a finite sum, each term being a constant times a product of one or more variables raised to powers. With only one variable the general form of a polynomial is a0xn+a in sin [theta]. A mean exposure rate from a seed can be computed by averaging f(sin [theta]) over all solid angles so that
[X.sub.n] = [[integral].sup.[pi].sub.o] [X.sub.n] sin [theta]d[theta]/[[integral.sup.[pi].sub.o] sin [theta]d[theta]
The values for the constants and for [X.sub.n] are given in table 20. Higher order polynomials provide equations which fit the data better but the mean value for [X.sub.n] does not change when they are used. The mean exposure rates given in table 20 are for normalized data and are fractions of the exposure rates perpendicular to the long axis of the seeds. The results are in general agreement with the results of other authors (7,8).
Uncertainties for Iodine-125 Exposure Standardization
The uncertainty for an exposure-rate calibration of an iodine-125 seed is dependent on the seed type. For each type, all random uncertainties, represented by standard deviations, and all remaining estimated uncertainties, which are treated like standard deviations, are added in quadrature quadrature, in astronomy, arrangement of two celestial bodies at right angles to each other as viewed from a reference point. If the reference point is the earth and the sun is one of the bodies, a planet is in quadrature when its elongation is 90°. to provide an estimate of the combined uncertainty. The random. uncertainties in the determination of the re-entrant chamber calibration factor are the standard deviation of the mean for the exposure rate measurements of the group of sources, and the standard deviation of the mean for the total current in the reentrant re·en·trant also re-en·trant
Reentering; pointing inward.
A reentrant angle or part.
Adj. 1. reentrant - (of angles) pointing inward; "a polygon with re-entrant angles"
re-entrant chamber. These are given in table 21 for the first and second series of measurements. In this table, n is the number of sources in the array. The percentage given is the square root of the sums of the squares of the percent standard deviations of the means for each source. The standard deviation of the mean for each source is calculated from source measurements for random positions in the re-entrant tube.
The estimated uncertainties for the standard free-air chamber and re-entrant chamber are 1.2% (2) and 0.2%, respectively. The random uncertainties and the estimated uncertainties are added in quadrature to form a combined uncertainty. To the combined uncertainty for the re-entrant chamber calibration factor must be added the random uncertainty for the mean of several measurements of a seed in the re-entrant chamber during a calibration. This is estimated to be 0.4%.
The combined uncertainty is multiplied by 2 to give an overall uncertainty that corresponds roughly to a 95% confidence interval confidence interval,
n a statistical device used to determine the range within which an acceptable datum would fall. Confidence intervals are usually expressed in percentages, typically 95% or 99%. . The combined and overall uncertainties are given in table 22.
The author wishes to acknowledge the many helpful suggestions of Dr. Robert Loevinger in editing this report.
About the Author: T. P. Loftus is a physicist in the Dosimetry dosimetry /do·sim·e·try/ (do-sim´e-tre) scientific determination of amount, rate, and distribution of radiation emitted from a source of ionizing radiation, in biological d. Group of the NBS Radiation Physics Division.
(1.) Figures in brackets brackets: see punctuation. indicate the literature references at the end of this paper.
(2.) The Si unit of exposure is the coulomb coulomb (k`lŏm) [for C. A. de Coulomb], abbr. coul or C, unit of electric charge. The absolute coulomb, the current U.S. per kilogram kilogram, abbr. kg, fundamental unit of mass in the metric system, defined as the mass of the International Prototype Kilogram, a platinum-iridium cylinder kept at Sèvres, France, near Paris. (C/kg). This Unit is not in general use, and the quantity exposure will probably be replaced by the quantity air kerma. which has the unit gray (Gy). As a result, the special unit roentgen roentgen /roent·gen/ (rent´gen) the international unit of x- or ?-radiation; it is the quantity of x- or ?-radiation such that the associated corpuscular emission per 0. (R) is used for exposure in this paper.
(1.) National Council on Radiation Protection and Measurements Report No. 41,23 pages; April 1974. Specification of gammaray brachytherapy sources. NCRP (Network Computer Reference Profile) The specification for network computer compliance established by Oracle and endorsed by Sun, IBM and others. The first version of this specification was known as the NC1 Reference Profile. See network computer. Publications, 7910 Woodmont Ave., Suite 1016, Washington, DC 20014.
(2.) Ritz, V. H. Standard free-air chamber for the measurement of low energy x rays (20 to 100 kilovolts-constant-potential). J. Res. Natl. Bur. Stand. (U.S.). 64C: 49-53; 1960.
(3.) Hubbel, J. H. Photon mass attenuation and mass energy-absorption coefficients from 1 keV to 20 MeV. Int. J. Appl. Radiat. Isot. 33: 126-1290; 1982.
(4.) Loftus, T. P. Standardization of iridium-192 gamma-ray sources in terms of exposure. J. Res. Natl. Bur. Stand. (U.S.). 85: 19-25; (1980).
[5.) National Council on Radiation Protection and Measurements Report No. 58, 506; 1978. A handbook of radioactivity measurements procedures. NCRP Publications, 7910 Woodmont Ave., Suite 1016, Washington, DC 20014.
(6.) Krishnaswamy, V. Dose distribution around an iodine-125 seed source in tissue. Radiology radiology, branch of medicine specializing in the use of X rays, gamma rays, radioactive isotopes, and other forms of radiation in the diagnosis and treatment of disease. 126: 489-491; 1978.
(7.) Sondhaus, C. A. Modern interstitial In a separate window. See interstitial ad.
(World-Wide Web) interstitial - A World-Wide Web page that appears before the expected content page. Interstitials can be used for advertising (intermercial, transition ad) or to confirm that the user is old enough to view the and intracavitary radiation management. New York New York, state, United States
New York, Middle Atlantic state of the United States. It is bordered by Vermont, Massachusetts, Connecticut, and the Atlantic Ocean (E), New Jersey and Pennsylvania (S), Lakes Erie and Ontario and the Canadian province of : MASSON Publishing, Inc., 1981. Chap. 9, pp. 83-101.
(8.) Ling ling: see cod. , C. C., et al. Physical dosimetry of [blank.sup.125.I] seeds of a new design for interstitial implant implant /im·plant/ (im-plant´) to insert or to graft (tissue, or inert or radioactive material) into intact tissues or a body cavity. . Int. J. Radiat. Oncol. Biol. & Phys. In press.
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[Figure 3 omitted]
Table 1. FAC dimensions, and corrections for measurement of iodine-125 radiation. Diaphragm area 0.7867 [cm.sup.2] Collector plate length 7.0003 cm Air attenuation length 12.7 cm Corrections: Air attenuation 1.018 Photon scattering 0.995 Electron loss 1.000 Recombination 1.000 Table 2. Iodine-125 seeds used for exposure measurements. Number Apparent Assay Type of seeds activity (1) date ("mCi") First series Gold-marker 4 60 1980/12/01 No marker 4 142 1982/09/06 Silver-wire 5 114 1981/06/01 Second series Gold-marker 6 84 1983/02/21 No marker 4 103 1983/03/07 Silver-wire 6 95.4 1983/02/21 (1)The apparent activity, as determined by the 3M Co., is a measure of the effective radiation output from the seeds and is independent of seed wall thickness or source self-absorption. The actual activity in the seed is greaser than the apparent activity. The apparent activities are taken from the labels on the 3M lead acontainers. The 3M Co. has used the term "mCi Camp." for apparent activity. Table 3. First series exposure rate measurements for gold-marker type seeds ([micro]R [m.sup.2]/s) FAC Mean dist. Number of Array for Array (m) measurements (1) means arrays 1 0.50 4 2.281 2 0.50 1 2.350 2.320 3 0.30 7 2.328 3 0.25 5 2.350 4 0.25 1 2.359 2.353 5 0.25 3 2.351 (1)Each measurement consists of an independent series of current measurements taken at different times. Table 4. First series gold-marker type iodine-125 seed measurements in the re-entrant ionization chamber. Data corrected so STP and reference time 1981 June 1.5. Mean current Standard Source for random Number of seed deviation of number seed position (pA) positions the mean (%) 1 18.50 6 1.0 2 18.91 5 0.3 3 18.76 5 1.2 4 19.52 4 1.9 Table 5. Nuclear-decay and energy-absorption data used to calculate [[GAMMA].sub.0] for iodine-125. Energy Number per Radiation E transition Radionuclide type (keV) P Tellurium-125 x ray K[[alpha].sub.2] 27.2 0.398 Tellurium-125 x ray K[[alpha].sub.1] 27.5 0.742 Tellurium-125 x ray K[beta] 31 0.258 Iodine-125 [gamma] ray 35.5 0.0667 Energy-absorption coefficient [[micro].sub.en]/ E P [[micro].sub.en]/ [rho] [rho] Radionuclide ([m.sup.2]/kg) (keV [m.sup.2]/kg) Tellurium-125 0.0212 0.230 Tellurium-125 0.0204 0.416 Tellurium-125 0.0140 0.112 Iodine-125 0.0094 0.022 Table 6. First series exposure-rate measurements of Ag-wire type seeds measured at 0.25 m from the FAC ([micro]R [m.sub.2]/s Standard deviation Array Number of Mean for Mean for of the mean number measurements 1981 June 1.5 all arrays (%) 1 12 4.639 2 5 4.593 3 2 4.699 4.633 0.8 4 2 4.560 5 2 4.762 6 14 4.543 Table 7. First series Ag-wire type iodine-125 seed measurments in the re-entrant ionization chamber. Data corrected to STP and reference time 1981 July 23.5. Standard Source Mean current deviation of number (pA) the mean (%) 11 115.8 0.1 12 112.4 0.5 13 110.4 0.4 14 110.3 0.9 15 111.3 0.7 Table 8. First series exposure data for No-market type seeds measured at 0.25 m from the FAC ([micro]R [m.sup.2]/s). Mean exposure Standard Array Means for rate ([micro]R deviation of number 1982 Sept. 12.5 [m.sup.2]/s) the mean (%) 1 5.040 2 5.059 3 5.060 5.068 0.2 4 5.070 5 5.092 6 5.086 Table 9. First series No-marker type seed measurements in the re-entrant chamer. Data are corrected to STP and reference time 1982 Sept 12.5. Mean current for Standard Source Seven measurements deviation of number (pA) the mean (%) 1 338.8 0.2 2 316.5 0.3 3 338.4 0.4 4 369.9 0.3 Table 10. Second series gold-marker type iodine-125 source exposure measurements at 0.25 m from the FAC ([micro]R [m.sup.2]/s). Mean Standard Number of exposure data deviation of Type of measurement measurements 1983 Feb 1.5 the mean (%) Single source, sum. 4 4.047 0.9 Arrays 5 4.054 0.7 Table 11. Re-entrant chamber current measurements for second series gold-marker iodine-125 seeds. Mean current Standard Source Number of 1983 Feb 1.5 deviation of number measurements (pA) the mean (%) 1 17 170.8 0.7 2 15 167.4 0.5 3 16 181.9 0.4 4 16 179.5 0.3 5 16 175.7 0.4 6 16 166.4 0.5 Sum 10.42 nA Table 12. Second series exposure measurements for silver-wire type iodine-125 sources at 0.25 m from the FAC ([micro]R [m.sup.2]/s). Mean Standard Number of exposure data deviation of Type of measurement measurements 1983 Feb 1.5 the mean (%) Single source, sum. 5 5.613 1.3 arrays 8 5.688 1.0 Table 13. Re-entrant chamber current measurements for second series silver-wire sources. Mean current Standard Source 1983 Feb. 1.5 Number of deviation of number (pA) measurements the mean (%) 1 208.6 6 0.2 2 219.5 6 0.5 3 195.0 6 0.2 4 228.9 7 0.7 5 230.4 11 0.8 6 198.7 6 0.9 Table 14. Second series exposure measurements for No-marker type iodine-125 sources at 0.25 m from the FAC ([micro]R [m.sup.2]/s). Mean Standard Number of exposure data deviation of Type of measurement measurements 1983 Feb 1.5 the mean (%) Single source, sum. 3 5.490 0.3 Arrays 9 5.595 0.3 Table 15. Second series re-entrant chamber current measurements for the No-marker type sources. Mean current (1) Standard Source 1983 Feb 1.5 deviation of number (pA) the mean (%) 1 367.4 0.2 2 388.4 0.2 3 361.8 0.2 4 381.1 0.3 (1)Current for nine random source positions in re-entrant tube. Table 16. Re-entrant chamber calibration factors (kR [m.sup.2]/C). Source type First series Second series Mean calibration factor Gold-marker 3.842 3.872 3.857 Silver-wire 4.467 4.400 4.433 No-marker 3.716 3.701 3.708 Table 17 Measurements of gold-marker iodine-125 source spectrum (1983 Feb 21.44) and energy-absorption data. E n [[micro].sup.en]/[rho] Radiation type (keV) ([s.sup.-1]) ([m.sup.2]/kg) Te k[alpha] 27.4 2.402X[10.sup.6] 0.0210 Te k[beta] 31 5.867X[10.sup.5] 0.0140 [gamma] 35.5 1.340X[10.sup.5] 0.00935 nE [[micro].sup.en]/[rho] Radiation type (keV [m.sup.2]/kg) Te k[alpha] 1.382X[10.sup.6] Te k[beta] 2.564X[10.sup.6] [gamma] 4.448X[10.sup.4] Table 18 Measurements of silver-wire iodine-125 source spectrum (1983 Feb 20.45) and energy-absorption data. E n [[micro].sub.en]/[rho] Radiation type (keV) ([s.sup.-1]) ([m.sup.2]/kg) Te K[alpha] 27.4 2.836 X [10.sup.6] 0.0210 Te K[beta] 31 7.028 X [10.sup.5] 0.0140 [gamma] 35.5 1.962 X [10.sup.5] 0.00935 Ag K[alpha] 22.1 6.59 X [10.sup.5] 0.0400 Ag K[beta] 25.2 1.61 X [10.sup.5] 0.0268 nE [[micro].sub.en]/[rho] Radiation type (keV [m.sup.2]/kg) Te K[alpha] 1.632 X [10.sup.6] Te K[beta] 3.050 X [10.sup.5] [gamma] 6.512 X [10.sup.4] Ag K[alpha] 5.826 X [10.sup.5] Ag K[beta] 1.087 X [10.sup.5] Table 19. Comparison of exposure calibration data, X(FAC) and exposure data computed from measurements of spectrum, X(A). Exposure Exposure from calibration spectrum Ratio Source type (nR [m.sup.2]/s) (nR [m.sup.2]/s) X(FAC)/X(A) Gold-marker 3.271 3.117 1.049 Silver-wire 5.138 4.936 1.041 Table 20. Constants for the relation [X.sub.n] = a + b sin [theta], and the normalized mean exposure rates for two seed types. The mean exposure rate is derived from data which were normalized to the exposure rate from the array in a direction perpendicular to the plane of the array. Source array a b [X.sub.n] Silver-wire 0.20 0.85 0.87 Gold-marker 0.09 0.92 0.81 Table 21. Summary of standard deviations of the mean. [[sigma].sub.m], for exposure and re-entrant chamber current summations. First series [[sigma].sub.m] [[sigma].sub.m] No. of FAC re-ent. Source type arrays (%) n (%) Gold-marker 5 0.6 4 2.5 Silver-wire 6 0.8 5 1.3 No-marker 6 0.2 4 0.6 Second series [[sigma].sub.m] [[sigma].sub.m] No. of FAC re-ent. Source type arrays (%) n (%) Gold-marker 9 0.5 6 1.8 Silver-wire 13 0.8 6 1.5 No-marker 12 0.3 4 0.2 Table 22. Combined and overall uncertainties for iodine-125 seed calibrations. The first and second series uncertainties are averaged to from the combined uncertainties. Combined Overall Seed type (%) (%) Gold-marker 2.6 5 Silver-wire 2.1 4 No-marker 1.5 3