Health Physics Pergamon Press 1976. Vol. 30 (Jan.), pp. 113-118. Printed in Northern Ireland
STANDARD FIELD METHODS FOR DETERMINING 1 3 7 c s AND 1311 IN VIVO* H. E. PALMERt B. M. BRANSON,$. S. H. COHN,§ P. N. DEAN,II J. A. ECKART,? R. D. LLOYD** and C. E. MILLER$$
(Received 23 June 1975; accepted 25 June 1975) Abstract-Methods have been developed for measuring the body content of 197Csand the thyroid content of 1311using rather simple, compact equipment. Procedures for calibrating the equipment were designed to provide reproducible results with a variety of counting equipment and environmental conditions. The methods and calibrations were tested by interlaboratory comparison among seven laboratories throughout the U.S. These methods should find application for measuring 137Cs and I3lIin those people who do not have access to an establishedwholebody counting facility. INTRODUCTION
SOMEradionuclides released to the environment gain access to the human body through ingestion with food or by inhalation and are incorporated into body tissues. Many of these radionuclides emit gamma rays which can be detected and used to determine their content in the body. Two of the more common gamma emitting radionuclides from nuclear weapons fallout are 137Cs and 1311. Although these two radionuclides are routinely measured in the many whole-body radioactivity monitors located throughout the world (IAEA, 1970), such facilities cannot be readily available everywhere that a need develops. Whole-body radioactivity monitors mounted on trucks can reach areas accessible by roads within a few days if a serious emergency should arise, but in many instances a simple method using unsophisticated counting
CRITERIA FOR METHOD DESIGN
* This paper is based on work performed under U.S. Energy Research and Development Administration Contract AT(45-1)-1830. t Battelle Northwest Laboratory, Richland, Washington. Nuclear Medicine Lab., BRH, FDA, Cincinnati General Hospital, Cincinnati, Ohio. $ Medical Dept., Brookhaven National Laboratory. 1) Biomedical Div., Lawrence Livermore Laboratory, Livermore, California. 7 National Environmental Research Center, Environmental Protection Agency, Las Vegas, Nevada. ** Radiobiology Div., Univ. of Utah, Salt Lake City, Utah. t t Purdue University, Hammond, Indiana. 8
equipment would be preferred over the expense and trouble of either transporting a large counting facility to a certain location or transporting people from a contaminated location to a wholebody counter. The purpose of this paper is to describe the calibration and testing of methods for measuring the total body content of 137Cs and 13lI in the thyroid which are portable, rapid and can be used in areas which are remote to whole-body counting facilities and can be performed by personnel who have not had previous wholebody counting experience. The need for these methods was suggested by the American Public Health Association and the information presented here was developed by a committee on Standard Methods of in-vivo Gamma Ray Spectrometry organized by APHA. The basic procedures for these methods have already been published (PALMER, 1965; WELLMAN et al., 1967). The committee used these procedures to develop methods which would meet the following criteria: (1) the counter is to be used without shielding from environmental radioactivity if necessary, (2) the counter can be calibrated by standard sources placed in an easily fabricated phantom, (3) the phantom material should be readily availiable and a standard size of material chosen which will probably be available for at least 20 yr, (4) the method can use a range of detector sizes, and
113
114
STANDARD FIELD METHODS FOR DETERMINING 137~sAND
( 5 ) the calibration procedure should provide a method by which one can obtain a result which would be the same as that obtained by any other investigator when measuring the same person regardless of detector size and type of instrumentation. The main emphasis has been placed on the reproducibility of results in any location by any person using any type of counting equipment and secondary emphasis was given to the absolute accuracy over the range of human body sizes. The method is presented with the assumption that the subject being measured is free of external radioactive contamination which may be contained in the clothes or on the skin surface. T o eliminate this problem, the subject should thoroughly wash his entire body, including the hair, in a shower and put on clean clothes which are non-radioactive. The subjects should remove wrist and pocket watches or jewelry which may contain radioactivity. Subject measurements should be made with other people kept a t least 8 ft away from this detector to avoid extraneous counts from their body, clothing and watches. EQUIPMENT
The instrumentation for counting consists of a NaI (Tl) scintillation detector, multichannel or single channel pulse-height analyzer, and associated high-voltage supply, amplifier and information readout components. The detector can be almost any size but detectors having a crystal diameter between 5 and 7.5 cm and a length between 5 and 12.5 crn have been tested using this procedure and found to be satisfactory. Discussions on the use of this equipment have been published elsewhere (DOUGLAS, 1967; USPHS, 1968). The use of the following methods assume that this equipment and a n experienced operator are available. The usefulness and accuracy of these methods depend on the reliability of the countingsystems. This reliability is maintained by regularly checking the response to a standard radioactive source and the background counting rate of the system. The techniques for maintaining satisfactory performance from radioactivity counting systems have been published elsewhere (KOLDE,1965) and they should be applied to these methods.
1311
IN VIVO
TOTAL BODY CESIUM-137 METHOD
Principle, application, limitation and interference Cesium-I37 distributes rather uniformly throughout the human body. A measurement made near the thighs and torso can be used to estimate the total body content. The person to be counted is seated in a chair with knees together and a NaI(T1) scintillation detector is placed on top of the thighs and against his abdomen. The person then leans over the detector to provide self-shielding against the background and to obtain a maximum counting rate. The method is calibrated with a simple phantom and a point source of 137Csof known concentration. The counting rate from this phantom can be used to determine a calibration factor for use with humans. A non-contaminated person is used to determine the background count and the counts due to natural 40K and normal 13’Cs. This method is suitable for measuring 13’Cs body burdens of 50 nCi or more without shielding from environmental radioactivity. By using a calibration factor that is a function of weight, the body burden of a n adult can be measured within f15 %. The method has not been calibrated for children and the accuracy for people weighing less than 45 kg is unknown. However, phantom studies have shown that the weight dependent factor holds reasonably true for children down to 17 kg (RECHEN et al., 1968). I n areas where high levels of recently produced fallout has occurred, this method may not be useful because of the high background counting rate; however, this problem can probably be overcome by making the measurement in a basement or a freshly excavated hole in the ground. If other radionuclides are present in the body, correction for these interferences must be made (see p. 82, USPHS, 1968). Calibration procedure I n addition to the detector and pulse height analyzer the following materials are needed : (1) A standard calibrated I3’Cs source of about 1 pCi. The area of the source should not have a diameter greater than 2 cm and the source container should not absorb more than 1 % of the activity directed toward the crystal
H. E. PALMER et al. or the absorbtion should be accurately calculated from the known thickness of the source holder. ( 2 ) A 1-gal paint can. T h e common type available from most hardware stores is 19 cm high and has a n outside diameter of 16.7 cm at the edges. (3) A regular size tuna fish can with one end cut out. T h e can dimensions are 4.5 cm high and a n outside diameter of 8.6 cm at the edges. The calibration of this method is based on the comparison of the counting rate obtained in the 662 keV photopeak by this method on people having known body burdens of 13'Cs (PALMER,1966). These body burdens were determined by a more conventional and accurate method and included a variety of body burdens and body sizes. The detector response varies as a function of body weight. A detector can be calibrated by measuring the standard source of 137Cs in prescribed way. The detector should be connected to the multichannel analyzer by a signal cable at least 6 ft long for ease in moving the detector. Adjust the analyzer such that each channel represents a n increment of 10 keV, 137Csphotopeak falls in channel 66 and use channels 60-72 inclusive for data analysis. If a single-channel analyzer is used, set the energy window to accept pulses from 600 to 720 keV. Place the empty tuna fish can (open end down) on the inside of and on the center of the bottom of the paint can. LPlace
FIG.1. Calibration assembly for l3'Cs field method showing cutaway view of detector on tuna can inside paint can.
115
the detector on top of the tuna fish can as shown in Fig. 1. Place the standard source on the center of the outside bottom of the paint can and count for 1 min. Place the source on the outside of the paint can, up 7.62 cm from the bottom, count for 1 min, and then remove the source and take a background count. The standard source count is taken at these two positions to average out any differences in detector response due to differences in direction of incoming radiation which may exist between different detectors. A calibration factor is determined for each person according to his weight by the following equation : pCi of I3?Cs per countlmin in a n adult human 56,200 ( A ) 8190 - 53 W where A = the average net count/minlpu(=iof 137Cs obtained from the two standard source counts, and W = weight in kg of the person being counted.
Counting procedure To determine the 13'Cs content of a subject, he is weighed and seated on a box or chair with his legs together. The detector is placed on top of the thighs and against the abdomen and the subject leans over as far as possible while resting the hands on the knees and the forearms on the thighs as shown in Fig. 2. I n this manner, the body provides a significant amount of selfshielding against the environmental radioactivity. The time required for counting varies from 1 min for body burdens which exceed 1 pCi to 5 min for burdens under 0.1 pCi. For a background count a n uncontaminated person of about the same weight is counted which also corrects for the interference from 40K in the body. The net countslmin from the subject are multiplied by the calibration factor to obtain the total-body content. Precision and accuracy The measurements made by this method have an accuracy of 15 % or better for all adults except the very obese. The counting precision for repeated measurements on the same person is excellent and is mainly limited by counting rate statistics.
116
STANDARD FIELD METHODS FOR DETERMINING l37Cs AND 1311 I N VIVO Toble 1. Laboratory inlercomparison study for Is7 Cs method
determine whether the I3lI level in the thyroids of a population group is high and to decide _ Laboratarv __~ ____ (uci) which people should be referred to a laboratory Argonne N a t i o n a l L a b o r a t o r y 3 " x 5" 7.54 facility for a more accurate and sensitive meaBattelle-Northwest 1.59 3" 3" Rdcio!o8;ical Sciences D e p t . surement. Dattei;e-:rorthwert Thyroid burdens down to 0.3nCi can be 3" x 3" 1.53 Biology Ceparfment measured by this method in both adults and Erookhiven l l d t i o n a l L a b w a t o r y 3" x 2" 1.54 children. Interference from l3II, which is not Los AIa:,;: t l a t i o n a l L a b o r a t o r y 3" x 3" 1.64 in the thyroid but contained in soft tissues in U n i v e i - s i t y o: Utah 3" x 3" 1.54 the neck surrounding the thyroid, can be subU.S.F.I!.S./Las Vegas 3" x 3" 1.59 tracted out by using a background count with B P a r : CTncinnati General 3" x 2" 1.55 Has p i t a l the detector(s) on the thigh a t a point approxi___ A;e-we.. ........................................... 1.57 mating the circumference of the neck. When Staodai-d g e i i a t i o n ................................... 0.038 O P 2.4: other radionuclides are present, corrections for photopeak and Compton scatter interferences An interlaboratory comparison of the calimust be made (see pp. 1-46 in DOUGLAS, 1967). bration procedure and the application of the calibration factor to the measurement of a Calibration procedurt? phantom containing 137Csdistributed throughI n addition to the pulse height analyzer and out its volume was conducted a t several laboeither one or two detectors, the following materatories throughout the U.S. T h e calibration rials are needed: source and the 13?Csfilled phantom was supplied ( I ) An adjustable detector support; in turn to each of the laboratories. Each labo( 2 ) Two polyethylene bottles with the neck ratory provided their own detector and countcut off. One bottle must have a n outside diing equipment which was calibrated and then ameter of 14-15cm, the other 9-10cm. The used to measure the 137Cs content in the phanlower section of the bottle must be a t least tom. The results are shown in Table 1. 18 cm high. (3) Two bacterial glass test tubes, 25 mm THYROIDAL 1311 METHOD outside diameter and two tubes, 14-16 mm Princ$le, application, limitations and interferences outside diameter. Approximately 20% of 1311 in the body of a (4) 50 mls of a standardized lS1Isolution normal person will accumulate in the thyroid containing about 2 nCi/ml. gland which is the critical organ for radiation From these materials a simple neck phantom dose considerations. NaI (Tl) scintillation de- can be constructed. For the adult neck, use tectors ranging in size from 5.1 to 7.7 cm in the lower section of the larger polyethylene diameter and 3.8-7.7 cm in length are placed bottle with 14-15 cm outer diameter and two on the front of the neck for the measurement. bacterial test tubes with 25 mm outside diThe method can be used with a single detector ameter. For the child's neck, use the lower or a dual detector system using two detectors section of the smaller polyethylene bottle with mounted such that their central axes are a t 9-10 cm outside diameter and two bacterial test 90" angles with one another. Iodine-131 emits tubes with 14-16 mm outside diameter. gamma rays of several energies but the most The polyethylene container is filled with water prominent gamma ray is a t 364 keV and is the to a depth of 15 cm. Ten to 100 nCi of 1311 one used for the determination of thyroid bur- standard solution is placed in each test tube dens. and the total volume in each tube is adjusted This method is designed to be used in the to 15 ml for the adult neck and to 7 ml for the field without background shielding. It does child phantom. The test tubes are then innot correct for the depth of the thyroid which serted into the beaker so that the column of varies among different individuals and, there- standard 1311solutions is equidistant from the fore, the accuracy is good to only f 5 0 % . It top and bottom of the water (see Fig. 3). For is meant to be used as a screening method to the adult phantom, place test tubes 1 cm apart
-
Detector Size (diameter x 1 . M
Assay
FIG. 2. Subject b e q measured b\
13;C5
held method.
H. E. PALMER et al.
-
117
PSI TUBES CONTAINING ZOTO 1W nCi
Nd (11) CRYSTAL
PHOTO TUBE
FIG. 3. Calibration assembly for thyroid
and 1.0 cm from outside of water container. Test tube clamps attached to a ring stand are useful in positioning the test tubes. For the child’s phantom, separate the test tubes by 0.5 cm and place 0.8 cm from outside of beaker wall. These conditions simulate the human neck with radioiodine in a thyroid, which is located at the average depth for adults (2.3 cm; center of one lobe to neck surface) and children 10 yr old (1.6 cm; center to neck surface). Place the detector system against side of the water container with the apex of the dual crystals, or center of a single crystal, centered between the two test tubes, directly in front of the liquid in the test tubes. With the multichannel analyzer adjusted such that each channel represents 20 keV, the l31I principal photopeak at 364 keV will fall into channel 18 and channels 16-20 will be used for data analysis. A second energy, 637 kev, will be observed in channel 32, but this peak will not be used. If a single channel analyzer is used, set the channel to count over the energy range of 320-400 keV. Measure the phantom containing the lS1I standard for 10 min and then remove the test tubes and measure the phantom background for 10 min. Determine the net counts per minute and divide this number by the total number of nCi of 1311 placed in both test tubes. This value of counts/min/nCi is the calibration factor to be used for subject measurements and, if both adults and children are measured, the calibration on both the adult and child phantom must be done. For those subjects whose size is midway between that of a child and adult, a calibration factor should be used which is a n
1311
measurement.
extrapolation between the adult and child factors.
Counting procedure Seat the subject on a chair with hands in lap and looking straight ahead a t a point up on the ceiling which will tilt the heat about 45’ from the horizontal. This will extend the neck for the measurement. Position crystal(s) against lower part of neck and center crystal system between clavicle bones (located a t base of neck) and thyroid cartilage (sometimes referred to as the “Adam’s apple”). This cartilage will not be prominent on children or female adults, but it can easily be located by feeling the neck with the fingers. The position is important, particularly in adults where the neck may be long. O n a long neck, the positioning half-way between the clavicle bones and thyroid cartilage is important. O n short necks and children, the same attempt should be made. If a 3 x 3 in. NaI(T1) crystal system is being used, the lower edge placed against the clavicles is usually the best position obtainable for this circumstance. Make sure the detector system is against neck, but not pressed too hard. Measure thyroid for 10 min, unless a high counting rate indicates that only a short count is necessary. After the thyroid count, remove chair, replace with sturdy box, and have subject stand on the box with the detector system placed against the front of the thigh just about 3 in. above the top of the knee cap. This position represents a portion of the body similar in build and circumference to the neck. The background measurement taken on the thigh will be similar to a neck with no radioiodine present. If any
STANDARD FIELD METHODS FOR DETERMINING
118
radioactivity is in the body, other than the thyroid, then the background taken here will correct for that radioactivity. The net count per min obtained after subtracting the thigh count is multiplied by the factor obtained from the phantom calibration to obtain the nCi of 1311in the subject’s thyroid.
Precision and accuracy The validity of this measurement is subject to the variation of thyroid depth from subject to subject. In this procedure an “average effective depth” is utilized for “adults” and a 10-yr-old child.” The actual depth could cause differences in the result of about 50%. For field determination, this error is tolerable. Because thyroid depth in the neck increases with age, the thyroid lobes in children are closer to the surface of the neck than in adults. A factor of 0.95 should be applied to data obtained from children 8 yr old and younger as an added correction. An interlaboratory comparison of the Calibration procedure and the application of the calibration factor to the measurement of an unknown amount of 1311in a lucite neck phantom was conducted. The standard 1311solution and the 1311-filled neck phantom was supplied in turn to each laboratory, which provided their own detector, counting equipment and calibration materials. The results of measuring the unknown quantity of 1311in the lucite neck phantom are shown in Table 2. Table 2. Laboratory intcrcomparison study for
Detectors -Used
Detector Size (diameter x length) (cm)
m
No. o f
___ Laborat on-,,
1811 method
Assay
Battel le-Northwest
1
7.62 x 7.62
0.225
Erookhaven National Laboratory
1
7.62 x 5.08
0.219
Los Alamos National Laboratory
1
7.62 x 7.62
0.231
Lcs Alamos National Laboratory
2
7.62 x.7.62
0.233
University of iitdh
2
5.08 x 5.08
0.273
U.S.P.H.S./Las
2
7.62 x 7.62
0.240
2
7.62 x 5.08
0.228
Vegas
6RH and Cincinnati
General Hospital
~
................................................ Standard Deviation.. .................................... Average..
.0.227 ..0.0089 o r 3.9%
137cs
AND
1311
IN
vzvo
SUMMARY
The methods described in this paper are not certified as a standard method by any official organization, but they do represent the development of methods by a group of some of the most experienced people in whole-body counting in the U.S. If these instructions are carefully followed, the same estimates of 137Cs or 1311 content in a particular individual would be obtained by anyone using almost any size detector with any type of gamma ray spectrometry and the results would be reasonably close to the absolute value of the radionuclide content. These methods are not expected to be used in place of those at established whole-body counters, but will be primarily applicable to those who are not close to a whole-body counter but do have some radiation counting equipment. As an example, a subject at a university Iocated 300 miles from the nearest whole-body counting facility became contaminated with an unknown quanity of 13’Cs. The university had a 3 X 3 in. NaI detector and a multichannel analyzer and from instructions by one of the authors given over the telephone, the 137Cs burden in the subject was determined. A stepwise procedure of the above methods, which gives more details than can be presented here, is available from the first author REFERENCES
DOUGLAS G. S. (Ed), 1967, Radioassay Procedures for Environmental Samples, U.S. Public Health Service Technical Rept, 999-RH, pp. 1-38. IAEA, 1970, Directory of Whole-Body Radioactivity Monitors (Vienna: IAEA). KOLDE H. E., 1965, Quality Control of RadioactiveCounting Systems, US. Public Health Service Publication 999-RH-15. PALMER H. E., 1966, Health Phys. 12,605. H. J. L., MIKKELSON R. L., BRINOL 0. G. RECHEN and STEINER J. F., JR., 1968, Radiat. Hlth Data Reps, 9, 705. U.S. Public Health Service, 1968, Common Laboratory Instruments for Measurementof Radioactivity, Rept 999 RH, 32, WELLMAN H. N., KEREIAKES J. G., YEAGERT. B., KARCHES G. J. and SAENGER E. L., 1967, J. nucl. Med., 8 , 86.
STANDARD FIELD METHODS FOR DETERMINING 1 3 7 c s AND 1311 IN VIVO* H. E. PALMERt B. M. BRANSON,$. S. H. COHN,§ P. N. DEAN,II J. A. ECKART,? R. D. LLOYD** and C. E. MILLER$$
(Received 23 June 1975; accepted 25 June 1975) Abstract-Methods have been developed for measuring the body content of 197Csand the thyroid content of 1311using rather simple, compact equipment. Procedures for calibrating the equipment were designed to provide reproducible results with a variety of counting equipment and environmental conditions. The methods and calibrations were tested by interlaboratory comparison among seven laboratories throughout the U.S. These methods should find application for measuring 137Cs and I3lIin those people who do not have access to an establishedwholebody counting facility. INTRODUCTION
SOMEradionuclides released to the environment gain access to the human body through ingestion with food or by inhalation and are incorporated into body tissues. Many of these radionuclides emit gamma rays which can be detected and used to determine their content in the body. Two of the more common gamma emitting radionuclides from nuclear weapons fallout are 137Cs and 1311. Although these two radionuclides are routinely measured in the many whole-body radioactivity monitors located throughout the world (IAEA, 1970), such facilities cannot be readily available everywhere that a need develops. Whole-body radioactivity monitors mounted on trucks can reach areas accessible by roads within a few days if a serious emergency should arise, but in many instances a simple method using unsophisticated counting
CRITERIA FOR METHOD DESIGN
* This paper is based on work performed under U.S. Energy Research and Development Administration Contract AT(45-1)-1830. t Battelle Northwest Laboratory, Richland, Washington. Nuclear Medicine Lab., BRH, FDA, Cincinnati General Hospital, Cincinnati, Ohio. $ Medical Dept., Brookhaven National Laboratory. 1) Biomedical Div., Lawrence Livermore Laboratory, Livermore, California. 7 National Environmental Research Center, Environmental Protection Agency, Las Vegas, Nevada. ** Radiobiology Div., Univ. of Utah, Salt Lake City, Utah. t t Purdue University, Hammond, Indiana. 8
equipment would be preferred over the expense and trouble of either transporting a large counting facility to a certain location or transporting people from a contaminated location to a wholebody counter. The purpose of this paper is to describe the calibration and testing of methods for measuring the total body content of 137Cs and 13lI in the thyroid which are portable, rapid and can be used in areas which are remote to whole-body counting facilities and can be performed by personnel who have not had previous wholebody counting experience. The need for these methods was suggested by the American Public Health Association and the information presented here was developed by a committee on Standard Methods of in-vivo Gamma Ray Spectrometry organized by APHA. The basic procedures for these methods have already been published (PALMER, 1965; WELLMAN et al., 1967). The committee used these procedures to develop methods which would meet the following criteria: (1) the counter is to be used without shielding from environmental radioactivity if necessary, (2) the counter can be calibrated by standard sources placed in an easily fabricated phantom, (3) the phantom material should be readily availiable and a standard size of material chosen which will probably be available for at least 20 yr, (4) the method can use a range of detector sizes, and
113
114
STANDARD FIELD METHODS FOR DETERMINING 137~sAND
( 5 ) the calibration procedure should provide a method by which one can obtain a result which would be the same as that obtained by any other investigator when measuring the same person regardless of detector size and type of instrumentation. The main emphasis has been placed on the reproducibility of results in any location by any person using any type of counting equipment and secondary emphasis was given to the absolute accuracy over the range of human body sizes. The method is presented with the assumption that the subject being measured is free of external radioactive contamination which may be contained in the clothes or on the skin surface. T o eliminate this problem, the subject should thoroughly wash his entire body, including the hair, in a shower and put on clean clothes which are non-radioactive. The subjects should remove wrist and pocket watches or jewelry which may contain radioactivity. Subject measurements should be made with other people kept a t least 8 ft away from this detector to avoid extraneous counts from their body, clothing and watches. EQUIPMENT
The instrumentation for counting consists of a NaI (Tl) scintillation detector, multichannel or single channel pulse-height analyzer, and associated high-voltage supply, amplifier and information readout components. The detector can be almost any size but detectors having a crystal diameter between 5 and 7.5 cm and a length between 5 and 12.5 crn have been tested using this procedure and found to be satisfactory. Discussions on the use of this equipment have been published elsewhere (DOUGLAS, 1967; USPHS, 1968). The use of the following methods assume that this equipment and a n experienced operator are available. The usefulness and accuracy of these methods depend on the reliability of the countingsystems. This reliability is maintained by regularly checking the response to a standard radioactive source and the background counting rate of the system. The techniques for maintaining satisfactory performance from radioactivity counting systems have been published elsewhere (KOLDE,1965) and they should be applied to these methods.
1311
IN VIVO
TOTAL BODY CESIUM-137 METHOD
Principle, application, limitation and interference Cesium-I37 distributes rather uniformly throughout the human body. A measurement made near the thighs and torso can be used to estimate the total body content. The person to be counted is seated in a chair with knees together and a NaI(T1) scintillation detector is placed on top of the thighs and against his abdomen. The person then leans over the detector to provide self-shielding against the background and to obtain a maximum counting rate. The method is calibrated with a simple phantom and a point source of 137Csof known concentration. The counting rate from this phantom can be used to determine a calibration factor for use with humans. A non-contaminated person is used to determine the background count and the counts due to natural 40K and normal 13’Cs. This method is suitable for measuring 13’Cs body burdens of 50 nCi or more without shielding from environmental radioactivity. By using a calibration factor that is a function of weight, the body burden of a n adult can be measured within f15 %. The method has not been calibrated for children and the accuracy for people weighing less than 45 kg is unknown. However, phantom studies have shown that the weight dependent factor holds reasonably true for children down to 17 kg (RECHEN et al., 1968). I n areas where high levels of recently produced fallout has occurred, this method may not be useful because of the high background counting rate; however, this problem can probably be overcome by making the measurement in a basement or a freshly excavated hole in the ground. If other radionuclides are present in the body, correction for these interferences must be made (see p. 82, USPHS, 1968). Calibration procedure I n addition to the detector and pulse height analyzer the following materials are needed : (1) A standard calibrated I3’Cs source of about 1 pCi. The area of the source should not have a diameter greater than 2 cm and the source container should not absorb more than 1 % of the activity directed toward the crystal
H. E. PALMER et al. or the absorbtion should be accurately calculated from the known thickness of the source holder. ( 2 ) A 1-gal paint can. T h e common type available from most hardware stores is 19 cm high and has a n outside diameter of 16.7 cm at the edges. (3) A regular size tuna fish can with one end cut out. T h e can dimensions are 4.5 cm high and a n outside diameter of 8.6 cm at the edges. The calibration of this method is based on the comparison of the counting rate obtained in the 662 keV photopeak by this method on people having known body burdens of 13'Cs (PALMER,1966). These body burdens were determined by a more conventional and accurate method and included a variety of body burdens and body sizes. The detector response varies as a function of body weight. A detector can be calibrated by measuring the standard source of 137Cs in prescribed way. The detector should be connected to the multichannel analyzer by a signal cable at least 6 ft long for ease in moving the detector. Adjust the analyzer such that each channel represents a n increment of 10 keV, 137Csphotopeak falls in channel 66 and use channels 60-72 inclusive for data analysis. If a single-channel analyzer is used, set the energy window to accept pulses from 600 to 720 keV. Place the empty tuna fish can (open end down) on the inside of and on the center of the bottom of the paint can. LPlace
FIG.1. Calibration assembly for l3'Cs field method showing cutaway view of detector on tuna can inside paint can.
115
the detector on top of the tuna fish can as shown in Fig. 1. Place the standard source on the center of the outside bottom of the paint can and count for 1 min. Place the source on the outside of the paint can, up 7.62 cm from the bottom, count for 1 min, and then remove the source and take a background count. The standard source count is taken at these two positions to average out any differences in detector response due to differences in direction of incoming radiation which may exist between different detectors. A calibration factor is determined for each person according to his weight by the following equation : pCi of I3?Cs per countlmin in a n adult human 56,200 ( A ) 8190 - 53 W where A = the average net count/minlpu(=iof 137Cs obtained from the two standard source counts, and W = weight in kg of the person being counted.
Counting procedure To determine the 13'Cs content of a subject, he is weighed and seated on a box or chair with his legs together. The detector is placed on top of the thighs and against the abdomen and the subject leans over as far as possible while resting the hands on the knees and the forearms on the thighs as shown in Fig. 2. I n this manner, the body provides a significant amount of selfshielding against the environmental radioactivity. The time required for counting varies from 1 min for body burdens which exceed 1 pCi to 5 min for burdens under 0.1 pCi. For a background count a n uncontaminated person of about the same weight is counted which also corrects for the interference from 40K in the body. The net countslmin from the subject are multiplied by the calibration factor to obtain the total-body content. Precision and accuracy The measurements made by this method have an accuracy of 15 % or better for all adults except the very obese. The counting precision for repeated measurements on the same person is excellent and is mainly limited by counting rate statistics.
116
STANDARD FIELD METHODS FOR DETERMINING l37Cs AND 1311 I N VIVO Toble 1. Laboratory inlercomparison study for Is7 Cs method
determine whether the I3lI level in the thyroids of a population group is high and to decide _ Laboratarv __~ ____ (uci) which people should be referred to a laboratory Argonne N a t i o n a l L a b o r a t o r y 3 " x 5" 7.54 facility for a more accurate and sensitive meaBattelle-Northwest 1.59 3" 3" Rdcio!o8;ical Sciences D e p t . surement. Dattei;e-:rorthwert Thyroid burdens down to 0.3nCi can be 3" x 3" 1.53 Biology Ceparfment measured by this method in both adults and Erookhiven l l d t i o n a l L a b w a t o r y 3" x 2" 1.54 children. Interference from l3II, which is not Los AIa:,;: t l a t i o n a l L a b o r a t o r y 3" x 3" 1.64 in the thyroid but contained in soft tissues in U n i v e i - s i t y o: Utah 3" x 3" 1.54 the neck surrounding the thyroid, can be subU.S.F.I!.S./Las Vegas 3" x 3" 1.59 tracted out by using a background count with B P a r : CTncinnati General 3" x 2" 1.55 Has p i t a l the detector(s) on the thigh a t a point approxi___ A;e-we.. ........................................... 1.57 mating the circumference of the neck. When Staodai-d g e i i a t i o n ................................... 0.038 O P 2.4: other radionuclides are present, corrections for photopeak and Compton scatter interferences An interlaboratory comparison of the calimust be made (see pp. 1-46 in DOUGLAS, 1967). bration procedure and the application of the calibration factor to the measurement of a Calibration procedurt? phantom containing 137Csdistributed throughI n addition to the pulse height analyzer and out its volume was conducted a t several laboeither one or two detectors, the following materatories throughout the U.S. T h e calibration rials are needed: source and the 13?Csfilled phantom was supplied ( I ) An adjustable detector support; in turn to each of the laboratories. Each labo( 2 ) Two polyethylene bottles with the neck ratory provided their own detector and countcut off. One bottle must have a n outside diing equipment which was calibrated and then ameter of 14-15cm, the other 9-10cm. The used to measure the 137Cs content in the phanlower section of the bottle must be a t least tom. The results are shown in Table 1. 18 cm high. (3) Two bacterial glass test tubes, 25 mm THYROIDAL 1311 METHOD outside diameter and two tubes, 14-16 mm Princ$le, application, limitations and interferences outside diameter. Approximately 20% of 1311 in the body of a (4) 50 mls of a standardized lS1Isolution normal person will accumulate in the thyroid containing about 2 nCi/ml. gland which is the critical organ for radiation From these materials a simple neck phantom dose considerations. NaI (Tl) scintillation de- can be constructed. For the adult neck, use tectors ranging in size from 5.1 to 7.7 cm in the lower section of the larger polyethylene diameter and 3.8-7.7 cm in length are placed bottle with 14-15 cm outer diameter and two on the front of the neck for the measurement. bacterial test tubes with 25 mm outside diThe method can be used with a single detector ameter. For the child's neck, use the lower or a dual detector system using two detectors section of the smaller polyethylene bottle with mounted such that their central axes are a t 9-10 cm outside diameter and two bacterial test 90" angles with one another. Iodine-131 emits tubes with 14-16 mm outside diameter. gamma rays of several energies but the most The polyethylene container is filled with water prominent gamma ray is a t 364 keV and is the to a depth of 15 cm. Ten to 100 nCi of 1311 one used for the determination of thyroid bur- standard solution is placed in each test tube dens. and the total volume in each tube is adjusted This method is designed to be used in the to 15 ml for the adult neck and to 7 ml for the field without background shielding. It does child phantom. The test tubes are then innot correct for the depth of the thyroid which serted into the beaker so that the column of varies among different individuals and, there- standard 1311solutions is equidistant from the fore, the accuracy is good to only f 5 0 % . It top and bottom of the water (see Fig. 3). For is meant to be used as a screening method to the adult phantom, place test tubes 1 cm apart
-
Detector Size (diameter x 1 . M
Assay
FIG. 2. Subject b e q measured b\
13;C5
held method.
H. E. PALMER et al.
-
117
PSI TUBES CONTAINING ZOTO 1W nCi
Nd (11) CRYSTAL
PHOTO TUBE
FIG. 3. Calibration assembly for thyroid
and 1.0 cm from outside of water container. Test tube clamps attached to a ring stand are useful in positioning the test tubes. For the child’s phantom, separate the test tubes by 0.5 cm and place 0.8 cm from outside of beaker wall. These conditions simulate the human neck with radioiodine in a thyroid, which is located at the average depth for adults (2.3 cm; center of one lobe to neck surface) and children 10 yr old (1.6 cm; center to neck surface). Place the detector system against side of the water container with the apex of the dual crystals, or center of a single crystal, centered between the two test tubes, directly in front of the liquid in the test tubes. With the multichannel analyzer adjusted such that each channel represents 20 keV, the l31I principal photopeak at 364 keV will fall into channel 18 and channels 16-20 will be used for data analysis. A second energy, 637 kev, will be observed in channel 32, but this peak will not be used. If a single channel analyzer is used, set the channel to count over the energy range of 320-400 keV. Measure the phantom containing the lS1I standard for 10 min and then remove the test tubes and measure the phantom background for 10 min. Determine the net counts per minute and divide this number by the total number of nCi of 1311 placed in both test tubes. This value of counts/min/nCi is the calibration factor to be used for subject measurements and, if both adults and children are measured, the calibration on both the adult and child phantom must be done. For those subjects whose size is midway between that of a child and adult, a calibration factor should be used which is a n
1311
measurement.
extrapolation between the adult and child factors.
Counting procedure Seat the subject on a chair with hands in lap and looking straight ahead a t a point up on the ceiling which will tilt the heat about 45’ from the horizontal. This will extend the neck for the measurement. Position crystal(s) against lower part of neck and center crystal system between clavicle bones (located a t base of neck) and thyroid cartilage (sometimes referred to as the “Adam’s apple”). This cartilage will not be prominent on children or female adults, but it can easily be located by feeling the neck with the fingers. The position is important, particularly in adults where the neck may be long. O n a long neck, the positioning half-way between the clavicle bones and thyroid cartilage is important. O n short necks and children, the same attempt should be made. If a 3 x 3 in. NaI(T1) crystal system is being used, the lower edge placed against the clavicles is usually the best position obtainable for this circumstance. Make sure the detector system is against neck, but not pressed too hard. Measure thyroid for 10 min, unless a high counting rate indicates that only a short count is necessary. After the thyroid count, remove chair, replace with sturdy box, and have subject stand on the box with the detector system placed against the front of the thigh just about 3 in. above the top of the knee cap. This position represents a portion of the body similar in build and circumference to the neck. The background measurement taken on the thigh will be similar to a neck with no radioiodine present. If any
STANDARD FIELD METHODS FOR DETERMINING
118
radioactivity is in the body, other than the thyroid, then the background taken here will correct for that radioactivity. The net count per min obtained after subtracting the thigh count is multiplied by the factor obtained from the phantom calibration to obtain the nCi of 1311in the subject’s thyroid.
Precision and accuracy The validity of this measurement is subject to the variation of thyroid depth from subject to subject. In this procedure an “average effective depth” is utilized for “adults” and a 10-yr-old child.” The actual depth could cause differences in the result of about 50%. For field determination, this error is tolerable. Because thyroid depth in the neck increases with age, the thyroid lobes in children are closer to the surface of the neck than in adults. A factor of 0.95 should be applied to data obtained from children 8 yr old and younger as an added correction. An interlaboratory comparison of the Calibration procedure and the application of the calibration factor to the measurement of an unknown amount of 1311in a lucite neck phantom was conducted. The standard 1311solution and the 1311-filled neck phantom was supplied in turn to each laboratory, which provided their own detector, counting equipment and calibration materials. The results of measuring the unknown quantity of 1311in the lucite neck phantom are shown in Table 2. Table 2. Laboratory intcrcomparison study for
Detectors -Used
Detector Size (diameter x length) (cm)
m
No. o f
___ Laborat on-,,
1811 method
Assay
Battel le-Northwest
1
7.62 x 7.62
0.225
Erookhaven National Laboratory
1
7.62 x 5.08
0.219
Los Alamos National Laboratory
1
7.62 x 7.62
0.231
Lcs Alamos National Laboratory
2
7.62 x.7.62
0.233
University of iitdh
2
5.08 x 5.08
0.273
U.S.P.H.S./Las
2
7.62 x 7.62
0.240
2
7.62 x 5.08
0.228
Vegas
6RH and Cincinnati
General Hospital
~
................................................ Standard Deviation.. .................................... Average..
.0.227 ..0.0089 o r 3.9%
137cs
AND
1311
IN
vzvo
SUMMARY
The methods described in this paper are not certified as a standard method by any official organization, but they do represent the development of methods by a group of some of the most experienced people in whole-body counting in the U.S. If these instructions are carefully followed, the same estimates of 137Cs or 1311 content in a particular individual would be obtained by anyone using almost any size detector with any type of gamma ray spectrometry and the results would be reasonably close to the absolute value of the radionuclide content. These methods are not expected to be used in place of those at established whole-body counters, but will be primarily applicable to those who are not close to a whole-body counter but do have some radiation counting equipment. As an example, a subject at a university Iocated 300 miles from the nearest whole-body counting facility became contaminated with an unknown quanity of 13’Cs. The university had a 3 X 3 in. NaI detector and a multichannel analyzer and from instructions by one of the authors given over the telephone, the 137Cs burden in the subject was determined. A stepwise procedure of the above methods, which gives more details than can be presented here, is available from the first author REFERENCES
DOUGLAS G. S. (Ed), 1967, Radioassay Procedures for Environmental Samples, U.S. Public Health Service Technical Rept, 999-RH, pp. 1-38. IAEA, 1970, Directory of Whole-Body Radioactivity Monitors (Vienna: IAEA). KOLDE H. E., 1965, Quality Control of RadioactiveCounting Systems, US. Public Health Service Publication 999-RH-15. PALMER H. E., 1966, Health Phys. 12,605. H. J. L., MIKKELSON R. L., BRINOL 0. G. RECHEN and STEINER J. F., JR., 1968, Radiat. Hlth Data Reps, 9, 705. U.S. Public Health Service, 1968, Common Laboratory Instruments for Measurementof Radioactivity, Rept 999 RH, 32, WELLMAN H. N., KEREIAKES J. G., YEAGERT. B., KARCHES G. J. and SAENGER E. L., 1967, J. nucl. Med., 8 , 86.
