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Machine-read from the scan, so expect some errors. Lines that couldn’t be read are left out.
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Health Physics Pergamon Press 1963. Vol. 9, pp. 1325-1331. Printed in Northern Ireland THYROID METABOLISM IN CHILDREN AND ADULTS USING Los -4lamos Scientific Laboratory, University of California, Los Alamos, New Mexico Abstract-In addition to its medical importance, is of interest because it is produced in large amounts in nuclear explosions and can deposit in the human thyroid via the food chain. Thyroid radio-iodine content is detectable by current human counting systems if over 50 pc. These facts, plus the need for uptake data on children, have prompted our investigation of a technique for measuring thyroid metabolism using doses of about 10 nc. An 8 x 4-in. NaI (Tl) spectrometer is used. Correction for thyroid depth in neck tissue is made using a mixture detected by the crystal is a function of thyroid depth. Results on six normal adults show uptake at 7 days ranges from 15 to 29 per cent and on eight normal children from 9.5 to 27 per cent. Biological half-times are similar (roughly 100 days) at this writing. IN ORDER to estimate properly the dose to the human thyroid resulting from ingestion of radioiodine, it is necessary to know the retention of iodine by the thyroid as a function of time. I n the case of children, there is a scarcity of such data. Recent improvements in technique have made it possible to measure much less than 1 nc of fallout 1131 in people.(1.2) These facts have encouraged us to set up an experiment to accumulate radioiodine retention data for children using doses in the IO-nc range. One of the problems encountered in measuring thyroid radioiodine is the uncertainty of thyroid depth in neck tissue. This uncertainty can lead to error due to the unknown attenuation of radiation by overlying tissue and unknown thyroid-to-detector geometry. Errors due to geometry can be large because of the small neck-to-crystal distance necessary at these low dose levels. This paper describes a method of measuring thyroid radioiodine at the nc level which determines and corrects for variations in thyroid depth. Also presented are preliminary results obtained by this technique. * Work performed under the auspices of the U.S. Atomic Energy Commission. The method employs a mixture of two isotopes of iodine: 60-day 1125 and 8-day 1131. Iodine-125 decays solely by electron capture to the first excited level (35.4 keV) of TeI25, which in turn decays to its stable ground state. In grations are produced, of which 96 per cent are Iodine- 13 1 decays by B-emission to various excited states of Xe131; decay to the ground state results in emission of 364-keV y-rays (79 per yield), among others.(4) Thus, a mixture of 1125 and 1131 constitutes a tracer with tvm abundant photon yields (27 keV and 364 keVj widely separated in energy. Because of this energy separation, these photons have very 2.2 cm and 6.3 cm of water, respectively). The relative attenuation of the 27-keV X-rays and the 364-keV y-rays gives a measure of thyroid depth. To establish this relationship, a polystyrene neck mockup similar to the ORINS standard(5) was constructed. It is shown in Fig. 1. Tissue overlying the gland was simulated by layers of lean beefsteak pressed to uniform thickness and frozen to the contour of the neck
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1326 THYROID METABOLISM IN CHILDREN AND ADULTS FIG.1. Polystyrene neck mockup. mockup. The uniform “tissue” layers were added to obtain a relationship between X / y * and thickness of overlying tissue and the corresponding correction factor. The results are given in Figs. 2 and 3, from which can be determined both depth and the factor needed to correct the observed count rate to zero The solutions of 1125 and 1131 were calibrated with an 1125 standard obtained from the National Bureau of Standards, and 1131 standards FIG.2. Relationship of to Q * More precisely, the ratio of photopeak areas for a 2 source containing equal activities of IlZ5and II3l. If UJ unequal activities are employed in a given experiment, correct the observed X-ray photopeak area by thyroid depth in neck tissue for equal activities of FIG.3. Relationship of to correction factor for equal aciivities of 1125and obtained from the Radiochemical Center, England, and Nuclear-Chicago. The latter two agreed to 2 per cent. The detector chosen was 8 x 4 in. NaI crystal (with a 0.005-in. entry window to minimize attenuation of the soft X-rays) mounted in a steel room. The decision to use a large crystal rather than a pair of smaller (3 x 3 in.) PERPENDICULAR DISTANCE FROM AXIS OF CRYSTAL ( Cm 1 FIG.4. Response of large (9 x 6 in.) crystal and two small (3 x 3 in.) crystals to perpendicular displacement of source from axis. crystals was based on the following considerat “Minimum detectable amount” is that quantity of 1131 which can be detected with a standard deviation of 50 per cent for source and background counting times of 30 min.
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FIG. 5. Subject in tilting chair during background count (with lead shield). 7. Subject in tilting chair during thyroid count (no lead shield).
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FIG. 8. Subject in tilting chair during thyroid count (no lead shield). i o 5 t 0 7 S FIG.9. Subject in tilting chair viewing television.
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was found to be similar for both systems, the smaller crystals being only 20 per cent more (2) A simple water mockup was used to measure the effect of perpendicular displacement of the source from the crystal axis, since actual thyroid position in the neck is usually uncertain. Figure 4 shows the result (a 9 x 6 in. NaI crystal was used, the 8 x 4 in. crystal being unavailable). Note that uncertainty in thyroid position introduces considerably less error in the case of the large crystal. (3) Another advantage of the single large sently in operation (and equipped with large crystals) would he able to adopt the procedure described in this paper. The pulses were analyzed by a RIDL 400channel pulse height analyzer. In order to delineate clearly both peaks, the system was calibrated at 1.05 keV per channel, and all 400 channels were used. To account for room and subject (nonthyroid) background, a thyroid eclipsing lead The shield was placed between the crystal and the subject, eclipsing the thyroid area of the neck from the view of the crystal, as shown in Fig. 5 . The dimensions are 4 X 4 x 6 in. for adults and 3 x 3 x 4 in. for children. That the shield was effective can be seen from Fig. 6, which shows the uneclipsed and eclipsed spectra for subject LVD 3 days after ingession. The contributions to background, sensitivity to radioiodine, and minimum detectable amount of 1131are shown in Table 1. The subjects recline in a tilting chair so that the head is tipped back fully exposing the thyroid area. During the thyroid count, neckto-crystal distance is maintained constant by a Lucite spacer rod 3$ in. long fixed to the center of the crystal (Figs. 7 and 8). The crystal is positioned so that the spacer rod lightly touches the subject’s neck. Counting children in the age group of 4-9 years is complicated by the need to keep them still during the 15-min count. To attract and to hold their attention, a small Sony television set is mounted above the crystal in such a way that good viewing requires the head be kept in the desired position (Fig. 9). This has proven very successful, although occasionally it is difficult to remove the child during a particularly fascinating cartoon. With this arrangement, considerably longer counting SPECTRA OF THYROID AND BACKGROUND . NO ECLIPSING SHIELD . WITH ECLIPSING SHIELD
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1328 THYROID METABOLISM IN CHILDREN &ID ADULTS times appear to be entirely feasible. During the measurement, an adult remains in the steel room shielded from the crystal by a steel partition. With the aid of a mirror, he is able to see the Lucite spacer rod lightly touching the child’s neck. If the child moves, a reminder to sit still and to watch the “TV” is usually Before trying this system out, it was thought desirable to measure anyone available that had taken recently for other purposes. When is quite prominent and gives a good X/y ratio. Two such adults and three children were found, the 1131having been administered for circulation studies or thyroid diagnosis. The results are summarized in Table 2. The two adults were of quite opposite build, BS being short and stocky and JL quite slender, which is consistent with the thyroid depth values found. Although uptake for these two adults could not be measured because the administered dose was unknown, the correction factors were appreciably different, being in the ratio 1.37. None of the three children can be considered normal from the viewpoint of thyroid function. PM had a history of hyperthyroidism ; Tapazole treatment stopped about 10 days before taking an oral dose of about 0.5 pc P1.His relatively high uptake may be the result. PB and DD had small goiters but no treatment during (and for several weeks preceding) the measurements. Their uptakes were normal, although reaching the extremes of the euthyroid range, as stated depths and correction factors. Retention as a function of time after oral administration for the three children is shown in Fig. 10. Note Table 1. Background and sensitivity of the 8 x 4-in. NaI thyroid spectrometer Steel room background 54 countslmin 144 counts/min Subject contribution to background (LVD) 1 day before dose 20 counts/min 45 countslmin Subject contribution to background (LVD) with lead shield 3 days after only) with no shield 3 days after dose 394 countslmin Net subject thyroid count (thyroid Counting sensitivity via neck * The minimum detectable amount is based on the criterion that the signal must equal twice its standard deviation. Counting time is 30 min for both signal and backTable 2. Thyroid depth measurement with only Thickness Uptake Uptake .4ge Weight Height overlying correction at 7 days
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TIME ( DAYS AFTER ORAL INGESTION 1 FIG. 10. Retention of 113' by thyroid glands of three children (age 8-15 years). that another administration of Tapazole to PM at 15 days seems to have decreased the biological half-life of the P3lthyroid burden to the shortest value of the three (22 days), the other values being 34 and 42 days. These results encouraged us to try out the 1125/1131 double tracer system, but before using children it was thought advisable to work out the defects first on a group of six adults. The 1131 dose administered was 17.3 nc. Difficulties were encountered with the carrier-free solution and hence this dose is not certain; 6.2 nc is the best estimate. Unfortunately, this makes the X/y ratios of little value, and an uptake correction cannot be derived from the mined from the two isotopes are valid and should agree. This was found to be the case. The results are summarized in Table 3, and the data for two subjects are shown graphically in Fig. 11. I t is to be noted that these subjects all have similar (exceptJP) and normal uptakes, normal biological half-time is about 90 days, so that the values for two subjects (MJF and DW) are larger than normal, while the value for JP is quite small. It is interesting that JP has a history-of low basal metabolism rate which is consistent with his relatively low uptake and Having obtained reasonable results with adults, only the quantitative difficulty with the stood in the way of extending the experiment to children. This problem was solved by adding 0.5 pg NaI carrier to each dose and then measuring the dose actually ingested by each child. Eight children, ranging in age from 4 to 10 years, each received a nominal 11 nc 1125 ments ( 1 0 min with n o shield, 5 min with lead shield) began the following day. I t is planned to count three times weekly for 6 weeks, and then at less frequent intervals until the 1125 decays away. At the present writing (June, 6 1963), we have about 2 weeks of data processed (summarized Table 4). Note that uptakes are very similar to those for the adults. Thickness Table 3. Radioiodine metabolism in six adults Age Weight Height at 7 days* Tsio TEi*
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1330 THYROID MET.AF5OLISM IN CHILDREN AND ADULTS TIME (DAYS AFTER ORAL INGESTION FIG. 11. Retention of 1131 and by thyroid glands of two adults. of overlying tissue is small in all cases, resulting in similar correction factors. Figure 12 shows that biological half-lives will be long compared to the 8-day physical half-life, and perhaps similar to adults. Thus far, no differences in radioiodine metabolism have been observed between children and adults. The uptake results reported here are consistent with those obtained by SAXENAet al.(*) TIME ( DAYS AFTER ORAL INGESTION FIG. 12. Retention of by thyroid glands of four children (age 4-10 years). children with normal thyroid function were 15-25 per cent. According to MORRISONet a1.,C9) mean uptake in newborn infants receiving oral radioiodine was 45 per cent at 25 hr and 50 per cent a t 30 hr; turnover studies indicated a biological half-life of the order of 15-25 days. These authors also review the literature and find conflicting reports on uptake by newborn Table 4. Radioiodine metabolism in eight children Weight Height overlying correction at 7 days
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infants, varying between adult values and much higher values. The situation with regard to newborn infants is unclear. uptake measurements on 20 normal adults using “very small doses of Il3l.” Uptake at 24 hr ranged from 8 to 39 per cent, with a mean of 21 per cent; dose was 100-200 nc. There is good agreement on uptake in children (newborn infants excepted) and adults. (1) A new technique is described for measuring the radioiodine content of the human thyroid in vivo with nc amounts of 1131 and 1125. (2) The technique gives a measure of the thickness of neck tissue over the thyroid ; values of 0.4-2.0 cm have been found in the subjects studied thus far. (3) Results on six normal adults show radioiodine uptake at 7 days after oral ingestion varies between 15 and 29 per cent, with biological half-times between 27 days and infinity. 4-10 years) show little or no difference from adults as far as the data extend at the present writing (15 days post oral ingestion). Radioiodine uptakes vary between 9.5 and 27 per cent, and biological half-times are long compared to mination of Low Level in Human Thyroids Resulting from Nuclear Test Fallout, Hanford, Atomic Products Operation, Report HW-SAAtom in Frequent Use, U.S. Atomic Energy ment, Oak Ridge Institute of Xuclear Studies RICHMOND,Los Alamos Scientific Laboratory