Vol, VII, Ne 3-4 135 RADIOIODINE UPTAKE MEASUREMENT an extended source in a simulated clinical condition, most laboratories are not able to achieve a result that is even reasonably accurate, It was not intended that the work of the Thyroid Uptake Calibration Committee be confined only to the determination of the variation in thyroid uptake measurements. This was only the first phase of the program. A second phase has been set up that is concerned with the determination of why there was a variation and what factors influenced the measurement. Included in the second pha- se is the determination of whetherit is possible to set up an intercalibration program that could be fairly universally followed and would be acceptabie to most workers in the field. The third phase of the program will then be to initiate an intercalibration pro- gram. The determination of the causes for variation with different instrumentation has been a complex study. It involves such things as investigating the variations due to different detectors, counting systems, distances, filtra- tions, size and shape of standards, variations in body background, scatter background, and a host of other causes for variance. A detailed explanation of each of these variances would be too long for this paper, but an illustration can be given of one of the major causes for variation, This involves the changes in the spectrum of energies emitted from the body from the standards when iodine 131 is distributed throughout an extended body. A point source of iodine gives off many gamma ‘photons ranging from an 80 kev to a 722 kev gammaphoton.It gives these off in definite proportions. When the iodine is distributed in an extended source, each of these gamma photons has a different proba- bility of absorption and scatter. Further, the higher-energy protons will tend to interact with the surrounding media with a Comp- ton interaction that will cause the production of a spectrum of degraded energies. The very TABLE 2? DISTRIBUTION OF RUSULTS OF ORINS THYROID UPTAKE CALIBRATION SURVEY First Phase (250 Laboratories) Measured % Hyperthyroid Hypothyroid Hypothyroid Uptake Manikins Manikins Manikins O10 20 - 10 20 30 l % 0 0 0 3 % 13 % 44 46 % 5% 30 - 40 2% 57 % 2 50 - 60 9 % 7 %& 60 - 70 15 % 40 - 70 80 90 100 110 120 130+ 50 80 90 100 110 120 130 2% 35 % 17 % 10 % 3 % 2% 2 % Ll % 17 % 3 % 0 0 0 0 0 0 — 2% 0 % 0 0 0 0 0 1% 0 0 low energies will tend to interact with a photoelectric absorption. Different measuring instruments have different sensitivities to different portions of the spectrum. A thin-walled Geiger-Muller tube, for example, has a very high sensitivity for the lower-energy radiation and a de- creased sensitivity for the high-energy radiation. A thick-walled Geiger-Muller tube has a different energy-sensitivity. The energy-sensitivity of a scintillation crystal will depend upon its size, shape, material, and the way in which it is canned. Even the size, shape, and the material of the collimating system will changethe spectral sensitivity of the counting system. Therefore, a given amount of iodine 131 existing as a point source in air will give a different number of counts with differnt systems of detection. When this given amount of iodine distributed in a small gelatin capsule, the number of counts will change when the gelatin capsule is put into different kinds of phantoms. When the iodine is distributed in the thyroid gland in a neck, it will give a still different number of counts. Oneof the methods of investigating how the detector will respond to these differences in