140 M. Brucer does is to distort the spectrum of radioactivity so that the detector sees a different kind of radiation from what it would see if the source were in free air. If the phantom distorts the spectrum in a mannerdifferent from the way the neck distorts the spectrum, then the measurements of the total number of counts detected will be different. Therefore, the phantom must be designed to give off a spectrum similar to that of the patient's neck.. The misuse of phantoms and the use of improperly designed phantoms accounts for about one quarter of the potential error in those laboratories where spectrometers are used for thyroid uptake. In some laboratories formulas have been devised with very careful attention to detail so that measurements on all of their hyper- thyroid patients are correctly calibrated to a reasonable answer. However, when a hypo- thyroid patient with a high body background Is inserted into this system of measurement, a very false answer is achieved. The use of queer and unusual formulas accounts for a significant portion of the error in those laboratories where spectrometers are used for thyroid uptake. Among those persons who used spectro- meter it was unusual that the size, shape, or kind of standard caused any error. Usually the laboratories sufficiently advanced to use a spectrometer made very good calibrations of their standards. The only argument that would ensue in a laboratory using a spectro- meter would be the argument on the estimate of absolute microcurie values. But very few laboratories used a spectrometer for the measurement of thyroid uptake. Most laboratories used instruments in which there was no possible control over the spectrum seen by the instrument. In these laboratories the major source of error in thy- roid uptake measurements was in the use of an improperly designed phantom or the use of no phantom at all. The phantom design alone accounts for the largest portion, about 50 per cent, of the error. Even whena stan- Acta Radiolégica Interamericana dard phantom is used, however,it is still possible to make considerable error in the thyroid uptake measurement. It would appear that the formula used to determine the per cent uptake in the neck accounts for the next largest portion-about 25 per cent. With many instruments the size and shape of the 100 per cent total dose standard is very impor- tant. Those persons who used a very small standard, for example a gelatin capsule in air, invariably gave very incorrect answers. Those persons who used very large standards, for example 100 cc of water in a paper cup, usually gave fairly accurate answers in the high-thyroid, low body-background type of patient. They invariably, however, had very incorrect answers in the low-thyroid, high body-background type of patient. The size and shape of the standard appears to account for about 20 per cent of the error when all types of machines are taken into considera- tion. Most of the other elements of variation accounted for a very small portion of error in thyroid uptake measurement. example, the area covered by the filters, type of collimation, the distance used, the For the the thickness of various filters, the materials used in manufacturing the filter — all of these factors were relatively minor. The distance factor, however, brought up a different ques- tion. In many laboratories the most important single source of error was the sloppiness with which the distance was measured. Almost any distance, provided it is neither too close nor too far from the patient, can be used adequately to measure thyroid uptake. However, the distance between the detector and the patient and the detector and the standard must beeither identical or in a precisely constant ratio. This is a very critical measurement and is one that is not appreciated by most of the technicians who are doing thyroid uptake measurement. During the survey of thyroid uptake measurments it was found that many laboratories did not use any device for determining distance.