92 Harold L. Atkins 28 patients failed to control the hyperthyroid state in 25. It is interesting that 3 patients did become euthyroid with doses of 400, 600, and 800 rads. The trial was then abandoned. The advantage of external beam irradiation is the rather precise control of radiation dose. This permits the depletion of a specified fraction of thyroid cells, a situation not possible with an internally deposited radionuclide. Trotter and Willoughbyhave demonstrated a biphasic effect of radiation on thyroidal radioiodine uptake following a dose of 400 to 800 rads of ®°Co gamma rays on a portion of the thyroid. There is an initial 30 to 50 percent reduction in uptake, demonstrated on scans, which occursat an interval of 24 hours between irradiation and administration of the dose. A slight compensatory increase in uptake is then noted for 2 weeks followed by a second decrease in uptake at 3 weeks lasting for about 2 weeks. Fifty patients were treated with radioiodine following irradiation of a portion of the gland with 800 rads in the hope that the portion of the gland temporarily suppressed by ®°Co would take up less of a therapeutic dose of I and thus be spared destruction. However, no difference in the rate of hypothyroidism was noted in the early results at less than I year. Further details have not been published. With the development of accurate methods for localizing charged particle beams, there is the possibility of using the Bragg peak for precise delineation of a target area within the thyroid while sparing normal tissue. With such a beam of heavy ions it would be possible to selectively irradiate a portion of the thyroid while sparing the remainder, as well as adjacent normal tissues. This approach has not yet been undertaken. Use of 125] The highly energetic beta emission combined with the gamma photons of!"] results in a rather uniform distribution of the radiation dose in the individual thyroid cell. Since, as stated above, the reproductive capacity of thyroid cells is more radiosensitive than the functional aspect, it seems very unlikely that reduc- tion of the functional integrity can occur following ''I without subsequentcell death and eventual hypothyroidism. With "5] the situation is theoretically quite different. Most of the radiation effect is probably due to the very low energy conversion and Auger electrons.***! The lower energy Augerelectrons (0.8-2.9 kev) are particularly abundant (Table 6-1). The range of these electrons in tissue is very small (from < 0.4 to 20 wm). Since the radioiodine is primarily in the colloid, the radiation dose distribution is markedly nonhomogeneous, affecting the cellular cytoplasm at the apex ofthe cell to a much greater extent than the nucleus (Fig. 6-2). Dose estimates for whole-gland distribution can be in serious error relative to the microscopic distribution. Difficulties in calculation are evident because of the marked variation in follicle size and cell size in the normal and hyperthyroid gland. Several authors have made such calculations,**5* and these have shown ratios of about 3-4 to 1 for the colloid-cell interface dose to the dose at the nucleus (Table Ci a crn 6-2). Theoretically this concentration of dose at the colloid-cell interface should