The Study On Radiation Protection Dosimetry Of Radioactive ¹³¹I Therapy For Differentiated Thyroid Carcinoma

Radiation safety is an integral part of targeted radionuclide therapy. The radioactive isotope 131 I was selected to treat patients for differentiated thyroid carcinoma, mainly due to the specific ability of thyroid tumor cells to take up this isotope I3I I, which emits beta rays to kill thyroid cancer cells and eliminate the focus. However, 131 I also emits gamma rays, whose strong ionizing radiation effects can cause damage to nearby individuals. In addition, the treated patient could spread radioactive contamination and pollute the surroundings. Therefore, before discharging the patients, the external dose rate and retained body activity must be reduced to an acceptable level.The aim of this work was to study the normalized external dose rate and retained body activity as functions of time in differentiated thyroid carcinoma patients receiving 131 I therapy. Seventy patients were stratified into two groups:the ablation group (A) and the follow-up group (FU).The patients’external dose rate was measured. In this study, the normalized external dose rate of group (A) at 1,6,18,24,30,42,48,54,66 and 72 h at 1 m were 0.052 ±0.006,0.032 ±0.009,0.015 ±0.005,0.009± 0.003,0.006 ±0.003,0.003 ±0.002, 0.002 ± 0.001,0.002 ± 0.001,0.001 ± 0.001 and 0 ± 0 μSv·h-1·MBq-1, respectively; the normalized external dose rate of group (FU) at 1,6,18,24,30,42,48,54,66 and 72 h at 1 m were 0.047 ± 0.007,0.029 ± 0.006,0.013 ± 0.003,0.009 ± 0.003,0.006 ± 0.002,0.003 ± 0.002,0.002 ± 0.001,0.001 ± 0.001,0.001 ± 0.001 and 0 ± 0 μSv·h-1·MBq-1, respectively. And simultaneously, their retained body radiation activity was monitored at various time points. The excretion of 131 I was fast during the first 24 h and became increasingly slow afterwards. Nevertheless, it was obvious that most of the activity was eliminated within the first 72 h.The equations of the external dose rate and the retained body activity, described as a function of hours post administration, were fitted. Additionally, the release time for patients was calculated. The reduction in activity in the group receiving a second or subsequent treatment was more rapid than the group receiving only the initial treatment. Most important, an expeditious method was established to indirectly evaluate the retained body activity of patients by measuring the external dose rate with a portable radiation survey meter. This method proved to be simpler and will result in less radiation exposure than with the urine assay method. Also noteworthy, compared to the whole-body counting method, this method is superior cost wise. By this method, the calculated external dose rate limits are 19.2,8.85,5.08 and 2.32 μSv·h-1 at 1,1.5,2 and 3 m, respectively, according to a patient’s released threshold level of retained body activity<400 MBq. This study is beneficial for radiation safety decision-making.