Changes In Radiation Dose Levels After ¹³¹I Treatment In Patients With Differentiated Thyroid Cancer

Objective:To study the changes of radiation dose in patients with differentiated thyroid cancer（DTC）after different pretreatment ¹³¹I treatment,and to explore the radiation protection in the clinical diagnosis and treatment process of DTC patients and self-management after discharge.Methods:Seventy-three patients with DTC who underwent bilateral total thyroidectomy at the First Hospital of Shanxi Medical University from April 2021 to November 2022 and were first treated with ¹³¹I"nail clearing"were prospectively collected and randomly divided into four groups:rh TSH group（0.9 mg,intramuscular gluteal injection,1time/day for 2 days）,23 cases,male/female（7/16）;L-T₄discontinuation group（no vitamin C,hormone and excretory drugs were given）,12 cases,male/female（5/7）;L-T₄discontinuation followed by vitamin C and glucocorticoid group:18 cases,male/female（4/14）;L-T₄discontinuation followed by vitamin C,glucocorticoid and excretory drugs group:20 cases,male/female（6/14）.All patients had TSH>30μIU/ml before ¹³¹I treatment.2h,6h,24h,32h,48h,56h,72h after taking 3700 MBq ¹³¹I,respectively,the dose equivalent rate was measured continuously by mobile robot at the patient’s neck（immediately adjacent）and body（at a distance of 1m）,and the standardized dose equivalent rate as a function of time was fitted by a nonlinear curve;and Outlining the48h whole-body iodine scan neck and abdominopelvic ROI and calculating the whole-body percentage of radioactive counts in the local area.By prospectively collecting 7 DTC patients who were treated with two different therapies in the intramuscular injection exogenous rh TSH group and the discontinuation of L-T₄group to form a before-and-after self control,all were given an oral diagnostic dose of ¹³¹I activity of 12.58±1.48MBq（3.4±0.4m Ci）,and the intra-serum radioiodine counts were measured at 2h,24h and 48h after oral ¹³¹I administration,respectively.The whole-body iodine scan abdominopelvic ROI was outlined at 2h,24h and 48h,respectively,and the whole-body percentage of radioactive counts in the local area was calculated.Results:1.The standardized dose equivalent rate as a function of time in the body parts of patients taking ¹³¹I were:（1）in the group injected with recombinant rh TSH:H_{body part}=0.1710e^-0.21616t+0.00501（P<0.05,R²=0.89）;（2）in the group discontinuing L-T4:H_{body part}=0.1268e^-0.0.1618t+0.00726（P<0.05,R²=0.92）;（3）on vitamin C and hormones:H_{body part}=0.1280e^-0.1375t+0.00345（P<0.05,R²=0.92）;（4）on vitamin C,hormones and excretory drugs:H_{body part}=0.12098e^-0.1017t+0.00101（P<0.05,R²=0.94）.In the four groups at 24h,the dose equivalent rates around 1m at 24h were 20.79μSv/h,36.22μSv/h,30.16μSv/h,42.73μSv/h,respectively;and Kruskal-Wallis test results showed that the difference in the percentage of radiation counts in the neck and abdominal basin department regions of the whole body iodine scan at 48h was not statistically significant in the four groups of DTC patients（all P>0.05）.2.Changes in radioactive counts in blood at the diagnostic dose of 131I at the time of intramuscular injection of rh TSH and discontinuation of L-T₄in patients with DTC.The results of paired t-test showed that patients with intramuscular injection of rh TSH had lower radioactive iodine counts in blood at 2h compared with the discontinuation of L-T₄group,and the difference was statistically significant（t=-4.52,P=0.04<0.05）;radioactive iodine counts in blood at 24h and 48h There was no difference（P>0.05）.3.The percentage of radioactive iodine counts in the local area of DTC patients that the percentage of radioactive counts in the abdominopelvic region at 24 h was lower in the analysis of patients with intramuscular rh TSH than in those who stopped taking L-T₄（t=-2.634,P=0.039<0.05）;the percentage of radioactive counts in the abdominopelvic region at all other time points was not significant（all P>0.05）.The Kruskal-Wallis test showed no statistically significant difference in the percentage of radiation counts in the cervical and abdominopelvic regions of the whole-body iodine scan at 48 h in patients with DTC（P>0.05）.Conclusion:1.In DTC patients treated with oral 3700 MBq 131I remnant ablation,the equivalent dose rate around 1m at 24h in patients treated with pre-treatment intram uscular rh TSH was 20.79μSv/h（<25μSv/h）,suggesting a faster clearance of radioa ctive iodine in patients and a shorter hospital restriction time.2.Intramuscular injection of rh TSH-prepared ¹³¹I for the treatment of patients with DTC reduces the retention time of ¹³¹I in the bloodstream,accumulation in the gastrointestinal tract,and the radiation effects in patients and their surrounding population with the addition of pro-discharge drugs if necessary.