Determination of blood dose in patients administered radioiodine therapy

Internal dosimetry is a controversial issue in the therapy of thyroid carcinoma with I-131 (radioiodine). Radiation absorbed dose to the red bone marrow, a critical organ in the therapy of thyroid carcinoma, has to be less than < 2 Gy. The aim of this study was to calculate bone marrow radiation dose, as it is highly radiosensitive, by using a modified approach based on the formalism of the Medical International Radiation Dose Committee (MIRD) and MIRDOSE3 software. Method: 3700 MBq, 5550 MBq and 7400 MBq radioiodine ( 131I ) are administered to patients for tyroid carcinoma treatment. In this study, there were total 83 patients whom was 3 group according to administered radioiodine dose. There were 48 patients (14 male, 34 female with age range from 22 to 79 years) in the first group that was administered 3700 MBq. There were 18 patients (3 male, 15 female with age range from 28 to 67 year) in the second group that was administered 5550 MBq. There were 17 patients (5 male, 12 female with age range from 23 to 71years) in the third group that was administered 7400 MBq. After the radioiodine threapy, urine samples were collected 1st d, 2nd d, 3rd d, 4th d, 5th d. on 51 patients and 6th h, 12th h, 18th h, 24th h on 32 patients of all patients. Also blood samples were taken in 2, 19, 24, 48, 72, 96, 120 and 144th hours. Samples were counted well-type gamma detector and time-activity curves have been generated. Dose-rate measurements were done by Geiger-Müller counter from 1 meter distance. Data on urine and blood clearance rate of radioiodine, absorbed radiation dose to bone marrow of patients calculated by using MIRD method. The amount of activity relationship with radiation dose and radioiodine effective half-life was determined by ANOVA test. The relationship with data from MIRD method and MIRDOSE3 software was determined by Unpaired t test Results: Effective half life that is calculated from urine sample measurements has found 18 ± 1.8 h within the first 24 h and 23±5.74 h between 24?120 h. It?s found that 56% of RAI are excreted by urine after 24-hour. Retention activity of patients who had 3700 MBq, 5550 MBq and 7400 MBq RAI were measured as 1608 MBq, 2503 MBq and 3100 MBq respectively 24-hour later. Dose-rate were 33.7 µSv/hr, 85 µSv/hr and 183 µSv/hr in order. Effective half-life of radioactive iodine from blood samples in patients receiving 3700MBq, 5550MBq, 7400MBq were calculated as 20.57±5.4, 17.8±5.8 and 18.7±3.9 hours, respectively. The average of the effective half-life for all the given doses was calculated as 20,4±7.01 hours. Bone marrow doses from bladder for patients receiving 3700MBq, 5550MBq, 7400MBq were found as 0.15±0.04 Gy, 0.27±0.09 Gy and 0.26±0.05 Gy, respectively. Bone marrow doses from total body for patients receiving 3700MBq, 5550MBq, 7400MBq were found as 0.32 ± 0.08 Gy, 0.42 ± 0.14 Gy and 0.60 ± 0.24 Gy, respectively. No statistically significant difference was detected between the effective half-life and the dose of administered radioactivity. According to MIRDOSE3 software, bone marrow doses was calculated as 0.04±0.016 mGy/MBq. Statistically, bone marrow doses from MIRD method and MIRDOSE3 software was no significant difference (p=0.07).Conclusion: In case of the recommended dose-rate was accepted as 50 µSv/hr to discharge of patients, 24-hour after 3700 MBq administration and 48-hour after 5550 MBq and 7400 MBq administration we can discharge patients. Blood biokinetics of the given radioactive iodine is not associated with the amount of activity. The amount of activity was correlated with bone marrow doses and our findings can be used as an extrapolation coefficient to determine the total bone marrow dose.Key Words: Thyroid carcinoma, internal dosimetry, MIRD method, I-131, bone marrow