Lesion-Wise Comparison of Pre-Therapy and Post-Therapy Effective Half-Life of Iodine-131 in Pediatric and Young Adult Patients with Differentiated Thyroid Cancer Undergoing Radioiodine Therapy / 대한핵의학회잡지

PURPOSE: The effective half-life of radioiodine is an important parameter for dosimetry in differentiated thyroid cancer patients, particularly in children. We determined the pre-therapy and post-therapy effective half-life in different types of lesions, i.e., remnant, node, or lung metastases.METHODS: Of 84 patients recruited, 27 were < 18 years (group 1) and the remaining 57 were between 18 and 21 years (group 2). A total of 114 studies were conducted and 253 lesions were analyzed. Serial whole-body scans were acquired at 24, 48, and ≥ 72 h after administration of iodine-131. Region of interests was drawn over lesions to determine counts in the lesion. Time versus counts graphs were plotted and mono-exponentially fitted to determine effective half-life.RESULTS: The post-therapy effective half-life was found to be lesser than pre-therapy effective half-life in all types of lesions and in all groups. Median effective half-life was found maximum in intact lobe, minimum in the lung, and intermediate in remnant and nodes. In the assessment of all lesions together, pre- and post-therapy median and interquartile range (IQR) effective half-life were 59.8 (37–112) h and 48.6 (35.2–70.8) h (p < 0.0001) in group 1, 73.9 (46.2–112.7) h and 60 (57.4–85.9) h (p < 0.0001) in group 2, and 68.6 (41.53–112.36) h and 54.7 (36–80.6) h (p < 0.0001) in combined group, respectively. Importantly, the pre- and post-therapy median effective half-life serially dropped after each successive cycles of iodine-131.CONCLUSIONS: There was a significant difference in pre-therapy and post-therapy effective half-life in all types of lesions. These results may have implications in calculating the correct therapeutic dose in children and in young adults.

Effective Half-life of I-131 in Patients with Differentiated Thyroid Cancer Treated by Radioactive I-131 / 핵의학분자영상

PURPOSE: Effective half life of I-131 (T(eff)) in patients with differentiated thyroid cancer treated by I-131 is must-know value for dose calculation and determination of release time from isolation room. There has been no report about T(eff) in Koreans. Thus, author tried to measure dose rate without radiation exposure to faculty members and calculated T(eff). METHODS: Probe of radiation survey meter was fixed at the wall of isolation room, and body of survey meter was placed outside the room. With this simple arrangement, author could measure radiation frequently without radiation exposure to faculty members in 68 patient (F=55, M=13, age=47+/-13.7) treated by I-131 (3.7~7.4 GBq) for differentiated thyroid cancer from Jan 2006 to Dec 2006. From this data, T(eff), 48 hr retention rate, and the time necessary to whole body retention of I-131 become less than 1.1 GBq were calculated. Serum creatinine levels were measured before and after thyroid hormone withdrawal. RESULTS: T(eff) was 15.4+/-4.3 hr (9.4~32.5 hr). There was a loose correlation between T(eff) and serum creatinine concentration (r=0.45). 48hr retention was 4.9+/-4.2% (1~23%). Time necessary to whole body retention of I-131 become less than 1.1 GBq was calculated as 47.1+/-13.2 hr for 9.25 GBq, 42.1+/-11.9 hr for 7.4 GBq, 35.7+/-10.0 hr for 5.55 GBq, and 26.7+/-7.5 hr for 3.7 GBq dose of I-131. CONCLUSION: Author successfully measured radiation dose rates in isolated patients treated by high dose of I-131 without radiation exposure to the faculty members with simple arrangement of survey meter probe. Using those data, T(eff) and some other indices were calculated.