Radioiodine Therapy of Hyperthyroidism - Simplified patient-specific absorbed dose planning

Abstract: Radioiodine therapy of hyperthyroidism is the most frequently performed radiopharmaceutical therapy. To calculate the activity of 131-I to be administered for giving a certain absorbed dose to the thyroid, the mass of the thyroid and the individual biokinetic data, normally in the form of uptake and biologic half-time, have to be determined. The biologic half-time is estimated from several uptake measurements and the first one is usually made 24 hours after the intake of the test activity. However, many hospitals consider it time-consuming since at least three visits of the patient to the hospital are required (administration of test activity, first uptake measurement, second uptake measurement plus treatment). Instead, many hospitals use a fixed effective half-time or even a fixed administered activity, only requiring two visits. However, none of these methods considers the absorbed dose to the thyroid of the individual patient. In this work a simplified patient-specific method for treating hyperthyroidism is proposed, based on one single uptake measurement, thus requiring only two visits to the hospital. The calculation is as accurate as using the individual biokinetic data. The simplified method is as patient-convenient and time-effective as using a fixed effective half-time or a fixed administered activity. The simplified method is based upon a linear relation between the late uptake measurement 4-7 days after intake of the test activity and the product of the extrapolated initial uptake and the effective half-time. Treatments not considering individual biokinetics in the thyroid result in a distribution of administered absorbed dose to the thyroid, with a range of -50 % to +160 % compared to a protocol calculating the absorbed dose to the thyroid of the individual patient. Treatments with a fixed administered activity of 370 MBq will in general administer 250 % higher activity to the patient, with a range of -30 % to +770 %. The absorbed dose to other organs than the thyroid is also influenced. These doses should also be considered in estimating the risk of late radiation effects in the patients. This is becoming more important as an increasing number of younger patients are treated with radioiodine. If all Swedish hospitals considered the individual biokinetic data the total administrated activity of 131-I would decrease by 10 % (100 GBq) corresponding to a yearly collective effective dose of 17 manSv, thyroid excluded. Seventeen different methods to determine the administered activity of 131-I are in use in 23 Swedish hospitals. Only nine hospitals calculate the administered activity of 131-I using individual biokinetic data. More effort should be done to consider the individual biokinetic data when calculating the administered activity of 131-I and thus decrease unnecessary radiation dose to individual patients, their families and the public.