Accuracy considerations when using early (four- or six-hour) radioactive iodine uptake to predict twenty-four-hour values for radioactive iodine dosage in the treatment of Graves' disease

Although literature has offered methods to predict 24-hour radioactive iodi ne uptake values from early (4- to 6-hour) measurements, the resultant dosa ge errors have not been examined. Potential errors include underdosage, ove rdosage, and a failure to recognize rapid turnover patients (early-to-late uptake ratios greater than or equal to 1) who are at high risk for treatmen t failure and full-body radiation exposure. We developed and tested a novel method for minimizing error involved in using a single early uptake measur ement to derive late uptake. From a retrospective analysis of 203 Graves' d isease patients, receiver operating characteristic (ROC) curve analysis ena bled us to identify patients likely to experience rapid turnover and theref ore should receive 24-hour studies. Twenty-four-hour uptake measurements ar e necessary with 77% or more 4-hour uptake values and 80% or more 6-hour va lues. After eliminating these patients, we developed linear regression equa tions to predict the 24-hour uptake from 4-hour (n = 61) and 6-hour (n = 22 ) rule groups, testing their efficacy on separate 4-hour (n = 61) and 6-hou r (n = 21) patient groups. We also used our test population to measure erro r in four early-to-late uptake conversion formulas presented in the literat ure. Error involved in these predictions ranged from a 10.6% overestimate f or 4-hour calculations to a 5.9% underestimate for 6-hour calculations. Whe n applied to two dosage formulas incorporating gland size, absorbed dose, a nd 24-hour uptake, average dosage error was 7%. In comparison to the other sources of error radioactive iodine (I-131) dosimetry, potential error in p redicting 24-hour uptake from 4- or 6-hour uptake values is low.