Alternative Means of Estimating 131 I Maximum Permissible Activity to Treat Thyroid Cancer

To protect bone marrow from overirradiation, the maximum permissible activity (MPA) of I to treat thyroid cancer is that which limits the absorbed dose to blood (as a surrogate of marrow) to less than 200 cGy. The conventional approach (method 1) requires repeated γ-camera whole-body measurements along with blood samples. We sought to determine whether reliable MPA values can be obtained by simplified procedures. Data acquired over multiple time points were examined retrospectively for 65 thyroid cancer patients, referred to determine I uptake and MPA for initial treatment after thyroidectomy ( = 39), including 17 patients with compromised renal function and 22 patients with known ( = 16) or suspected ( = 6) metastases. The total absorbed dose to blood (D ) was the sum of mean whole-body γ-ray dose component (D ) from uncollimated γ-camera measurements and dose due to β emissions (D ) from blood samples. Method 2 estimated D from D alone, method 3 estimated D from D alone, and method 4 estimated D from a single 48-h γ-camera measurement. MPA was computed as 200 cGy/D for each D estimate. Method 2 had the strongest correlation with conventional method 1 ( = 0.98) and values similar to method 1 (21.0 ± 13.7 cGy/GBq vs. 21.0 ± 14.1 cGy/GBq, = 0.11), whereas method 3 had a weaker ( = 0.001) correlation ( = 0.94) and method 4 had the weakest ( < 0.0001) correlation ( = 0.69) and lower dose (16.3 ± 14.8 cGy/GBq, < 0.0001). Consequently, correlation with method 1 MPA was strongest for method 2 MPA ( = 0.99) and weakest for method 4 ( = 0. 75). Method 2 and method 1 values agreed equally well regardless of whether patients had been treated with I previously or had abnormal renal function. Because MPA based on blood measurements alone is comparable to MPA obtained with combined body counting and blood sampling, blood measurements alone are sufficient for determining MPA.