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

To protect bone marrow from overirradiation, the maximum permissible activity (MPA) of131 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. Methods: Data acquired over multiple time points were examined retrospectively for 65 thyroid cancer patients, referred to determine131 I uptake and MPA for initial treatment after thyroidectomy ( n = 39), including 17 patients with compromised renal function and 22 patients with known ( n = 16) or suspected ( n = 6) metastases. The total absorbed dose to blood (DTotal ) was the sum of mean whole-body γ-ray dose component (Dγ ) from uncollimated γ-camera measurements and dose due to β emissions (Dβ ) from blood samples. Method 2 estimated DTotal from Dβ alone, method 3 estimated DTotal from Dγ alone, and method 4 estimated DTotal from a single 48-h γ-camera measurement. MPA was computed as 200 cGy/DTotal for each DTotal estimate. Results: Method 2 had the strongest correlation with conventional method 1 ( r = 0.98) and values similar to method 1 (21.0 ± 13.7 cGy/GBq vs. 21.0 ± 14.1 cGy/GBq, P = 0.11), whereas method 3 had a weaker ( P = 0.001) correlation ( r = 0.94) and method 4 had the weakest ( P < 0.0001) correlation ( r = 0.69) and lower dose (16.3 ± 14.8 cGy/GBq, P < 0.0001). Consequently, correlation with method 1 MPA was strongest for method 2 MPA ( r = 0.99) and weakest for method 4 ( r = 0. 75). Method 2 and method 1 values agreed equally well regardless of whether patients had been treated with131 I previously or had abnormal renal function. Conclusion: 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.