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

To protect bone marrow from overirradiation, the maximum permissible activity (MPA) of ¹³¹I 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 determine ¹³¹I 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 (D_Total) was the sum of mean whole-body γ-ray dose component (D_γ) from uncollimated γ-camera measurements and dose due to β emissions (D_β) from blood samples. Method 2 estimated D_Total from D_β alone, method 3 estimated D_Total from D_γ alone, and method 4 estimated D_Total from a single 48-h γ-camera measurement. MPA was computed as 200 cGy/D_Total for each D_Total 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 with ¹³¹I 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.