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

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

J Nucl Med. 2017 Apr 13;:

Authors: Nichols KJ, Robeson W, Yoshida-Hay M, Zanzonico PB, Leveque FC, Bhargava KK, Tronco GG, Palestro CJ

Abstract
To protect bone marrow from over-irradiation, maximum permissible activity (MPA) of iodine-131 ((131)I) to treat thyroid cancer is that which limits absorbed dose to blood (as a surrogate of marrow) to < 200 cGy. The conventional approach (Method-1) requires repeated gamma-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 (131)I uptake and MPA for initial treatment following thyroidectomy (N = 39), including 17 patients with compromised renal function and 22 patients with known (N = 16) or suspected (N = 6) metastases. Total absorbed dose to blood (DTotal) was the sum of mean whole-body γ ray dose component (Dγ) from un-collimated gamma-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-hour gamma-camera measurement. MPA was computed as 200 cGy/DTotal for each DTotal estimate. Results: Method-2 had strongest correlation with conventional Method-1 (r = 0.98) and values similar to Method-1 (21.0±13.7 cGy/GBq versus 21.0±14.1 cGy/GBq, P = 0.11), while Method-3 had a weaker (P = 0.001) correlation (r = 0.94), and Method-4 had 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 (131)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.

PMID: 28408530 [PubMed - as supplied by publisher]

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