A simple method for auditing thyroid absorbed dose from 131I in the treatment of benign thyroid disease

Objectives There are widely available guidelines for recommended absorbed thyroid doses and administered activities of I-131 for different benign thyroid diseases. It is useful to be able to assess if these target absorbed doses are being achieved. This work aimed to develop a simple method, and assess its value, to calculate achieved absorbed doses when using recommended administered activities and assess deviation from recommended target absorbed doses.

Methods 30 patients from January 2013 onwards (23 female, 7 male) were analysed; Graves’ n=22, autonomous adenoma n=5, toxic multinodular goitre n=3. Using published data, uptake of the ¹³¹I therapy activity was estimated from routine prior TcO₄^- thyroid uptake scan results. Administered activities and published disease specific effective half-life values (T_DS) and a fixed half-life of 5 days (T_FIXED) were then used to calculate cumulated activities for both half-lives. Standard MIRD adult male and female S-Factors were generated and used to calculate respective absorbed doses to the thyroid, D_DS and D_FIXED. In addition, administered activities required to achieve the recommended target absorbed doses, A_DS and A_FIXED, were calculated using the different half-lives and compared to those administered.

Results For patients with Graves disease, D_DS were 8% higher than D_FIXED. Both sets of doses were on average 73% and 60% higher than the target absorbed dose respectively. A_DS and A_FIXED to achieve 300Gy to the thyroid are 60% and 65% lower than administered activities on average respectively. For patients with autonomous adenoma, D_DS were 14% higher than D_FIXED. Both sets of doses were well matched to the target absorbed dose range. A_DS and A_FIXED to achieve the upper limit of this range (400Gy to the thyroid) are 10% and 20% higher than administered activities on average respectively.

Conclusions A simple method incorporating published data and routine thyroid uptake scan measurements was developed and used to provide a useful assessment of absorbed doses for auditing purposes. Absorbed dose calculations using both fixed and disease specific half-lives indicate a reduction in administered activities could be considered for patients with Graves disease. Further work is required incorporating patient follow-up blood test results to further inform any possible change in dosing regime.