Thyroid uptake and radiation dose after (131)I-lipiodol treatment: is thyroid blocking by potassium iodide necessary?

Abstract

In radionuclide therapy with iodine-131 labelled pharmaceuticals, free (131)I may be released and trapped by the thyroid, causing an undesirable radiation burden. To prevent this, stable iodide such as potassium iodide (KI) can be given to saturate the thyroid before (131)I is administered. The guidelines of the European Association of Nuclear Medicine do not, however, recommend special precautions when administering (131)I-lipiodol therapy for hepatocellular carcinoma. Nevertheless, some authors have reported (131)I uptake in the thyroid as a consequence of such therapy. In this study, the influence of prophylactic KI on the thyroid uptake and dose (MIRD dosimetry) was prospectively investigated. (131)I-lipiodol was given as a slow bolus selectively in the proper hepatic artery or hyperselectively in the right and/or left hepatic artery. Patients were prospectively randomised into two groups. One group received KI in a dose of 100 mg per day starting 2 days before (131)I-lipiodol administration and continuing until 2 weeks after therapy (KI group; n=31), while the other group received no KI (non-KI group; n=37). Thyroid uptake was measured scintigraphically as a percentage of administered activity 7 days after (131)I-lipiodol ( n=68 treatments). The absorbed radiation dose to the thyroid was assessed by scintigraphy after 7 and 14 days using a mono-exponential fitting model and MIRD dosimetry ( n=40 treatments). The mean activity of (131)I-lipiodol administered was 1,835 MBq in a volume of 2 ( n=17) or 4 ( n=51) ml. Thyroid uptake was lower in the KI group, being 0.23%+/-0.06% of injected activity ( n=31) compared with 0.42%+/-0.20% in the non-KI group ( n=37); the mean thyroid dose was 5.5+/-1.6 Gy in the KI group ( n=19) versus 11.9+/-5.9 Gy in the non-KI group ( n=21). These differences were statistically significant ( P<0.001). No effect of the amount of added cold lipiodol (4 vs 2 ml total volume) or selectivity of (131)I-lipiodol administration was evident ( P>0.1). (131)I-lipiodol is associated with a generally low thyroid uptake and dose that may be significantly decreased by KI premedication. Given the low cost and the very good tolerance of the KI treatment, we believe the use of KI should be recommended in the majority of the patients.