Modeling the absorbed dose to the common carotid arteries following radioiodine treatment of benign thyroid disease

Abstract

External fractionated radiotherapy of cancer increases the risk of cardio- and cerebrovascular events, but less attention has been paid to the potential side effects on the arteries following internal radiotherapy with radioactive iodine (RAI), i.e. 131-iodine. About 279 per million citizens in the western countries are treated each year with RAI for benign thyroid disorders (about 140,000 a year in the EU), stressing that it is of clinical importance to be aware of even rare radiation-induced side effects. In order to induce or accelerate atherosclerosis, the dose to the carotid arteries has to exceed 2Gy which is the known lower limit of ionizing radiation to affect the endothelial cells and thereby to induce atherosclerosis. To estimate the radiation dose to the carotid arteries following RAI therapy of benign thyroid disorders. Assuming that the lobes of the thyroid gland are ellipsoid, that the carotid artery runs through a part of the lobes, that there is a homogeneous distribution of RAI in the lobes, and that the 24h RAI uptake in the thyroid is 35% of the (131)I orally administrated, we used integrated modules for bioassay analysis and Monte Carlo simulations to calculate the dose in Gy/GBq of administrated RAI. The average radiation dose along the arteries is 4-55Gy/GBq of the (131)I orally administrated with a maximum dose of approximately 25-85Gy/GBq. The maximum absorbed dose rate to the artery is 4.2Gy/day per GBq (131)I orally administrated. The calculated radiation dose to the carotid arteries after RAI therapy of benign thyroid disorder clearly exceeds the 2 Gy known to affect the endothelial cells and properly induce atherosclerosis. This simulation indicates a relation between the deposited dose in the arteries following RAI treatment and an increased risk of atherosclerosis and subsequent cerebrovascular events such as stroke.