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Cancer mortality risk of radioactive iodine for hyperthyroidism examined

By Nicole MacKee
Radioactive iodine treatment of hyperthyroidism is associated with a modest, lifetime risk of cancer death, with the risk increasing with greater doses, say researchers in JAMA Internal Medicine.

The 24-year extension of the Cooperative Thyrotoxicosis Therapy Follow-Up Study, which had followed US and UK patients with hyperthyroidism for almost 70 years, assessed 18,805 patients with hyperthyroidism who were treated with radioactive iodine and had no history of cancer.

During a mean follow up of 26 years, the researchers calculated a 6% increased risk of death from all solid cancers per 100-milligray (mGy) dose to the stomach, a 12% increased risk of breast cancer death per 100-mGy dose to the breast, and a 5% increased risk of all solid cancers (excluding breast cancer) per 100-mGy dose to the stomach.

Professor Duncan Topliss, Director of the Department of Endocrinology and Diabetes at the Alfred Hospital and Professor of Medicine at Monash University, said the research was the best available data on the dose-response relation between low-dose radioactive iodine treatment and risk of cancer death. But he emphasised that the risk was longterm – among patients in their 40s, only 8% of excess solid cancer deaths occurred in the first 20 years after treatment – and didn’t take account of the risks of other treatments or not treating at all.

‘[This research] doesn’t talk about all-cause mortality,’ he said. ‘On [the researchers’] calculations, people may get slightly more cancer deaths, but there may be fewer cardiovascular deaths, such as deaths from embolic stroke. This is giving us an idea about risk, but it’s not telling us about benefit of radioactive iodine.’

Professor Topliss said in the real world there were no perfect treatments, noting that agranulocytosis was a rare, but potentially life-threatening, side effect of antithyroid medications, and surgery also carried risks, including a very low risk of death.

‘There might be an increased cancer death rate over a lifetime, but that doesn’t mean that people might not be doing better overall,’ he said.

Professor Topliss said the findings were unlikely to change current practice, but did provide additional information to allow a more informed choice by the patient in discussion with their doctor.

‘It does tell us that we should try to minimise the dose, but I think we do that anyway,’ he said.
JAMA Intern Med 2019; doi:10.1001/jamainternmed.2019.0981.