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expert reaction to study looking at radioactive iodine treatment for overactive thyroid and risk of cancer

Research, published in JAMA Internal Medicine, reports that patients that have received radioactive iodine treatment for an overactive thyroid are at greater risk of developing breast and stomach cancer. 


Dr Heather Williams, Principal Medical Physicist, Nuclear Medicine, The Christie, said:

“This is a large study monitoring over 18,000 patients who received radioactive iodine treatment for hyperthyroidism over 50 years ago, and suggests this is linked to a slightly higher risk of developing breast cancer and stomach cancer.  However, the researchers also say that they cannot be very certain about how big the risk is.  This is because it is very difficult to calculate these risks as there is variation between individual patients in how iodine is processed by the body, the alternative treatments available at any point in time, and how each person responds to the various different treatments.  All treatments for hyperthyroidism carry risks as well as benefits, and I would advise patients to discuss all the options with their doctor before making a decision before or against a particular type of treatment.”


Prof Jim Smith, Professor of Environmental Science, University of Portsmouth, said:

“This study provides further evidence that radiation can cause cancer, but that cancer risks from radiation are relatively small.  Extremely high doses to organs required for radiation therapy can cause other parts of the body also to receive much lower, but still significant doses.

“This study conducted a long follow up of patients exposed to extremely high radiation doses for treatment of hyperthyroidism and found that the lower exposure to other tissues resulted in a “small long-term increase in number of solid cancer deaths … amongst patients”.  Despite the extremely high thyroid doses, no increase in thyroid cancer was observed: this was attributed to the low sensitivity of already developed thyroids in adults and to the low number of thyroid cancer deaths in the group of patients, making statistical detection of an effect difficult.  Children are known to be more susceptible to radiation-induced thyroid cancer than adults.

“This study does not in any way mean that radiation therapy is not the best treatment for hyperthyroidism, but it helps evaluate the risks.”


Prof Saad Shakir, Director of the Drug Safety Research Unit (DSRU), said:

“This is an excellent observational study from the largest registry in the world.  The data go back in decades and the number of patients is large.  The modest increase in the incidence of solid tumour related to dose of iodine absorbed by organs is credible.

“The influence of the finding of this study on clinical decisions has to be put in context with the benefits of the use radioactive iodine for hyperactive thyroid disease and with the benefits and risks of different therapies.  This fine balance can be discussed by doctors with patients when choosing the best therapy for the treatment of hyperactive thyroid disease.”


David Dommett, Chair of the Medical Committee of The Society for Radiological Protection, said:

“The study itself is of great relevance – our understanding of cancer causes and organ response to radiation has changed greatly over the years, and since the activity / dose associated with sodium-iodide 131 can be considered significant this is worthwhile.

“The methodology, assumptions and conclusions appear sound, taking into account the acknowledged difficulties in such a large scale study and associated confounding issues.  The data provided appears to support the conclusions within the study.  The calculation of relative risk to organs other than the thyroid is very beneficial and may have relevance to other high dose therapies.  The lack of thyroid cancer death and leukaemia mortality despite the high dose radiation exposure was interesting, but perhaps also highlights the difficulty in identifying causes of cancer when so many other factors can attribute or contribute to this.

“Under the Ionising Radiation (Medical Exposure) Regulations 2017 (English law) patients must have the benefits and risks of any therapy explained to them.  The quantitative estimates of the risks of radiation-associated cancer deaths in RAI-treated patients with hyperthyroidism provided by this study and evidence that the risk of death from solid cancer (including breast cancer) increases with the greater absorbed dose to exposed organs and tissues has to be noted and made available to those involved in I-131 therapies.  Certainly I support the conclusion that further studies are needed to provide more detail on this issue.

“As with any area of treatment where there is an inherent risk, especially when that risk is cancer, a topic such as this must be approached sensitively and the risks properly understood so that people can properly balance the benefits of the treatment against what may be an increased risk factor – but not a certainty.”


Jim Thurston, radiation protection expert, Institute of Physics and Engineering in Medicine, said:

Does the press release accurately reflect the science?


Is this good quality research?  Are the conclusions backed up by solid data?

“Yes, but there are limitations on the estimation of the radiation doses received by the cohort.

How does this work fit with the existing evidence?

“It fits well, and is a significant addition to the evidence.

Have the authors accounted for confounders?  Are there important limitations to be aware of?

“The author has tried to account for confounders, but the most significant one is the lack of data on actual radiation doses received based on imaging calculations rather than more basic estimations.

What are the implications in the real world?  Is there any overspeculation?

“There is no overspeculation – however this paper would support other research suggesting that such therapies are not optimised – that some patients in the cohort received more radiation dose than was required to treat the thyroid, whilst others may not have received enough.  This is due to the fact that such therapies are still mainly given based on a fixed quantity of iodine being given, rather than planned and adjusted to the specific disease in the individual patients.  The implication is that the excess incidence rate of cancer in patients treated could be more than it should be if the treatments were properly planned.

“The paper shows a relationship between radiation dose to the thyroid and stomach, and the excess cancer risk.  However, such treatments with radioactive Iodine continue to be given in most countries, including the UK, on a fixed administered activity basis – i.e. that all adults are given a fixed amount of Iodine, irrespective of the resulting radiation doses delivered to the target based on the extent or state of their disease.

“This goes against one of the fundamental principles of radiation therapy set out in a European Directive and in UK Regulations [1,2] – that all such radiation therapies should be individually planned, optimised and verified so that the minimum radiation dose is given to the patient whilst still achieving the therapeutic effect.  This is the basis of all external beam radiotherapy and brachytherapy carried out in the UK, however it is not the basis for radionuclide therapies carried out such as iodine treatment for thyrotoxicosis, and for thyroid and other cancers.  This is despite the fact that Nuclear Medicine / PET imaging and quantitative analysis is readily available that could be used to individually plan and verify such treatments.

“This fact is highlighted in the article in the dose estimates given for the thyroid gland amongst the cohort of patients studied.  The intended (target) absorbed dose to the thyroid for treating Grave’s Disease is quoted elsewhere as being 60Gy (60,000mGy) [1].  However the patients in this cohort received a mean absorbed dose to the thyroid of up to 130Gy, over twice that target dose.  This is because the centres involved in this study, as we do in the UK, give fixed radioactivity to adult patients without calculating before the treatment how active the thyroid gland is in taking up iodine.

“I would suggest that this paper provides more evidence that carrying out radionuclide therapies without appropriate treatment planning first is most certainly not optimised (some patients may receive too much, and others too little) and may be considered to not meet the requirement of the EU Directive and UK Regulations.”

1 Dosimetry-based treatment for Graves’ disease.  Hyer, S. L; Pratt, B; Gray, M; Chittenden, S; Du, Y; Harmer, C. L; Flux, G. D.  Nuclear Medicine Communications: June 2018 – Volume 39 – Issue 6 – p 486–492.

2 Council Directive laying down basic safety standards for protection against the dangers arising from exposure to ionising radiation. (2013/59/EURATOM).

3 Ionising Radiation (Medical Exposures) Regulations 2017.  (SI1322, 2017).



* ‘Association of radioactive iodine treatment with cancer mortality in patients with hyperthyroidism’ by Cari M. Kitahara et al. was published in JAMA Internal Medicine on Monday 1 July 2019. 


Declared interests

Dr Heather Williams: “I have nothing to declare.”

Prof Jim Smith: “I have recently had a Natural Environment Research Council (NERC) grant (ca. £450k) studying the effects of radiation on fish at Chernobyl. This was part of a programme jointly funded by NERC, the Environment Agency and Radioactive Waste Management Ltd. I have done a small (about £10k, paid to the university) consultancy job for Horizon Nuclear Power (completed 2012) and other small contracts to customers including the Japan Atomic Energy Agency and Radioactive Waste Management Ltd.. I have also in the past carried out a variety of consultancy to independent regulatory bodies (including Environment Agency; Food Standards Agency, International Atomic Energy Agency) worth about £100k to my institution. I don’t do consultancy in a personal capacity. I currently lead a small (£100k) research project funded by NERC on re-use of radioactively contaminated lands in Ukraine.”

None others received.


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