The minimum radioactive dose for treating thyroid cancer

The approach to treating thyroid cancer has changed over time. One method is using radioactive iodine to kill left over cells after surgically removing the thyroid gland fully or partially. The American Thyroid Association is in the process of publishing new guidelines, with significant changes in the amount of radioactive material to be given to a patient. The guidelines are expected to impact many countries, including India, where doctors follow the American guidelines in the absence of their own protocol. An Indian research, led by Chandrashekhar Bal of the Department of Nuclear Medicine, All India Institute of Medical Sciences (AIIMS), Delhi, is at the heart of these changes in protocol. He spoke to Jyotsna Singh about the studies conducted at AIIMS. Some excerpts:

By Jyotsna Singh
Published: Saturday 02 February 2013

Chandrashekhar Bal What was your study about and what were the conclusions reached?

We first conducted a study from 1990-95 among 149 patients to analyse the differences of outcome when different amounts of radioactive dosages were given for remnant ablation. The patients were divided into four groups and were administered radioactive iodine of 30, 50, 100 and 200 mCi (millicurie) each. We found that the success rate of removing left over cancerous cells was the same in the latter three groups. There was no difference in treatment outcome for a person given 50 mCi compared to the one given 200 mCi. The success rate for first group was 65 per cent and 78, 77 and 77 percent for second, third and fourth groups respectively.

The second study was conducted in late 2000s, for which the results were published in May 2012 in the journal Nuclear Medicine Communications. The first study had established 50 mCi as the limit. We wanted to see if the dose can be further brought down. We studied 509 patients in eight groups giving them radiation of 15, 20, 25, 30, 35, 40, 45 and 50 mCi each. Success rate was the same for groups which were given 25 to 50 mCi of radioactive material. Thus, we established that the minimum dose is 25 mCi to treat thyroid cancer. Due to small sample size in the first study, success rate of 30 mCi was not equivalent to other amounts.

Have there been some corroborative studies? How has this changed the approach in treatment of thyroid cancer?

Yes. Two studies in France and the United Kingdom reached the same conclusion. They added more factors due to difference in treatment. But the conclusion for radiation dose was the same.

Conventionally, it was believed that high dose of radioactive iodine is good to treat thyroid cancer. Doctors would give 300-350 mCi to a single patient. The argument was: high dose would kill not only cancerous cells in the vicinity of the gland, but also those which would have escaped and were circulating in the blood. But this had many side-effects. In 1990s people started questioning this method. The approach of ALARA or As Low As Reasonably Achievable was being promoted. With these studies, we have changed the way thyroid cancer is treated.

What is the rationale for reducing the amount of radioactive iodine dose?

Patients have to face many side-effects. Iodine affects many parts of the body. It permanently damages the mouth and eye as it is flushed out through them. It destroys cells in the salivary gland leading to a condition of permanent dry mouth. The patient has to take water with any solid food throughout life. The patient also suffers from dryness in eyes.

Ovaries in women and testicles in men also receive unnecessary radiation, leading to complications. Further, high radiation can also result in second cancer later in life as well as anaemia.

What has been the impact of this study?

The American Thyroid Association (ATA) is in the process of revising its guidelines. Presently, it recommends that doctors should use low dose of radiation. The revised guidelines will fix the dose at 30 mCi in recommendations. From the beginning, people have shown interest in the work. Director of Swedish Karolinska Institute had wondered in late 1990s if we could find the minimum dose needed to treat remnant ablation. That is how the second study came out. Now it will impact a large number of countries which follow the ATA recommendations because of absence of guidelines in their own system.


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