CLINICAL THYROIDOLOGY FOR PATIENTS
A publication of the American Thyroid Association

Summaries for Patients from Clinical Thyroidology by Ernest Mazzaferri, MD MACP
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COMPLICATIONS OF RADIOIODINE THERAPY

What is the study about? There is a small risk of second primary nonthyroidal malignancies in patients with differentiated thyroid cancer

The full article title: “The risk of second primary malignancies up to three decades after the treatment of differentiated thyroid cancer”. It is in the February 2008 Issue of the Journal of Clinical Endocrinology and Metabolism, (volume 93 Issue 2, pages 504-15) The authors are AP Brown, J Chen, YJ Hitchcock, A Szabo, DC Shrieve, and JDTward. The abstract can be obtained at:

http://www.ncbi.nlm.nih.gov/pubmed/
18029468?dopt=Citation

What is known about the problem being studied? Papillary and follicular thyroid cancers are common in young and middle-aged adults and are associated with cancer-specific survival rates that exceed 90%. Because they are typically treated with total thyroidectomy and often with radioiodine postoperatively, studies raise concerns about the risk of second non-thyroid malignancies.

What was the aim of the study? This study was done to identify the risk of second cancers in patients treated with radioactive iodine.

Who was studied? The study subjects were 26,517 patients with papillary and 3761 with follicular thyroid cancer treated between 1973 and 2002; 76% were female, and 24% were male. Median age was 43 years at the time of diagnosis.

How was the study done? Information was obtained from the U.S. National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) program that has the largest cancer database in the world with the longest period of follow-up. The risk of second cancers in patients with thyroid cancer was compared with cancers among persons in the general population matched for age, gender and calendar periods of study (control group).

What were the results of the study? Between 1973 and 2002, 2158 thyroid cancer patients had 2338 second malignancies of almost all types. The risk was greatest for patients aged 25 to 29 years and for a period of 5 years after diagnosis of thyroid cancer, after which it declined rapidly and remained low. Compared with the general population, there was an excess risk for some but not all cancers. An analysis of patients treated between 1988 and 2002, when information concerningradioiodine was available, found the risk of second malignancies was significantly greater in the radioisotope-treated group than in the general population.

How does this compare with other studies? Elevated risk of second primary malignancies among thyroid cancer survivors have been found in numerous studies in the past 10 years. An analysis of 13 such studies found a 20% increase in cancer risk among patients with thyroid cancer as compared with the general population. However, a study from the Netherlands found that disease-free patients who were treated for thyroid cancer achieved a normal life span in contrast to patients with persistent thyroid cancer who had a median life-expectancy of only 60% predicted for their age. The study concluded that overall, treatment including radioiodine is safe.

What are the Limitations of this study? This and most other studies fail to describe how much radioiodine is necessary to cause second primary malignancies. However, a study from France found a linear relationship between the cumulative exposure to radioiodine and the risk of solid cancers and leukemia. The study found that one might expect 172 excess cancers per 10,000 people surviving 20 years after treatment of thyroid cancer with the mean of 163 mCi of radioiodine. The amount of radioiodine given postoperatively to destroy occult thyroid cancer and the normal thyroid remnant is usually 30 to 100 mCi, while larger doses are usually given only when thyroid cancer metastases are found.

What are the implications of this study? There is a small but not inconsequential increased risk of second cancers in patients with thyroid cancer treated with and without radioiodine. The risk of breast cancer is increased in patients not treated with radioiodine, whereas certain types of leukemia and lymphatic tumors and stomach cancer are increased in patients treated with radioiodine. Patients with thyroid cancer should thus undergo routine screening measures as recommended by the U.S. Public Health Service.

Details of informed decision-making to promote cancer screening are available on the website for the Center for Disease Control http://www.thecommunityguide.org/cancer/idm/

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