Thyroid Cancer Survivors at Increased Risk for Second Cancers

Survivors of thyroid cancer are at an increased risk for developing a second cancer, according to the results of a study published recently in the Journal of the National Cancer Institute.

Researchers led by Chung-Jen Teng, of Far Eastern Memorial Hospital, New Taipei City, Taiwan, found an association for increased risk for second primary malignancy (SPM) and leukemia among survivors of thyroid cancer, especially those patients who were treated with radioactive iodine (RAI) doses of greater than 150 mCi.

“Overall, it is recommended that fewer patients receive high doses of RAI for the treatment of thyroid cancer; a low fixed dose or dosimetric method would be more favorable strategies,” the researchers wrote. “For those whose disease burden necessitates a higher RAI dose, we suggest a more thorough survey to investigate clinically suspicious symptoms of SPM.”

In their analysis, Teng and colleagues looked at data from patients aged 20 years or older who were diagnosed with thyroid cancer. Patient data were taken from the Taiwan National Health Insurance database from 1997 to 2010. Patients had no prior history of cancer.

The researchers identified 692 patients with a SPM among more than 20,000 patients with thyroid cancer in the database. Compared with the general population, these patients had a statistically significant increased risk (SIR) for developing cancer (SIR, 1.41 [95% CI, 1.31–1.52]; P < .001). The researchers found that this increased risk for SPM was highest in patients aged 20 to 39 at age of diagnosis (SIR, 2.05 [95% CI, 1.54–2.65]).

Leukemia was the most frequently observed SPM among the thyroid cancer survivors (SIR, 2.72 [95% CI, 1.65–4.28]). Higher rates of non-Hodgkin lymphoma (2.38), prostate (2.30), lung and mediastinum (1.93), pancreas (1.83), kidney (1.81), breast (1.38), and colorectal cancers (1.31) were also observed.

The researchers also evaluated factors among these survivors that might predict their increased risk for SPM. They found that the cumulative RAI dose and external beam radiotherapy dose were predictive of all cancer combined.

The cumulative RAI dose per 30 mCi increase was associated with an adjusted hazard ratio (HR) of 1.01 (P < .001) for SPM and 1.03 (P < .001) for leukemia. In addition, a cumulative dose of greater than 150 mCi significantly increased a survivor’s risk for all cancers (adjusted HR, 1.30) and leukemia (adjusted HR, 6.03).

“The wide range of 95% confidence intervals for the hazard ratios of leukemia at higher dose levels implies small event numbers,” the researchers explained. “In general, RAI is commonly administered at a dose of 30 to 100 mCi, and this is considered a dose that does not increase the risk of SPM.”