Radioactive iodine therapy associated with increased second malignancies in low-risk thyroid cancer

Iyer NG. Cancer. 2011;doi:10.1002/cncr.26070.

Issue: May 25, 2011

Patients with well-differentiated thyroid cancer developed excess cases of salivary gland malignancies and leukemia after treatment with radioactive iodine, according to an analysis of more than 37,000 patient records collected in the SEER database.

Because well-differentiated thyroid cancer is rarely fatal, the researchers said these results suggest that the use of radioactive iodine should be rationed in this patient population.

Researchers at Memorial Sloan-Kettering Cancer Center reviewed data on 37,176 patients diagnosed with low-risk thyroid cancer from 1973 to 2007. During that time, use of radioactive iodine as first-course therapy increased from 6.1% to 48.7%. Among patients aged younger than 45 years with low-risk tumors, use of radioactive iodine increased from 3.3% in 1973 to 38.1% in 2006. However, the OS rate has remained constant during that time.

Approximately 15,000 patients were treated with radioactive iodine. Researchers expected to find 3,029 secondary primary malignancies; however, they discovered 3,223 secondary primary malignancies.

"During the era when radioactive iodine use was uncommon, there was no statistically significant elevated risk of secondary primary malignancies among patients who were diagnosed with well-differentiated thyroid cancer," they wrote. "However, the risk of secondary primary malignancies subsequently has increased in parallel with the escalating frequency of radioactive iodine use."

The overall risk for secondary primary malignancies at any site increased from 1.00 (95% CI, 0.94-1.16) during 1973 to 1981 to 1.22 (95% CI, 1.10-1.35) during 1999 to 2006 for all patients. At the same time, excess absolute risk increased from 0 to 14.3 excess cases per 10,000 person-years at risk.

Risk for hematologic malignancies had the greatest increase. For leukemia, the standarized incidence ratio increased from 1.09 (95% CI, 0.72-1.57) during 1973 to 1981 to 2.40 (95% CI, 1.40-3.85) during 1999 to 2006. There was a corresponding increase in excess absolute risk, from 0.2 to 2.0 excess cases per 10,000 person-years at risk.

Among all patients in the cohort who received radioactive iodine from 1973 to 2007, SIR for secondary primary malignancies at any site was 1.18 (95% CI, 1.10-1.25). There was no increased risk for patients who did not receive radioactive iodine therapy (SIR=1.02; 95% CI, 0.98-1.06). There was an excess absolute risk for 11.9 excess cancers per 10,000 person-years at risk among patients assigned to radioactive iodine.

Follow HemOncToday.com on Twitter.