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.