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Radioactive iodine for well-differentiated thyroid cancer increases risk for acute, chronic myeloid leukemias

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January 29, 2018

Patients treated with radioactive iodine for well-differentiated thyroid cancer demonstrated an increased risk for developing acute myeloid leukemia and chronic myeloid leukemia, study data showed.

“Although adjuvant radioactive iodine improves overall and disease-free survival in advanced-stage well-differentiated thyroid cancer, most studies report little or no benefit from radioactive iodine in low-risk and intermediate-risk tumors, where 5-year [RFS] is already [more than] 97% without radioactive iodine,” Sudipto Mukherjee, MD, PhD, MPH, of the Taussig Cancer Institute, Cleveland Clinic, and colleagues wrote. “Because the widespread use of adjuvant radioactive iodine has not improved survival, its clinical benefit in the treatment of well-differentiated thyroid cancer is controversial. Furthermore, several meta-analyses have reported an increase in the incidence of second primary malignancies [among] patients with well-differentiated thyroid cancer treated with radioactive iodine.”

The researchers used SEER registries to identify patients with well-differentiated thyroid cancer, and used competing risk regression analysis to assess risks of second hematologic malignancies after treatment. Mukherjee and colleagues also evaluated outcomes after patients developed second malignancies.

A total of 148,215 patients had well-differentiated thyroid cancer. Of these, 53% underwent only surgery, and 47% received radioactive iodine.

Second hematologic malignancies developed in 783 patients. The median time to second malignancy was 6.5 years after thyroid cancer diagnosis.

Treatment with radioactive iodine appeared associated with an increased early risk for both AML (HR = 1.79; 95% CI, 1.13-2.82) and CML (HR = 3.44; 95% CI, 1.87-6.36). The increase occurred with all thyroid cancers, even low- and intermediate-risk disease.

AML appeared associated with a shorter median survival compared with matched controls (8 years vs. 31 years; P = .001), although CML did not.

Patients who developed AML after being treated with radioactive iodine demonstrated worse survival than matched controls who had de novo AML (1.2 years vs. 2.9 years; P = .06).

“Our results demonstrate the importance of avoiding treatment with radioactive iodine [among] patients with low-risk or intermediate-risk disease, in whom radioactive iodine has shown no or questionable benefit,” Mukherjee and colleagues wrote. “Furthermore, our results support using the least effective dose to treat patients who have high-risk features to avoid excess bone marrow exposure, because the risk [for] second hematologic malignancy is dose dependent.” – by Andy Polhamus

Disclosures: Mukherjee reports consulting or advisory roles with Bristol Myers Squibb, Novartis and Takeda Pharmaceuticals; honoraria from Novartis and research funding from Celgene and Novartis.