High radioactive iodine-131 exposure dose linked to aggressive thyroid cancer

Issue: December 10, 2014

Children and adolescents exposed to high doses of radioactive iodine-131 after the Chernobyl nuclear power plant accident experienced greater rates of more aggressive thyroid cancers, according to study results.

“Our group has previously shown that exposures to radioactive iodine significantly increase the risk of thyroid cancer in a dose-dependent manner,” researcher Lydia Zablotska, MD, PhD, associate professor in the department of epidemiology and biostatistics at the University of California, San Francisco, said in a press release. “The new study shows that radiation exposures are also associated with distinct clinical features that are more aggressive.”

Zablotska and colleagues evaluated data from 11,664 children and adolescents who lived in contaminated areas of Belarus and were aged 18 years or younger at the time of the 1986 Chernobyl accident. All participants underwent three rounds of thyroid screening between 1997 and 2008.

Researchers compared the screening results with domestic questionnaire data and estimated radioactive iodine-131 thyroid activity measures taken within 2 months of the accident.

Screening identified 158 individuals with thyroid cancer, most of whom (93.7%) had T1a and T1b tumors. A majority (60.6%) had regional lymph node-positive disease, but fewer than 1% had distant metastases.

An additional 52 study participants presented with confirmed thyroid cancer before their first screening.

Study participants who had thyroid cancer detected prior to screening were significantly younger at the time of the accident (P˂.01) and at the time they underwent surgery (P˂.0001), and they also were more likely to have lived in a rural area (P=.02). Seventy-one percent of patients whose thyroid cancer was detected prior to screening had ≥T3 tumors, which represented a significantly higher rate of advanced tumors compared with those who had screening-detected thyroid cancers (P˂.0001).

Patients with screen-detected thyroid cancer were exposed to a median radiation dose of 529 mGy (range, 1-17,472).

Exposure to higher radiation doses was significantly associated with the frequency of lymphatic invasion (P=.03) and intrathyroidal infiltration (P=.02). Patients exposed to higher doses also were significantly more likely to have a solid or diffuse sclerosing variant of papillary thyroid cancer compared with any other disease variant (P˂.01).

However, researchers observed no significant association between radiation dose and presence of tumor capsule and vascular invasion, thyroid capsule invasion and extrathyroidal spread.

Patients exposed to ≥950 mGy — the highest radiation dose category in the analysis — were at significantly increased risk to have more than one distinct cancer at pathology compared with patients exposed to doses ˂200 mGy (RR=4.86; 95% CI, 1.3-18.1).

Sally E. Carty

“Those exposed as children or adolescents to the fallout are at highest risk and should probably be screened for thyroid cancer regularly, because these cancers are aggressive and they can spread really fast,” Zablotska said in the release. “Clinicians should be aware of the aggressiveness of radiation-associated tumors and closely monitor those at high risk.”

Additional molecular profiling of these patients may help determine if there are genetic drivers of radiation-induced thyroid cancer, Linwah Yip, MD, and Sally E. Carty, MD, both of the University of Pittsburgh Medical Center, wrote in an accompanying editorial.

“Although dose-related effects likely contribute to the variable histology observed … alternatively there may be a yet-to-be-defined genetic predisposition that modifies the increased thyroid cancer risk,” Yip and Carty wrote. “This clinical effort provides a precious resource for comprehensive genetic profiling to investigate possible predispositions to genetic rearrangements.”

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Disclosure: The study was funded by the NCI. Carty reports consultant roles for a book chapter for Jaypee Brothers Publishing and UpToDate.