Researchers who have followed the children and adolescents exposed to radioactive fallout from the the 1986 Chernobyl nuclear power plant accident have highlighted the fact that the thyroid cancers seen in these individuals are particularly aggressive. They say their work has implications for individuals affected by the 2011 nuclear reactor incidents in Fukushima, Japan.
The study is published online October 28 in Cancer.
The research team screened nearly 12,000 individuals from Belarus several times over a course of 9 years. Study participants were all under age 18 years when they were exposed to the Chernobyl radioactive fallout.
All individuals underwent up to three cycles of thyroid screening from 1997 to 2008. Radioactive iodine-131 (I-131) thyroid doses were estimated from individual thyroid activity measurements taken within 2 months after the accident, along with dosimetric data from questionnaires.
A total of 158 thyroid cancers were diagnosed in the 11,664-person cohort during 3 rounds of screening, which were conducted at roughly 2-year intervals.
The researchers found that higher I-131 thyroid doses were associated with higher frequency of solid or diffuse sclerosing variants of thyroid cancer (P < 0.01), as well as histologic features of cancer aggressiveness, such as lymphatic vessel invasion, intrathyroidal infiltration, and multifocality (all P < .03).
First author Lydia Zablotska, MD, PhD, associate professor in the Department of Epidemiology and Biostatistics at the University of California, San Francisco, explained that her group has previously shown that radioactive iodine exposure significantly increases the risk for thyroid cancer in a dose-dependent manner. But this new study demonstrates that exposure to I-131 is associated with distinct clinical features indicating more aggressive tumors.
"Our study has very important implications for clinical practice," Dr Zablotska told Medscape Medical News, especially as they relate to radiation exposure that resulted from the 2011 nuclear reactor incidents in Fukushima, Japan.
"The radiation exposures were similar, and currently the population of the areas around the Daiichi nuclear plant and other contaminated areas in the Fukushima and other prefectures are being screened for thyroid diseases. Our study adds to that protocol because we have shown that you need to screen continuously."
"These cancers are very aggressive, so this is not something that you screen for once, or screen for every ten years," she said. "Our first screening was 11 years after the accident, and the radiation risks have not gone down."
In 1997, at the first screening, research identified 71 cases of thyroid cancer. Since that time, during two additional rounds of screening, 87 additional new ones have been diagnosed, Dr Zablotska pointed out.
Uniqueness of Study
Other studies have investigated the risk for Chernobyl-related thyroid cancer among persons who were exposed to radiation as children or adolescents, Dr Zablotska noted. "But our study is unique, in that we uniformly screened according to standardized protocol and irrespective of the thyroid radiation dose," she explained. "For each screening we have complete data, and every case of thyroid cancer was diagnosed according to the standardized protocol and confirmed by the international pathology review panel based on pathological findings of thyroid tumor tissues collected during surgery."
Other studies did not have the individual radiation doses, she pointed out. "There were scientists in the field immediately after the accident, taking thyroid radioactivity measurements," she said. "Radioactive iodine disintegrates quickly, so it needs to be measured within 2 months. So we have that data for each person in our study."
The cancers detected in this cohort were caught early and are largely treatable, so there have been no deaths specific to thyroid cancer, Dr Zablotska said. This group is still being followed but will not be screened as frequently now, she added.
More-Advanced Tumors in Prescreened Patients
At the initial 1997 screening, 52 patients had already been diagnosed with thyroid cancer in the years since the Chernobyl accident in 1986. These "prescreening cases" tended to be significantly younger at the time of their exposure (P < .01), significantly younger at the time of surgery (P < .0001), and significantly more likely to have lived in rural areas at the time of the accident (P = .02).
They were also more likely to have a more advanced tumor stage at histology (71% with tumors staged as T3 and above) compared with the screening-detected cancers (94%), who had T1a and T1b tumors (P < .0001). Stage of metastases to the regional lymphatic nodes or distant metastases did not differ between the prescreening and screening thyroid cancers.
The authors also found that the frequency of lymphatic invasion (P = .03) and intrathyroidal infiltration (P = .02) increased significantly with higher radiation doses among screening-detected cancers.
However, other properties of invasive thyroid cancer, such as presence of tumor capsule and vascular invasion, thyroid capsule invasion, and extrathyroidal spread, did not appear to be associated with radiation dose.
The National Cancer Institute supported the study. The authors have disclosed no relevant financial relationships.
Cancer. Published online October 28, 2014. Abstract
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Cite this: Aggressive Thyroid Cancers Linked to Radiation Exposure - Medscape - Oct 28, 2014.