LAKE BUENA VISTA, FLA. – More cases of thyroid cancer are being seen in Japanese youth after the Fukushima Daiichi nuclear power plant accident, but the increased incidence may be an artifact of heightened surveillance.
“The thyroid cancers appear to have already occurred prior to radiation exposure,” said Dr. Shinichi Suzuki of the department of thyroid and endocrinology at Fukushima (Japan) Medical University. Radiation-induced thyroid cancers take about 5 years to become detectable, so physicians should just now be seeing the earliest cases of thyroid cancer related to Fukushima radiation exposure, according to Dr. Suzuki. He presented interim results of Japan’s universal screening protocol for children potentially affected by the Fukushima incident at the International Thyroid Conference.
The protocol, designed to screen everyone residing in the Fukushima prefecture and aged 19 years or younger at the time of the 2011 incident, has been highly successful, with over 80% of those eligible receiving a baseline screening that included a thyroid ultrasound exam.
Screening consisted of an initial thyroid ultrasound exam performed with a portable ultrasound device. If no cyst or nodule was found, then the patient would be seen at the next scheduled thyroid ultrasound exam, 2 years later. Patients with cysts 20 mm or less in greatest diameter or nodules 5 mm or smaller also were deferred to the next scheduled examination. Patients with cysts larger than 20 mm or nodules larger than 5 mm received confirmatory examination by detailed ultrasound examination, blood work, and fine-needle aspiration.
Of the 300,476 patients who received the preliminary baseline survey, 2,294 (0.8%) had an abnormality that warranted confirmatory examination and 91.9% of patients went on to have the confirmatory exams. Of these, 113 were assessed as malignant or suspicious for malignancy. Ninety-nine patients had surgery, with findings of 98 cases of thyroid cancer and one benign tumor.
Patients examined after April 2014 were part of an expanded protocol. Under this protocol, 169,455 patients (44.7% participation) were examined and 1,223 patients (0.8%) had suspicious findings on thyroid ultrasound exam. Participation rate for confirmatory testing for this group was 62.7%, with 25 patients’ thyroids having malignant or potentially malignant findings. Six of these patients had surgery, and thyroid cancer was found in all six cases.
Pooling data from the 138 malignant or suspicious cases from the two groups, 105 patients in total have had surgery, 13 patients with small, noninvasive masses are being watched, and a further 20 are awaiting surgery, Dr. Suzuki said at the meeting held by the American Thyroid Association, Asia-Oceania Thyroid Association, European Thyroid Association, and Latin American Thyroid Society.
Of the 97 patients with thyroid cancer who were treated at Fukushima University, 61 were female. The mean patient age at the time of the disaster was 14.8 ± 2.7 years (range, 6-18 years), while the mean age at diagnosis was 17.4 ± 2.8 years (range, 9-22 years). All patients were asymptomatic.
Tumors were unilateral in all but two patients. Mean tumor size was 15.1 ± 0.8 mm (range, 5-53 mm). Nearly all of the tumors (94/97) were papillary thyroid carcinoma, with 86 of those being classical-type papillary thyroid carcinoma. Three patients had poorly differentiated thyroid carcinoma. Fifty-eight patients (60%) had some intraglandular spread, while 71 (73%) had calcifications.
Dr. Suzuki and his collaborators compared these 97 cases with 37 cases of pediatric thyroid cancer in an historical Japanese cohort and to the 26 cases seen in a cohort from Belarus following the Chernobyl disaster. The Fukushima patients were significantly older than either comparison group, with mean age of 11.9 years for the historical Japanese cohort and 10.6 years for the children from Belarus. Tumor size was smaller than the historical Japanese cohort’s mean of 4.1 cm but about the same as that seen in Belarus (1.4 cm). Pulmonary metastases were more common in the historical Japanese cohort (19% vs. 4% in Belarus and 2% in Fukushima).
To have reference data that use similar techniques on a similar population, Japanese researchers are conducting thyroid ultrasound examsaccording to the Fukushima protocol concurrently in three other Japanese prefectures. This is especially important, Dr. Suzuki said, because rapid technological advances in ultrasound imaging mean that screening is much more likely to detect small abnormalities in the thyroid than would have been the case even a few years ago. For this reason, and also because much more radiation was released at the site of the Chernobyl nuclear disaster, only limited comparisons can be made between pediatric thyroid cancer rates from the two nuclear accidents.
Thyroid ultrasound exam “has the ability to detect a lot of thyroid cancers,” he said, so care must be taken to avoid overdiagnosis and overtreatment in this group of young people. Information to date from the Fukushima surveillance project does not yet “give us the clear view about the influence of radiation exposure after the accident on thyroid cancer occurrence,” he said.