To the Editor:
In a recent letter, Takahashi et al.1 questioned the use of a 4-year latency duration in the study by Tsuda et al.,2 which warns of the risk of thyroid cancer in Fukushima. Although Tsuda et al.2 analyzed data obtained from the first round of thyroid ultrasound examination, which was conducted between October 2011 and March 2014 and detected 113 malignancies (including suspicious cases, “malignancies,” hereafter), results from the ongoing second round of ultrasound examination (April 2014 to March 2016) that detected additional 51 malignancies as of December 20153 summarized in Table, should provide valuable information on the growth rate of thyroid tumors.
According to the 17th report,5 which includes data up to October 31, 2014, the minimum possible interval between the first and second round ultrasound examination is 7 months, 4 malignancies were detected with a maximum tumor size of 17.3 mm. Results of the first round ultrasound for the four cases “show that four of the four participants were categorized as A (A1: 2; A2: 2) and 0 as B.”5 Here, A1, A2, and B were defined as “no nodules or cysts,” “nodules ≤5.0 mm or cysts ≤20.0 mm,” and “nodules ≥5.1 mm or cysts ≥20.1 mm,” respectively. Assuming that the child with the largest tumor was diagnosed as A1 at the first round ultrasound, the maximum tumor growth rate can be estimated to be 17.3/7 (mm/month) or 29.1 mm/year. As reported in 22nd report, for 51 malignancies from the second round ultrasound, 49 of them were categorized as A (A1: 25; A2: 22) and four of them as B.3 Even if a few of these cases were possibly misdiagnosed at the first round ultrasound, 49 small nodules are unlikely to be missed by trained examiners with a high-resolution ultrasound device that detected tumors of 5 mm. Based on these results, assumption of the 4-year latency is reasonable.
Moreover, with an increasing age at diagnosis with subsequent ultrasound examination, an associated increase in tumor size is expected. In contrast, a decrease in mean tumor size might implicate the effects of radiation; ultrasound examination is conducted in contaminated areas where a greater prevalence of thyroid cancer is expected than in areas with less contamination.
Although we appreciate the effort of the Fukushima Medical University, their comment does not take into account the factors outlined in this letter. An evidence-based discussion is necessary for informed decision making regarding the risk of thyroid cancer among exposed children in Fukushima.
Faculty of Business and Commerce
1. Takahashi H, Ohira T, Yasumura S, et al. Re: Thyroid cancer among young people in Fukushima. Epidemiology. 2016. Doi: 10.1097/EDE.0000000000000467.