The author is sincerely grateful to Dr. Hidehiko Yamamoto et al. (hereafter H.Y.) for their reply1 to the letter2. The following citations from the reply should be further commented because they are essential for the argument.
H.Y.: Increased thyroid cancer (TC) risks were found after exposure to doses above 50 mGy3.
Author: In the cited review3 it is written: "The risk is significantly increased for radiation doses to the thyroid of 50-100 mGy"3 with reference to4, where it is stated: "For persons exposed to radiation before age 15 years, linearity best described the dose response, even down to 0.10 Gy."4 The low figures had primarily come from a study of Israeli children who received radiotherapy for scalp ringworm, whereas an estimated thyroid dose 90 mGy was linked to a fourfold increase of TC and a twofold increase of benign thyroid tumors5. Considering the low doses and pathogenetic differences between TC and benign tumors, the causality was questioned, the data being regarded as outstanding and needing experimental verification6,7. Apparently, these latter results could have been caused by an observation bias or screening effect with detection of thyroid nodules.
H.Y.: According to the UNSCEAR 2013 report8 (Appendix C-16), the thyroid dose to a 10-year old child increases linearly with the Cs-137 deposition by 49.2 mGy per every MBq/m2 Cs-137. The same report8 in its Appendix C-9 documents an estimated total Cs deposition in 1 km2 grid cells from 12 March-1 April 2011 of up to 9.8 MBq/m2… the realistic maximum thyroid doses certainly exceeded 500 mGy.
Author: If even there are correlations between deposition values and individual doses, they do not prove causality and hence do not justify extrapolations, the more so as thyroid doses are caused predominantly by I-131.
H.Y.: Cardis et al. demonstrate a relative risk for thyroid cancer of 5.0 per Gy or equivalently of 1.0016 per mGy9…
Author: This argumentation is based on the linear no-threshold theory (LNT) that has never been satisfactorily proven. In brief, the LNT postulates that linear dose-effect correlations, proven to some extent for higher doses, can be extrapolated down to minimal doses. However, the DNA damage and repair are permanent processes in a dynamic balance. Living organisms have probably been adapted to the natural radiation background (NRB) in a similar way as to other environmental factors. Natural selection is slow; adaptation to a changing factor would probably correspond to some average from the past. The NRB has been decreasing during the time of life existence. The conservative nature of mutation repair mechanisms in living organisms suggests that these mechanisms evolved in the past and that organisms may have retained some capability of efficient reparation of damage from a higher NRB than that existing today. Considering the above, with the dose rates tending to the wide range NRB level, radiation-related risks would tend to zero, and can even fall below zero in accordance with the concept of hormesis; details and references are in6,7.
H.Y.: Mathews et al. report significant relative risks for thyroid cancer in the range of 1.5 for CT-scans exposing children's thyroid glands to about 20 mGy external radiation per scan10,11.
Author: The causality is not proven, which can be seen from the following citations from the same sources10,11: "We cannot, however, necessarily assume that all the excess cancers seen during the current period of follow-up were caused by CT scans, because scanning decisions are based on medical indications and are not allocated at random… whereby symptoms of precancerous conditions (including genetic conditions) or early symptoms of the cancer itself might themselves prompt a CT scan."10 "Paralleling the increasing use of medical radiation is an increase in the incidence of papillary thyroid cancer. At present, it is unclear how much of this increase is related to increased detection of subclinical disease from the increased utilization of ultrasonography and fine-needle aspiration, how much is due to a true increase in thyroid cancer, and how much, if any, can be ascribed to medical radiation exposure."11
H.Y.: One strength of the Fukushima Health Management Survey is a uniform screening procedure covering all eligible residents. If this screening detected only insignificant cases, it would have detected them in all municipalities uniformly at random and irrespective of their geographic location, radiological contamination, or the timing of the examinations.
Author: Dose-effect correlations can be caused or overestimated due the screening-effect, dose-dependent quality of diagnostics, selection and self-selection2. There have been methodological differences of the screening in different areas after the Fukushima accident12. Both the screened people and medical personnel were informed about the contamination level in a given area, so that their action might have been consciously or subconsciously influenced by doses.
H.Y.: …the association between the TC increase and radiation has been clearly demonstrated13.
Author: the correlations by themselves do not prove causality being at least in part caused by factors and bias not related to radiation; commented in6,7,14 with references also to9,13 Please see also the preceding comment.
H.Y.: SV Jargin questioned the increase in TC after the Chernobyl accident….
Author: The TC incidence increase after the Chernobyl accident has never been questioned. Neither was it denied that TC could have resulted from radiation exposures. However, according to the author's hypothesis, the quantity of radiogenic cases after Chernobyl has been overestimated6,7,14.
H.Y.: However, the frequent occurrence of TC in contaminated regions after Chernobyl was evident and subsequent screenings of children born in the same regions after the decay of I-131 demonstrated the absence of frequent TC15.
It is written in the cited article: "Nowadays, 20 years after the Chernobyl tragedy, incidence of thyroid cancer in children in the affected countries decreased to the levels just somewhat elevated compared to the pre-accident rate"15, which is not exactly the same as the above citation from1; but this latter statement also needs a comment. Prior to the Chernobyl accident, the registered incidence of pediatric TC had been considerably lower in the former SU than in other developed countries15,16 probably due to an insufficient coverage of the population by medical checkups. Accordingly, there must have been neglected TC in the population 6,7,14. For the period 1981-1985, the TC incidence among children ≤15 years old in the northern regions of Ukraine (overlapping with the areas contaminated after the Chernobyl accident) was 0.1, and in Belarus – 0.3 per million per year16. After the accident, the TC incidence in Belarusians ≤18 years old has remained on an enhanced level - 15.7 per million per year (reported in 2012) or at least thrice the level of other countries17,18, although the radiation factor has no longer been present. This indicates that other mechanisms such as enhanced attention and improved diagnostics have contributed to the higher detection rate.
H.Y.: SV Jargin states 'The screening detected not only small nodules, but also late-stage TC interpreted as rapidly growing radiogenic cancers. Unlike Chernobyl, most cases after the Fukushima accident were of the classical papillary TC (PTC) type'. This perception is incorrect… In Fukushima, the percentage of PTC was 100/101 (99.0%) in the first screening and 49/50 (98%) in the second round, totaling 149/151 (98.7%), which is not much different from PTC after Chernobyl.
Author: If not the whole sentence is cited, dots of the ellipsis … are required. The complete sentence in2 was follows: "Unlike Chernobyl, most cases after the Fukushima accident were of the classical PTC type (not the less differentiated solid variant)19 which indicates that there were virtually no neglected advanced TC in the Japanese population"2. From the incomplete citation resulted a misunderstanding. The "less differentiated solid variant" of PTC and its high prevalence among first wave post-Chernobyl (diagnosed during ~10 years after the accident) TC is well known. The first wave TC following the Chernobyl accident were averagely larger and higher grade than those detected later20 presumably due to old neglected cases gradually sorted out by the screening 6,7,14.
H.Y.: Screening effects or overdiagnosis have yet to be proven unequivocally in sufficiently representative epidemiological studies in unexposed populations.
Author: For example, in the United States the incidence rate of thyroid tumors detected between 1974 and 1979 during a screening program was 21 times higher than that before the screening21. Obviously, an ultrasonic screening would find thyroid nodules. Among others, overdiagnosis means detection of thyroid tumors histologically diagnosed as cancers that would not, if left untreated, result in symptoms or death22.
H.Y.: In one of SV Jargin's references reportedly showing a "screening effect"23, 36 occult thyroid cancers were found in 101 (selected) autopsies, 34 of which in the age group 40-100.
Author: the autopsies were not selected but consecutive23. "The rate … did not correlate to the age of the patients."23 "According to the study, occult papillary carcinoma can be regarded as a normal finding which should not be treated when incidentally found."23
The inexact citations specified in this letter potentially interfere with objective debates. More argumentation is in6,7,14. In conclusion, a monitoring of populations exposed to low-dose radiation is important but will hardly add reliable information about health risks. It can be reasonably assumed that the screening effect and increased attention of exposed people to their own health will result in new reports on the elevated cancer and other health risks, which would prove no cause-effect relationships. Dose-response correlations at low doses can be further studied in large-scale animal experiments. The life duration is known to be a sensitive endpoint attributable to radiation exposures24. To enable extrapolations to humans, the doses and dose rates in experiments must be comparable to those in corresponding human populations, taking into account the radiosensitivity and life duration of animal species.
Sergei Jargin, MD, Peoples' Friendship University of Russia, Clementovski per 6–82, 115184 Moscow, Russia
 Yamamoto H, Hayashi K, Scherb H. Authors' reply: Letter to the Editor by SV Jargin: Association between the detection rate of thyroid cancer and the external radiation dose-rate after the Fukushima nuclear power plant accident. Medicine (Baltimore) Correspondence Blog, January 21, 2020 https://journals.lww.com/md-journal/Blog/MedicineCorrespondenceBlog/pages/post.aspx?PostID=113
 Jargin SV. Letter to Editor: Association between the detection rate of thyroid cancer and the external radiation dose-rate after the Fukushima nuclear power plant accident. Medicine (Baltimore) Correspondence Blog, January 15, 2020 https://journals.lww.com/md-journal/Blog/MedicineCorrespondenceBlog/pages/post.aspx?PostID=111
 Iglesias ML, Schmidt A, Ghuzlan AA, et al. Radiation exposure and thyroid cancer: a review. Arch Endocrinol Metab 2017;61:180-7.
 Ron E, Lubin JH, Shore RE, et al. Thyroid cancer after exposure to external radiation: a pooled analysis of seven studies. 1995. Radiat Res 2012;178:AV43-60.
 Ron E, Modan B, Preston D, et al. Thyroid neoplasia following low-dose radiation in childhood. Radiat Res 1989;120:516-31.
 Jargin SV. Hormesis and radiation safety norms: Comments for an update. Hum Exp Toxicol 2018;37:1233-43.
 Jargin SV. The Overestimation of Medical Consequences of Low-Dose Exposure to Ionizing Radiation. Newcastle upon Tyne: Cambridge Scholars Publishing; 2019.
 UNSCEAR. Report for the General Assembly. Sources, Effects and Risks of Ionizing Radiation. Annex A. Levels and effects of radiation exposure due to the nuclear accident after the 2011 Great East Japan Earthquake and tsunami. New York: United Nations; 2013.
 Cardis E, Kesminiene A, Ivanov V, et al. Risk of thyroid cancer after exposure to 131I in childhood. J Natl Cancer Inst 2005;97:724-732.
 Sinnott B, Ron E, Schneider AB. Exposing the thyroid to radiation: a review of its current extent, risks, and implications. Endocr Rev 2010;31:756-73.
 Mathews JD, Forsythe AV, Brady Z, et al. Cancer risk in 680,000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians. BMJ 2013;346:f2360.
 Ohira T, Takahashi H, Yasumura S, et al. Associations between childhood thyroid cancer and external radiation dose after the Fukushima Daiichi Nuclear Power Plant accident. Epidemiology 2018;29(4):e32-4.
 Tronko MD, Howe GR, Bogdanova TI, et al. A cohort study of thyroid cancer and other thyroid diseases after the chornobyl accident: thyroid cancer in Ukraine detected during first screening. J Natl Cancer Inst 2006;98:897-903.
 Jargin SV. Thyroid cancer after Chernobyl: Obfuscated truth. Dose Response 2011;9:471-6.
 Demidchik YE, Saenko VA, Yamashita S. Childhood thyroid cancer in Belarus, Russia, and Ukraine after Chernobyl and at present. Arq Bras Endocrinol Metabol 2007;51:748-62.
 Stsjazhko VA, Tsyb AF, Tronko ND, et al. Childhood thyroid cancer since accident at chernobyl. BMJ 1995;310:801.
 Fridman MV, Demidchik IuE, Papok VE, et al. Morphological features of spontaneous papillary carcinoma of the thyroid in children and adolescents in the republic of Belarus. Vopr Onkol 2012;58:578-81.
 Fridman MV, Kras'ko OV, Man'kovskaia SV, et al. The increase of non‑cancerous thyroid tissue in children and adolescents operated for papillary thyroid cancer: related factors. Vopr Onkol 2013;59:121-5.
 Suzuki S. Childhood and adolescent thyroid cancer in Fukushima after the Fukushima Daiichi Nuclear Power Plant accident: 5 years on. Clin Oncol (R Coll Radiol) 2016;28:263-71.
 Williams ED, Abrosimov A, Bogdanova T, et al. Thyroid carcinoma after chernobyl latent period, morphology and aggressiveness. Br J Cancer 2004;90:2219-24.
 Jaworowski Z. Observations on the Chernobyl Disaster and LNT. Dose Response 2010;8:148-71.
 Vaccarella S, Franceschi S, Bray F, et al. Worldwide thyroid-cancer epidemic? The increasing impact of overdiagnosis. N Engl J Med 2016;375:614-7.
 Harach HR, Franssila KO, Wasenius VM. Occult papillary carcinoma of the thyroid. A "normal" finding in Finland. A systematic autopsy study. Cancer 1985;56:531-8.
 Braga-Tanaka I 3rd, Tanaka S, Kohda A, et al. Experimental studies on the biological effects of chronic low dose-rate radiation exposure in mice: overview of the studies at the Institute for Environmental Sciences. Int J Radiat Biol 2018;94:42333.