Ludmila Borislavova IVANOVA, Mircho Ivanov VUKOV, Zdravka Gardeva VASSILEVA-VALERIANOVA

Thyroid Cancer Incidence in Bulgaria before and after the Introduction of Universal Salt Iodization: An Analysis of the National Cancer Registry Data

Background: Thyroid cancer is the most common malignancy of the endocrine system and it has become the fastest growing cancer among women. The suspected risk factors include increased exposure to ionizing radiation during childhood, environmental pollutants, possible iodine deficiency, and excessive iodine exposure.Aims: To analyze the thyroid cancer incidence between 1980 and 2013 in Bulgaria and to determine the incidence rate before and after the introduction of universal salt iodization in 1994 in regions with different iodine deficiency levels.Study Design: Retrospective cohort.Methods: The study was a retrospective analysis of the total number of thyroid cancer cases with all histological types in Bulgaria (thyroid cancer, ICD10 code C73), diagnosed between 01/01/1980 and 31/12/2013, and retrieved from the anonymous cancer registry database of the Bulgarian National Cancer Registry. Age-standardized rates of thyroid cancer per 100,000 persons were calculated for each year of the periods mentioned below by sex and age, utilizing the WHO world reference populations with a special statistical module of the Bulgarian National Cancer Registry’s software CancerRegBG, 2011. Incidence rates were reported by age, sex, and period of diagnosis(1980-86,1987-93,1994-99,2000-2006,2007-2013).Trends among males and females were analyzed separately, as well as by age category: 0-19, 20-44, 45-64, and 65+. Annual percentage changesofage-standardizedincidencerateswereanalyzedbyJoinpoint regression to determine trends using the Joinpoint statistical software SEER* Stat Software, Version 4.1.1, 2014.Results: The age-standardized rates of thyroid cancer in Bulgaria has been increasing since 1990, being higher among women compared to men (4.68 vs 2.81). The highest age-standardized rates of thyroid cancer was observed in women in the 2007-2013 period. The only significant joinpoint was recorded in 1990 for females and in 1991 for males. The highest incidence rates was in the Smolyan district, a region with historically existing iodine deficiency and relatively high post-Chernobyl radiation exposure.Conclusion: Our results showed that, in different regions, the age-standardized thyroid cancer rates between endemic and non-endemic differ greatly depending on the radiation dose from the Chernobyl accident. The role of iodine intake in thyroid cancer remains uncertain, but iodine deficiency could be a contributing factor to the increased risk of thyroid cancer.

PDF

___

1. European Network of Cancer Registries. ENCR Factsheets, January 2017. https:// www.encr.eu/sites/default/files/factsheets/ENCR_Factsheet_Thyroid_2017-2.pdf
2. Smailyte G, Miseikyte-Kaubriene E, Kurtinaitis J. Increasing thyroid cancer incidence in Lithuania in 1978-2003. BMC Cancer 2006;6:284.
3. Kilfoy BA, Zheng T, Holford TR, Han X, Ward MH, Sjodin A, et al. International patterns and trends in thyroid cancer incidence, 1973-2002. Cancer Causes Control 2009;20:525-31.
4. Vučemilo L, Znaor T, Kulis T, Sekerija M, Znaor A. Thyroid Cancer Incidence and Mortality Trends in Croatia 1988-2010. Acta Clin Croat 2015;54:30-7.
5. Li N, Du XL, Reitzel LR, Xu L, Sturgis EM. Impact of Enhanced Detection on the Increase in Thyroid Cancer Incidence in the USA: Review of Incidence Trends by Socioeconomic Status Within the Surveillance Epidemiology, and End Results Registry, 1980-2008. Thyroid 2013;23:103-10.
6. Topstad D, Dickinson JA. Thyroid Cancer Incidence in Canada: a national cancer registry analysis. CMAJ Open 2017;5:E612-6.
7. Pellegriti G, Frasca F, Regalbuto C, Squatrito S, Vigneri R. Worldwide Increasing Incidence of Thyroid Cancer: Update on Epidemiology and Risk Factors. J Cancer Epidemiol 2013;2013:965212.
8. Liu Y, Lei S, Xiao H. Review of Factors Related to the Thyroid Cancer Epidemic. Int J Endocrinol 2017;2017:5308635.
9. Parkin DM, Bray F, Ferlay J, Pisani P. Global Cancer Statistics, 2002. CA Cancer J Clin 2005;55:74-108.
10. Zimmermann MB, Galetti V. Iodine Intake as a risk factor for thyroid cancer: a comprehensive review of animal and human studies. Thyroid Res 2015;8:8.
11. Dal Maso L, Bosetti C, La Vecchia C, Franceschi S. Risk factors for thyroid cancer: an epidemiological review focused on nutritional factors. Cancer Causes Control 2009;20:75-86.
12. Pentchev, I et al. In: Endemic goiter in Bulgaria, Med. I fizk, 1961 (Bulga)
13. Peneva L, Lozanov B, Koev D. Status of Iodine Nutrition in Bulgaria. In: Delange F, Dunn JT, Glinoer D, editors. Iodine Deficiency in Europe. New York: Plenum Press; 1993:415-9.
14. Van der Haar F, van Ingen JWE, Laurberg P. Review of Progress towards Sustained Optimal Iodine Nutrition in Bulgaria. Report by a team of experts on behalf of the Network for Sustained Elimination of Iodine Deficiency. Ministry of Health of Bulgaria, UNICEF, 2006. http://kan-kaz.org/english/files/review_team_bulgaria.pdf
15. Bulgarian National Cancer Registry. https://rb.gy/ohkubt
16. Ahmad OB, Boschi-Pinto C, Lopez AD, Murray CJL, Lozano R, Inoue M. Age standardization of rates: a new WHO standard. GPE Discussion Paper Series Nr.31 WHO 2001. https://www.who.int/healthinfo/paper31.pdf
17. Cancer Registration: Principles and Methods. In: Jensen OM, Parkin DM, MacLennan R, Muir CS, Skeet RG, editors. Lyon: IARC Scientific Publications; 1991.
18. Altman GD, Bland MJ. How to obtain the P value from a confidence interval. BMJ 2011;343:d2304.
19. Bray F, Ren JS, Masuyer E, Ferlay J. Global estimates of cancer prevalence for 27 sites in the adult population in 2008. Int J Cancer 2013;132:1133-45.
20. Pathac KA, Leslie WD, Klonisch TC, Nason RW. The changing face of thyroid cancer in a population-based cohort. Cancer Med 2013;2:537-44.
21. Vigneri R, Malandtino P, Vigneri P. The changing epidemiology of thyroid cancer: why is incidence increasing? Curr Opin Oncol 2015;27:1-7.
22. Lukas J, Drabek J, Lukas D, Dusek L, Gatek J. The epidemiology of thyroid cancer in the Czech Republic in comparison with other countries. Biomed Pap Med Fac Univ Palacky Olomouc Czech Repub 2013;157:266-75.
23. Salamanca-Fernandez E, Rodriguez-Barranco M, Chang-Chan YL, Redondo-Sánchez D, Domínguez-López S, Bayo E, et al. Thyroid Cancer Epidemiology in South Spain: a population-based time trend study. Endocrine 2018;62:423-31.
24. Nagataki S, Nyström E. Epidemiology and primary prevention of thyroid cancer. Thyroid 2002;12:889-96.
25. Vasilev G. Bayrakova A. Predicted health effects in Bulgaria from the Chernobyl NPP accident: Objective assessments and public reactions.One decade after Chernobyl: Summing up the consequences of the accident. Poster presentations, 1997. https://inis. iaea.org/search/Search.aspx?q=28073810
26. Drozdovitch V, Bouville A, Chobanova N, Filistovic V, Ilus T, Kovacic M, et al. Radiation Exposure to the population of Europe following the Chernobyl Accident. Radiat Prot Dosim 2007;123:515-28.
27. Chobanova N, Vukov M, Yagova1 A, Ivanova R. Epidemiological Study on 2006; vol. XI No: 2: p 86-94.
28. Lozanov B, Tzachev K, Kirilov G, Kovatcheva R, Aceva E, Apostolova E, et al. Selenium and Thyroid Status in Children Living in an Endemic Region with adequate iodine supplementation. Endocrinologia J 2006;4:213-23.