ISSN : 2146-3123
E-ISSN : 2146-3131

Thyroid Cancer Incidence in Bulgaria Before and After The Introductıon of Universal Salt Iodization- An Analysis of National Cancer Registry Data
Ludmila Borislavova Ivanova1, Mircho Ivanov Vukov2, Zdravka Gardeva Vassileva-Valerianova3
1Neurology, Psychiatry, Physiotherapy and Rehabilitation, Preventive Medicine, and Public Health Sofia University "St. Kl. Ohridski", Faculty of Medicine, Bulgaria
2Statistition-Consultant Bulgarian National Cancer Registry, Bulgaria
3Bulgarian National Cancer Registry University Hospital of Oncology, Bulgaria
DOI : 10.4274/balkanmedj.galenos.2020.2019-10-5

Background: Thyroid cancer (TC) is the most common malignancy of the endocrine system and has become the fastest increasing cancer among women. Increased exposure to ionizing radiation during childhood, environmental pollutants, possible iodine deficiency, and excessive iodine exposure are among the suspected risk factors.
Aims: The study aimed to analyze thyroid cancer (TC) incidences between 1980 and 2013 in Bulgaria and to study the incidence rate before and after the introduction of universal salt iodization in 1994 in regions with different iodine deficiency levels.
Study Design: Retrospective analysis of the total number of cases with all histological types of thyroid cancer in Bulgaria (TC, ICD10 code C73) diagnosed between 01/01/1980 and 31/12/2013 retrieved from the anonymous cancer registry database of the Bulgarian National Cancer Registry (BNCR Age-standardized rates (ASR) of thyroid cancer per 100,000 of the population were calculated for each year of the periods mentioned below by sex and age utilized the WHO world reference populations with a special statistical module of the BNCR’s software CancerRegBG, 2011. Incidence rates (IR) 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 changes (APCs) of age-standardized incidence rates were analyzed to determine trends using Joinpoint regression, by a Joinpoint statistical software SEER * Stat Software, Version 4.1.1, 2014.
Results: The ASR of TC in Bulgaria has been increasing since 1990 being higher among women compared to men (4.68 vs 2.81). The highest ASR of TC was observed in women in the period 2007 to 2013. The only significant joinpoint was recorded in 1990 for females and in 1991 for males. The highest IR was in Smolyan district, a region with historically existing iodine deficiency and relatively high post-Chernobyl radiation exposure.
Conclusion: In Bulgaria, thyroid cancer incidence rates have increased since 1990 following global trends. The reasons might be improved diagnostics and registration, pre-existing iodine deficiency, and post-Chernobyl radiation exposure. Our results showed ASR of TC in different regions - endemic and non-endemic - largely differed depending on the Chernobyl accident-induced radiation dose, but the previously existing iodine deficiency might have also played a role as it increased also the risk in areas with relatively low exposure. The role of iodine intake in thyroid cancer remains uncertain but iodine deficiency could be a contributing factor to increased risk of thyroid cancer.

Keywords : Bulgaria, Cancer, Epidemiology, Incidence, Thyroid
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