Iodine intake and epidemiological characteristics of thyroid cancer: comparison between inland and littoral Croatia

Abstract

The aim of the study was to determine epidemiological characteristics of thyroid cancer in Dalmatia and Slavonia, to analyze regional differences in its incidence, and to determine whether iodine intake had an impact on the incidence. This epidemiological study was based on data published by the Croatian National Cancer Registry (CNCR) in the Croatian Health Service Yearbook and CNCR Bulletin. Data on 651 thyroid cancer patients operated on in Dalmatian hospitals between 1997 and 2006 were analyzed. Control group consisted of thyroid cancer patients operated on in Slavonia (N = 498). Incidence rates recorded in Dalmatia were compared with those in Slavonia. Iodine intake was measured in elementary schoolchildren in the Split-Dalmatia and Osijek-Baranya Counties. Iodine excretion levels were measured in 131 children. In the 10-year period, the mean age-standardized incidence rate (ASR) of thyroid cancer was 9.32 per 100,000 inhabitants in Dalmatia and 6.02 in Slavonia. The difference was statistically significant (P < 0.001). Incidence rates (World ASR, European ASR, crude incidence rate and research rates) showed an increase and were significantly higher in Dalmatia than in Slavonia. Patient sex structure showed the disease to be 4 times more common in women than in men. In Dalmatia sample, women accounted for 81.4% of all patients and papillary cancer accounted for 80.03% of all thyroid cancers. Median age of new patients was 50 in Dalmatia and 48 in Slavonia. Papillary cancer accounted for 63.7% of all patients in Slavonia. Follicular thyroid cancer accounted for 20.9% of patients in Slavonia and 12.4% in Dalmatia. Epidemiological characteristics of thyroid cancer in Dalmatia were found to be consistent with the characteristics of this cancer in iodine-sufficient areas: papillary carcinoma was the most common type and the papillary to follicular ratio was 6.4:1. Follicular cancer accounted for 12.4% of the total number of patients. In Slavonia, the papillary form predominated, the papillary to follicular ratio was 3:1, and follicular cancer accounted for 20.88% of all patients. Median urinary iodine excretion in elementary schoolchildren was 23.6 microg/dL in the Split-Dalmatia County and 28.1 microg/dL in the Osijek-Baranya County. The difference was not statistically significant (P < 0.050). In conclusion, the average iodine intake levels in Dalmatia and Slavonia were not the cause of the high thyroid cancer incidence in the 1997-2006 period. Improved thyroid cancer diagnosis may be one of the causes of the increased thyroid cancer incidence in Dalmatia.