Incidence of thyroid cancer in the selected areas of iodine deficiency in Poland.

The aim of the study was to evaluate the incidence rate (IR), trend and histotype of the differentiated thyroid cancer in the selected areas with varying iodine deficiency. The study was carried out in three areas: Krakow, (Carpathian endemic goiter area with 1.99 million mixed rural and urban population), Gliwice (Upper Silesia--moderate iodine deficiency area mostly industrial with 4.89 million inhabitants) and Olsztyn (slight iodine deficiency area, mainly rural with 0.77 million inhabitants). Between 1990 and 2001, in the study area 2691 newly diagnosed cases of malignant neoplasms of the thyroid gland were registered. In over 80% of patients it was differentiated thyroid cancer: mainly in women over 40 years, with F/M ratio 5.8. The highest percentage of papillary cancer 72.9% was observed in Olsztyn and lowest--50.0%--in Krakow and Nowy Sacz districts. In this period of time incidence rate of differentiated thyroid cancer in women increased in Kraków, Gliwice, and Olsztyn from 1.51 to 9.34 in 1998 1.27 to 5.74 in 1999 and from 2.52 to 11.35 in 2001 respectively. In the youngest (0-20 years) age group no significant increase of IR was observed. Between 1998 and 2001 the dynamics of increase of the thyroid cancer incidence markedly diminished. In conclusion it was hypothesised that an increase in IR of differentiated thyroid cancer in the study area was caused mainly by the suspension of iodine prophylaxis in 1980 and was diminished by the introduction of an obligatory model of iodine prophylaxis in 1996/1997. It was modified in terms of histotype and dynamics of increase by exposure to ionizing radiation. A very specific group at risk on the population level were women aged 20-40 years in the reproductive age exposed to iodine deficiency after suspension of iodine prophylaxis in 1980 and to radiation after the Chernobyl accident in 1986.