In Lithuania, as in many other countries, the incidence of thyroid cancer is increasing. Most probably, the trends reflect an environmental risk increase that is boosted by augmented diagnostic activity following more careful pathological examination.
The increase in thyroid cancer risk could be attributed to ionizing radiation exposure [2
]. Environmental and occupational exposures from the Chernobyl accident were studied in Lithuania – the recent estimates do not confirm an evident contribution to thyroid cancer rates for the Lithuanian population. In a study of Lithuania clean-up workers, no elevated thyroid cancers risk due to exposure was detected. There was no association with the level of radiation dose or duration of stay in the area of Chernobyl [16
]. Studies of individuals living in the Chernobyl areas have shown an increased risk among those exposed as children [13
]. In Lithuania in 1987 thyroid cancer incidence rates for males and females, in age group 0–19 years were 0.2 and 0.6 per 100 000 and in 2003 0.9 and 0.6 respectively. Ionizing radiation exposure, therefore cannot explain the recent increase in incidence of thyroid cancer in Lithuania; other factors should have contributed importantly to the changing pattern.
In addition to this well-known risk factor of thyroid cancer, the increase of diagnostic activity has been suspected to be of etiological importance in the observed increase of thyroid cancer incidence. Analysis of the impact of changes in medical practice found an association between the spread of fine-needle aspiration biopsy and the increase in thyroid cancer incidence [17
]. In Lithuania, the overall thyroid cancer increase was caused primarily by an increase in papillary thyroid cancer and in I stage thyroid cancer. Ultrasound guided fine-needle aspiration biopsy of thyroid nodules was first introduced in Lithuania in 1997. At about at the same time, the practice of follow-up of small thyroid nodules was changed accordingly. The method of ultrasound guided aspiration biopsy of asymptomatic thyroid nodules smaller than 1 cm in diameter was employed for the follow-up. If one takes into consideration the dynamic of fine-needle aspiration biopsy dissemination in the region, then the increase in thyroid cancer incidence would seem to be correlated well with the new diagnostic technology.
Studies from other countries confirm our suggestions of the possible impact of changes in the management of thyroid nodules and diagnostic activities. Increase in papillary thyroid cancer incidence in Geneva has been found to be related mainly to changes in histological diagnostic criteria, and, to a lesser extent, to increased diagnostic activity [18
]. The proportion of microcarcinomas and silent carcinomas increased from 17% to 24% between 1970–79 and 1990–98. A multicentric study in France showed a significant increase, from 1980 to 2000, in ultrasonographic (from 3 to 84.8%) and fine-needle aspiration biopsy (from 4,5 to 23%) of patients with thyroid nodules as well as a significant association between the increase in the prevalence of thyroid carcinomas among operated patients (from 12.5 to 37) [17
]. There was increased incidence of thyroid cancer in France, mainly due to papillary type, with an epidemic of microcarcinomas (43% of operated cancers, for the period 1998–2001) [9
The increasing incidence of thyroid cancer in the United States also was related to the increased detection of small papillary cancers. Between 1988 and 2002, 49% of the increase consisted of cancers measuring 1 cm or smaller; 87% consisted of cancers measuring 2 cm or smaller [10
]. The authors suggested that increasing incidence with stable mortality rates reflects increased detection of subclinical disease, not an increase in the true occurrence of thyroid cancer. Most probably, increased thyroid cancer incidence rates in Lithuania may also be attributed to diagnostic improvements and, consequently, to the discovery of smaller tumours.
The joinpoint analysis has recognized evident changes only by years 1999 and 2000 that can be a reflection of major changes in diagnostic and treatment paradigms and new diagnostic technology in the country. The results show changes in thyroid cancer incidence, and the relative importance of the diagnostic activities in recent years. Additional research on the risk factors for thyroid cancer and incidence changes related to diagnostic practices is needed to explain the peaking of the incidence rates in Lithuania.