Introduction - chernobyl irradiation and thyroid cancer
In April 1986, the explosion at the Chernobyl nuclear power plant resulted in widespread radioactive contamination in southern Belarus, northern Ukraine and southern Russia. The main radiation dose was to the thyroid and came from exposure to I131 from drinking contaminated milk. A substantial increase in thyroid cancer incidence in children was observed as early as 5 years after the accident. The first report in 1992 described 114 cases of thyroid cancer in children in Belarus during a 30 month period, the increase starting in 1990; diagnoses were confirmed in 102 of 104 cases examined. Similar increases were reported from Ukraine and Russia and by 2002, 1500 – 2000 cases had been recorded in children younger than 15 years who were exposed to radioactive fallout. World incidence of childhood cancer is approx 1/million children/year; incidence in Belarus in 1995 was 30 fold this figure. Incidence peaked in 1995 and has since declined. Of the Chernobyl related cancers, 98% were papillary. 50–90% of these cancers show a RET oncogene rearrangement; in contrast mutations in other genes (eg RAS) that are involved in other types of thyroid tumours were absent. A recent case-control study of 276 children with thyroid cancer has confirmed a strong relation between radiation dose and cancer risk, occurrence was relatively early after exposure compared with external radiation and risk was 3 times higher in iodine-deficient areas. The influence of increased surveillance and early detection has proved difficult to quantitate. Lessons and questions include age- and genetic-related susceptibility, the incorrect assumption that approximately 10 years is the latency period for radiation induced thyroid cancer and the value of administering iodine supplements after exposure.