Japanese doctors have begun long-term testing on 360,000 children to learn the extent to which radiation leaking from a crippled nuclear power plant may have exposed them to an increased risk of thyroid cancer.

The ultrasonic thyroid exams began Sunday at Fukushima Medical University, targeting children in two towns closest to the disabled Fukushima-Daiichi plant. The power plant has been leaking radiation since the March 11 earthquake and tsunami devastated much of coastal northeastern Japan, leaving 20,000 people dead or missing.

No one has yet died from radiation exposure. But the screenings were launched after a recent survey showed that at least 10 of 130 children evacuated from Fukushima prefecture had thyroid abnormalities. No direct link was established between the medical conditions and the leaking radiation.

Doctors plan to first examine 5,000 children who were living closest to the plant when meltdowns occurred at three of the plant's six nuclear reactors.

Authorities say follow-up exams will be performed every two years, until the children reach age 20.

Fukushima Medical University physician Shinichi Suzuki said it will take several years carry out the preliminary exams of those age 18 and under in the prefecture. He also noted that it can take a long time for thyroid irregularities to develop into cancer.