Repeat imaging in kids with cancer: a balancing act

The imaging tests widely used in children's cancer treatment can expose some kids to potentially concerning levels of radiation, according to a study published Monday.

The findings, based on 150 pediatric cancer patients at one Canadian medical center, offer a first look at the radiation doses these children can accumulate from various types of imaging tests during the course of their treatment.

A number of studies in the past few years have raised concerns about adults' increasing exposure to radiation from medical imaging, driven largely by the growing use of CT scans and nuclear medicine tests - where a small amount of radioactive material is injected into the bloodstream, concentrates in various tissues and is then read by special cameras.

The issue of radiation exposure from medical imaging is complex when it comes to children with cancer.

On one hand, childhood cancer survivors as a group already have a higher-than-average risk of developing another cancer later in life, and there is a concern that radiation exposure from medical imaging could further increase those odds.

However, imaging tests are a necessary part of diagnosing and managing a child's existing cancer - and any potential radiation risks need to be balanced against the immediate, and significant, benefits, experts say.

For the new study, Karen E. Thomas and colleagues at the Hospital for Sick Children in Toronto reviewed the records of 150 children who began cancer treatment at their center in 2001.

The researchers found a wide variation in the children's cumulative radiation dose from imaging tests done over five years - from less than 1 millisievert (mSv) to 642 mSv. The median, or midpoint, dose for the whole study group was 61 mSv, with CT scans and nuclear medicine tests accounting for nearly all of the total radiation exposure.

To put that in context, the average American is exposed to about 3 mSv of radiation per year from natural sources, like the sun's rays and radioactive substances in the ground and water. A single chest CT would expose a child to a comparable amount of radiation, while an abdominal CT would expose him or her to up to 5 mSv, or 20 months' worth of background radiation.

Based on a report on the biological effects of ionizing radiation from the National Academy of Science, Thomas and her colleagues estimate that a 10-year-old exposed to 61 mSv of radiation from medical imaging would see his or her lifetime risk of cancer go up by 1.2 percent. Children exposed to more than 100 mSv - as 41 percent of those in this study were - might see their lifetime cancer risk go up by more than 2 percent.

But that estimate is based on the entire population; any excess risk associated with radiation from imaging would have to be added to the already heightened risks found among childhood cancer survivors, the researchers point out.