ALARA principles

Exposure to ionizing radiation

Exposure to ionizing radiation is a natural consequence of living on Earth. We are exposed to ionizing radiation from the Earth beneath our feet, from the air we breathe, from materials within our own bodies, and from space. Background radiation from natural sources varies by location and is generally greater at higher elevations. For a person in the United States, the estimated average background radiation exposure from natural and manmade sources is approximately 3 mSv/year.

In 1982, natural background radiation contributed most of the public’s exposure to ionizing radiation, and the average annual diagnostic medical exposure per person in the United States was estimated to be 0.54 mSv. By 2006, the contribution of radiation exposure from diagnostic medical procedures in the United States had increased by almost six times, to approximately 3 mSv per person per year. This increased medical radiation exposure was largely due to computed tomography (CT) and nuclear medicine procedures. Although CT procedures represented only 15% of diagnostic procedures, CT contributed over half of the cumulative diagnostic procedure dose. At one hospital, although CT accounted for only 15% of the diagnostic radiology procedures, CT contributed approximately 75% of the total effective dose.

Risks of ionizing radiation exposure and ALARA

The primary concern for exposure of patients to ionizing radiation is the risk of cancer induction. Children are considered to be at greater relative risk from radiation exposure than adults are. Children are smaller and incur a greater effective dose per body mass unit than do adults for a given exposure. As children are growing, they have a greater proportion of dividing cells and are considered more sensitive to radiation-induced carcinogenesis. Children also have more life years ahead of them, giving them a longer time to develop the late effects of radiation exposure or to be further exposed to ionizing radiation. There is debate among experts over the risk of cancer induction from medical imaging, but there is agreement that we should act cautiously, assuming risk. Although there are no absolute defined limits for patient radiation exposure, the goal is that radiation exposure from diagnostic imaging should be kept “as low as reasonably achievable” (ALARA).