“What is radiation dose?”

Answering the question “What is radiation dose?” can be complicated. Health care professionals know that the answer depends on context.

Definitions of radiation dose have changed over time. Because everyone is concerned about radiation safety, it’s important to understand those contexts and how the terms describing each have evolved.

There are three ways of describing the effects of radiation dose that are most relevant to radiation dosimetry: absorbed, equivalent and effective dose.

Absorbed dose is straightforward. Absorbed dose describes the energy deposited by ionizing radiation (e.g. X-rays, electrons, alpha particles) into whatever was exposed to that radiation. Absorbed dose characterizes the energy deposited in mass. The units used are often Jules per kilogram, or gray (Gy). The smaller unit, milligray (mGy), is also often used to describe absorbed doses common in medical imaging. There are 1000 milligrays in one gray.

It’s important to note that different types of radiation can do more biologic damage with the same deposited energy. The concept of equivalent dose accounts for these differences.

Equivalent dose (formerly called “dose equivalent”) is a measure of the effectiveness of radiation in causing tissue damage. Here, the absorbed dose is adjusted by a weighting factor that depends on the type of radiation particle. For example, X-rays do comparatively less biologic damage per gray than alpha particles, and so their weighting factor is lower.

“Dose equivalent” is an older version of this concept yet uses earlier radiation weighting factors called “quality factors.” When a gray is weighted for radiation type, the new unit is called a sievert (Sv).

But equivalent dose doesn’t tell the whole story either. Some tissues are more negatively affected by radiation than others. So effective dose is examined

Effective dose (similar to the older term “effective dose equivalent”) weights the equivalent dose by a tissue’s, or organ’s, sensitivity to radiation. For example, lung tissue is far more sensitive to radiation than fatty tissue.

So, we’ve moved from absorbed dose to equivalent dose to understanding the radiation detriment by tissue type, where we weight the equivalent dose with tissue weighting factors. Effective dose permits us to compare the risks from partial body exposures (such as those in medical imaging) with those from whole body radiation exposures. Sieverts also serve as a unit for effective dose. Although sievert can refer to both equivalent dose and effective dose, it most often refers to effective dose.

Often, measures of effective dose are used in radiological protection, or for showing compliance with regulatory bodies. It’s worth noting that the Nuclear Regulatory Commission (NRC) still uses older tissue weighting factors and therefore refers to “effective dose equivalent.”

In summary, absorbed dose represents energy deposited per unit mass, equivalent dose weights by the resulting tissue damage from different radiation types, and effective dose accounts for differences among tissue types in sensitivity to radiation.

Medical physicists, radiologists and technologists understand that there may be risks associated with exposures to radiation from medical procedures. Because of radiation risks and regulations, it’s important for personnel to wear dosimeters to measure and track dose.