How are radiation doses measured in gray units?

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linear accelerator; external beam radiation therapy — radiation therapy External beam radiation therapy (also known as external beam teletherapy, or long-distance therapy) being delivered using a machine known as a linear accelerator.

The gray (Gy) is the standard unit used to measure the absorbed dose of ionizing radiation within the International System of Units (SI).1

One gray is defined as the amount of radiation energy absorbed per unit mass of matter, specifically equal to one joule of energy imparted to one kilogram of material, such as living tissue. In medical contexts, this unit quantifies the physical energy deposited by radiation into the body.12

The gray replaced the older unit known as the rad, which is still occasionally used in the United States. The conversion between these units is straightforward: one gray is equal to 100 rads. For clinical applications where smaller increments are necessary, the milligray (mGy), which is one-thousandth of a gray, is frequently employed.132