Which of the following is NOT a quantity of radiation in tissue?

• A. effective dose
• B. exposure
• C. absorbed dose
• D. equivalent dose

Answer: (B)

Understanding which quantities describe radiation in tissue versus in air helps you see why exposure isn’t a tissue dose. Exposure measures the amount of ionization produced in air by X-rays or gamma rays, not the energy actually deposited in tissue. It’s tied to how strong the beam is and is used for things like shielding design and generator output, with units like Coul per kilogram (or historically roentgen).

In contrast, absorbed dose directly quantifies what tissue receives: energy deposited per unit mass, measured in gray. Building on that, equivalent dose adds a radiation weighting factor to reflect the different biological effects of various radiations, giving units in sieverts. Then effective dose goes a step further by weighting those doses across tissues to reflect overall risk, also in sieverts.

So, the quantity that is not a tissue dose is the exposure, because it describes ionization in air rather than energy deposited in tissue.