What is the unit used for absorbed dose in radiology?

• A. Coulombs per kilogram
• B. Joules per kilogram
• C. Gray (Gy)
• D. Sieverts (Sv)

Answer: (C)

Absorbed dose is the energy actually deposited in tissue per unit mass. The standard SI unit for this quantity is the gray, defined as one joule of energy deposited per kilogram of matter. So gray directly expresses how much energy is deposited in the tissue, which is why it’s the correct choice.

Joules per kilogram expresses the same physical relationship, just in base units, but the radiology convention is to use gray as the unit name for absorbed dose. Sieverts, by contrast, describe dose equivalent or effective dose, which incorporate radiation quality and tissue sensitivity to assess biological risk rather than the actual energy deposited. Coulombs per kilogram is a charge-based unit linked to exposure or kerma concepts, not to energy deposited per mass, so it isn’t used for absorbed dose.