Absorbed Dose

Definition:

• The energy deposited per unit mass of any medium (air, tissue, water) by ionizing radiation.
• Represents the physical energy imparted to the material.

Unit:

• Gray (Gy) = joules per kilogram (J/kg)

Measurement/Calculation:

• D = (ΔE) / m, where ΔE is energy absorbed and m is mass of the medium.
• Measured directly in phantoms or derived from Air kerma with tissue conversion factors (e.g., f-factors, mass energy absorption coefficients).

Clinical/Protection Use:

• Quantifying patient dose in CT (CTDI_vol), fluoroscopy (DAP → estimated organ dose), nuclear medicine (organ doses), and external beam therapy (tumor dose).
• Used to predict Deterministic effects (e.g., skin erythema thresholds) and for treatment planning in radiotherapy.

see also Equivalent Dose vs Effective Dose (Clinical examples)