Absorbed dose

Absorbed dose (also known as total ionizing dose, TID) is a measure of the energy deposited in a medium by ionizing radiation per unit mass. It is equal to the energy deposited per unit mass of medium, which may be measured as joules per kilogram and represented by the equivalent SI unit, gray (Gy), or the antiquated CGS units, rad and rep. The absorbed dose depends not only on the incident radiation but also on the absorbing material: a soft X-ray beam may deposit four times more dose in bone than in air, or none at all in a vacuum.

The absorbed dose is equal to the kerma of the radiation beam times the ionization energy of the medium to be ionized. For example, the ionization energy of dry air at 20°C and 101.325 kPa of pressure is 33.97 ± 0.06 J/C.[1]:305 (33.97 eV per ion pair) Therefore a beam kerma of 2.58×10-4 C/kg (1 roentgen) would deposit an absorbed dose of 8.76×10-3 J/kg (0.00876 Gy or 0.876 rad) in dry air at those conditions.

Absorbed dose is used to rate the survivability of electronic components to nuclear environments, to gauge the risk of acute radiation syndrome, and as an intermediate figure in dosimetry calculations. The absorbed dose alone is not an adequate indicator of the likely health effects in humans. Consideration must also be given to the type of radiation, the dose rate, the affected tissues, and other factors. For example, 1 Gy of alpha radiation would carry a much greater risk of cancer than 1 Gy of photon radiation. Further calculation can be performed to find the equivalent dose for whole body external exposure, the effective dose for partial body external exposure, or the committed dose for internal exposures. These adjusted doses, measured in units of sievert (Sv) or rem, are much more representative of the stochastic risks to human health.

When the absorbed dose is not uniform, or when it is only applied to a portion of a body or object, an absorbed dose representative of the entire item can be calculated by taking a mass-weighted average of the absorbed doses at each point. More precisely,[2]

$\bar{D_T}=\frac{\int_{T}D(x,y,z)\rho(x,y,z)dV}{\int_{T}\rho(x,y,z)dV}$

Where

$\bar{D_T}$ is the mass-averaged absorbed dose of the entire item T
$T$ is the item of interest
$D(x,y,z)$ is the absorbed dose as a function of location
$\rho(x,y,z)$ is the density as a function of location
$V$ is volume

Non-uniform absorbed dose is common for soft radiations such as low energy x-rays or beta radiation. Self-shielding means that the absorbed dose will be higher in the tissues facing the source than deeper in the body.

The mass average can be important in evaluating the risks of radiotherapy treatments, since they are designed to target very specific volumes in the body, typically a tumour. For example, if 10% of a patient's bone marrow mass is irradiated with 10 Gy of radiation locally, then the absorbed dose in bone marrow overall would be 1 Gy. Bone marrow makes up 4% of the body mass, so the whole-body absorbed dose would be 0.04 Gy. The first figure (10 Gy) is indicative of the local effects on the tumour, while the second and third figure (1 Gy and 0.04 Gy) are better indicators of the overall health effects on the whole organism. Additional dosimetry calculations would have to be performed on these figures to arrive at a meaningful effective dose, which is needed to estimate the risk of cancer or other stochastic effects.

When ionizing radiation is used to treat cancer, the doctor will usually prescribe the radiotherapy treatment in units of gray. Medical imaging doses may be described in units of coulomb per kilogram, but when radiopharmaceuticals are used, they will usually be administered in units of becquerel. When health risk from ionizing radiation is being discussed, the sievert would be used. There is no universally applicable conversion constant between these different units because they measure different things.