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Radiation Quantities and Units

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Absorbed dose is the energy absorbed per unit mass at a given point. The unit is the joule per kilogram (J kg-1) and is given the special name gray (Gy). A more detailed description is given in ICRU Report 74 and in IAEA TRS-457.

Organ dose is a quantity defined in ICRP Publication 60 in relation to the probability of stochastic effects (mainly cancer induction) as the absorbed dose averaged over an organ, i.e., the quotient of the total energy imparted to the organ and the total mass of the organ. The unit is the joule per kilogram and is given the special name gray (Gy).

Equivalent dose to an organ or tissue is the organ dose corrected by a radiation weighting factor that takes account of the relative biological effectiveness of the incident radiation in producing stochastic effects. This correction factor is numerically 1 for X rays. The unit is the joule per kilogram (J kg-1) and is given the special name sievert (Sv).

Effective dose is a quantity defined in ICRP Publication 60 as a weighted sum of equivalent doses to all relevant tissues and organ with the purpose "to indicate the combination of different doses to several different tissues in a way that is likely to correlate well with the total of the stochastic effects". This is, therefore, applicable even if the absorbed dose distribution over the human body is not homogeneous. The unit is the joule per kilogram (J kg-1) and is given the special name sievert (Sv).

The use of effective dose for patients has to be done with caution, as indicated in the UNSCEAR 2000 report to the UN, "effective dose should not be used directly for estimating detriment from medical exposure …  by application of the nominal fatality probability coefficients. Such assessments would be inappropriate and serve no purpose in view of the uncertainties arising from potential demographic differences (in terms of health status, age and sex), between particular population of patients and those from general populations for whom ICRP derived the risk coefficients … effective dose could broadly underestimate the detriment from diagnostic exposures of young patients by a factor of 2 and, conversely, could overestimate the detriment from old patients by a factor of at least 5. … Notwithstanding the above caveat … practice in diagnostic radiology is summarized for comparative purpose, principally in terms of effective dose to the exposed individuals … taking into account the number of procedures, collective effective dose over exposed populations ...."

It is possible, therefore, to use effective dose and even collective dose for medical diagnostic exposure as long as this is done only for comparative purposes and for the same or similar patient populations, and it would require additional considerations or significant corrections if we try to use them to compare with other populations.

Air kerma in air is the sum of kinetic energy of all charged particles liberated per unit mass. A number of publications in the past have expressed measurements in terms of absorbed dose to air. Recent publications and an IAEA Code of Practice point out the experimental difficulty in determining the dose to air, especially in the vicinity of an interface, and that, in reality, what the dosimetry equipment registers is not the energy absorbed from the radiation by the air, but the energy transferred by the radiation to the charged particles resulting from the ionization. For these reasons the IAEA Code of Practice and ICRU Report 74 recommend the use of air kerma rather than absorbed dose to air. The unit is the joule per kilogram (J kg-1) and is given the special name gray (Gy)

This correction applies to the quantities determined in air, such as entrance surface air kerma (rather than entrance surface air dose), computed tomography air kerma index (instead of computed tomography dose index), kerma area product (rather than dose area product) and air kerma area length (rather than dose length product).

The above recommendation refers to air. When referring to tissues, it is also correct to estimate absorbed dose to the skin, by applying the necessary correction coefficient to obtain the absorbed dose to the tissue from the air kerma.


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