Relationship between absorbed dose and ambient dose equivalent

The International
Commission on Radiation Units and Measurements (ICRU) quantity ambient dose
equivalent, H*(d), was a quantity that was devised for purposes of operational radiation
field measurements. When d is equal to 10 mm in soft tissue equivalent
material, the ambient dose equivalent is commonly used as a surrogate for the
quantity effective dose equivalent (H_E in ICRP Report 26 or called
effective dose, E, in ICRP 60). It is the effective dose equivalent that we
would like to be evaluating for purposes of radiation protection. Since
determining effective dose requires knowing the doses delivered to all the
major organs in the body, it is not a practical dose quantity to attempt to
evaluate in routine operations involving external source exposures. Instead we
have devised alternate quantities such as the 1 cm deep dose that is used in
personnel dosimetry devices and the ambient dose equivalent that is used in
instrumental measurements.

The values of
ambient dose equivalent for a given radiation field are not easily calculated
using conventional deterministic methods. As you have noted, by definition, the
fluence of photons (when concerned with gamma or x-ray doses) is visualized as
an aligned field (rays all parallel) expanded so as to cover and be incident on
the ICRU 30 cm diameter tissue-equivalent sphere. The ambient dose equivalent
is determined at the depth of interest (10 mm for effective dose approximation)
measured along the radius that directly opposes the direction of the incident
field. The ambient dose at that defined point does indeed represent the
absorbed dose at the point, multiplied by the appropriate quality factor, which
is 1.0, as you note, for photons. The absorbed dose at the point includes
contributions from both primary photons and scattered photons in the sphere
and is best calculated by performing Monte Carlo
simulations. This has been done for photons (as well as neutrons) and results
have been published in the forms of various conversion factors. You can find
conversion factors at discrete energies and graphical representations covering
a wide energy range in ICRU Report 47, Measurement of Dose Equivalents from
External Photon and Electron Radiations, 1992, and ICRU Report 39, Determination
of Dose Equivalents Resulting from External Radiation Sources, 1985, and ICRU
Report 43, Determination of Dose Equivalents from External Radiation Sources –
Part 2, 1988. Report 39 includes graphs of the ratio of H_E/H*(10) as
a function of energy for an irradiation of an anthropomorphic phantom. For a
front-to-back irradiation, the ratio varies from about 0.2 at 15 keV to about 1
at 10 MeV. Thus, the use of ambient dose equivalent as a surrogate for
effective dose equivalent is conservative.

Since, when
instruments are being calibrated, it is common to set up the instrument in a
known radiation field from a defined source with both the source and the
instrument in air, one quantity that is often determined is the exposure rate
(for photons) or the air kerma rate. These quantities may be measured directly
by some instruments or they may be calculated from the known field conditions.
If one wants to calibrate a dose-measuring instrument to measure ambient dose
equivalent, one may multiply the exposure or air kerma by the appropriate
conversion factor available in the literature. Ambient dose conversion factors
listed in ICRU Report 47, for example, allow conversions to ambient dose
equivalent at 10 mm and 0.07 mm depths from photon fluence, exposure, or air
kerma.

Under
conditions of secondary charged particle equilibrium, a condition that normally
prevails in calibration procedures, the air kerma should be about equal to the
absorbed dose in air. The air kerma-to-ambient dose equivalent conversion
factor given in Table A.2 of ICRU Report 47 is about 1.20 Sv Gy^-1 at
a photon energy of 662 keV from ¹³⁷Cs, a commonly used calibration
radionuclide. This implies that one would multiply the ambient dose equivalent
by a factor of 1/1.20 = 0.83 to obtain the approximate absorbed dose in air. If
one wanted to convert from ambient dose equivalent to soft-tissue dose at the
same energy, one would have to multiply air kerma by the ratio of mass energy
absorption coefficients for soft tissue compared to air at the 662 keV energy.
This ratio has a value of about 1.1, and this results in an ambient dose
equivalent-to-tissue dose conversion factor of 1.09, implying having to
multiply ambient dose equivalent by 1/1.09 = 0.92 to obtain tissue absorbed
dose. The advice you got, namely to multiply the ambient dose equivalent by 0.7
to get absorbed dose (in tissue), would not generally be correct, although it
would apply at photon energies around 100 keV.

You are
correct in your observation that in real life radiation fields are often not
aligned and expanded, although most photon fields we measure are sufficiently
extensive to at least cover the dimensions of the detector being used. The
ambient dose equivalent was devised as a convenience more than as a
representation of reality. Such “conveniences” are often necessary compromises
in order to have a system that can be implemented practically, uniformly, and
reasonably easily. Consider how impractical, indeed impossible, it would be to
attempt to calibrate instruments to account for all possible variations in
radiation field that might actually be experienced in the field. So ambient
dose equivalent is an operational quantity intended for field use. What is
important is that it provides an acceptable approximation to the actual
dosimetric quantity of interest—e.g., effective dose equivalent, preferably
with errors being in the conservative direction. This is true for the quantity
ambient dose equivalent, H*(10).

Your
questions are completely legitimate, and I think it has been worth spending
time here in trying to address them. I wish you well in your continuing
studies.

George
Chabot,
PhD, CHP