Answer to Question #8949 Submitted to "Ask the Experts"

I have a question about ambient dose equivalent. One definition I found states, "The ambient dose equivalent H*(d) at a point of interest in the real radiation field is the dose equivalent that would be produced by the corresponding aligned and expanded radiation field, in the ICRU sphere at a depth d, on the radius vector opposing the direction of radiation incidence." I am not sure what the exact correlation between the dose absorbed at a point of interest and the ambient dose equivalent is. I find that the H*(d) does not equal the value of D*Q*N when we take the Q as 1 in the situation of measuring the gamma radiation. So, I want to know how to calculate the conversion factor to transform absorbed dose to ambient dose equivalent. Someone just told me that there is a factor of about 0.7 that you use to multiply the ambient dose equivalent to get the dose absorbed. If so, how is this factor obtained? Another question is that the ambient dose equivalent is defined in the aligned and expanded radiation field and is in the ICRU sphere, but we take it as the operational quantity for use and one thing is that the real radiation field is never aligned and expanded, so why do we use 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