Answer to Question #12078 Submitted to "Ask the Experts"

Category: Radiation Basics — Radiation Quantities and Units

The following question was answered by an expert in the appropriate field:

Q

I'm currently trying to compare two results using different quantities for radiation. One uses equivalent dose, i.e., absorbed dose with a radiation-weighting factor. The other one uses ambient dose equivalent, which to my understanding is the dose from an expanded radiation field to a point at a depth of 1 centimeter (cm) in the International Commission on Radiation Units and Measurements (ICRU) phantom.

I'm not sure how to go from one of these results to the other, and they also differ by several orders of magnitude, with ambient dose equivalent being the larger quantity. Is this because it takes the radiation from a point to a whole 30-cm-diameter sphere (the ICRU phantom), resulting in a dose over a larger area, while the equivalent dose just takes the dose in a point?

Another thing to point out is that I'm looking at the dose from electrons, neutrons, and photons separately, but the two quantities (equivalent dose and ambient dose equivalent) differ by different amounts for different particle types, especially for photons. I can't understand the reason for this.

A

Unfortunately, there is no simple analytical expression that describes the relationship between ambient dose equivalent and equivalent dose. The relative magnitudes of the two quantities depend on the types of incident radiation; the energies of the radiations; the characteristics of the target tissue for which the equivalent dose is evaluated, including such things as its mass and its location in the body; the geometry of the radiation field; and the orientation of the body in the field.

Your observation that differences may be associated with dose being evaluated over a larger area for the ambient dose equivalent vs. equivalent dose being evaluated at a point is not accurate. The ambient dose equivalent is evaluated at a specific point in the tissue-equivalent phantom; the dose at the point is affected by all radiation, direct or scattered, in the sphere that reaches the dose point. The equivalent dose may sometimes be determined at a point but is most often determined by averaging dose over the mass of a receptor tissue of interest in the medium of interest. When the body is simulated by an appropriate phantom, the locations of the respective dose receptor point (or points) and mass of interest are typically different.

The equivalent dose to a particular tissue is calculated by taking into account the direct radiation that penetrates to the target tissue, as well as the radiation that scatters in the body (simulated by an appropriate phantom for calculational purposes) and reaches the target tissue. The ambient dose equivalent, a quantity originally intended by the ICRU to be used in calibration of field instruments, as you imply, makes an artificial geometric correction to the radiation field so that all photons appear uniformly aligned so as to be unidirectional; the dose is evaluated at a point 1 cm below the surface of a 30-cm-diameter tissue-equivalent sphere and along a radius opposed to the direction of the incident field. The calculation of dose at the point includes the effects of direct and scattered radiation in the sphere. Fluence-to-dose conversion factors have also been determined by some authors for cylindrical phantoms. The two quantities being considered, equivalent dose and ambient dose equivalent are then quite different. One of the greatest factors contributing to differences between the two is the depth of the tissue of interest in the body for estimating the equivalent dose.

For example, if one were concerned with 25 kiloelectronvolt (keV) photons incident normally as a broad beam on the front side of a typical woman, the equivalent dose to the ovaries would be about 10 times less than would be the ambient dose equivalent. If the body were actually exposed to a planar isotropic source rather than a broad parallel beam, the equivalent dose would be an additional factor of two to three times lower than the ambient dose equivalent. If we replaced the 25 keV photons with 1 megaelectronvolt (MeV) photons having the same broad parallel-beam geometry, the equivalent dose to the ovaries would be within roughly 20% of the ambient dose equivalent, and the plane isotropic source equivalent dose would likely be within 10–15% of the broad beam equivalent dose. At the higher photon energy there is much less attenuation of photons that penetrate the body before reaching the ovaries.

The differences noted between ambient dose equivalent and equivalent dose for other radiations may be greater or less than those for photons, depending on the type and energy of the radiations, as well as on possible geometric variations.

George Chabot, PhD, CHP

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