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I often see different types of medical imaging effective doses compared to time required for an equivalent effective dose from background radiation.

For example, a dental x-ray is compared here:

... around 0.150 mSv for a full mouth series equivalent to a few days' worth of background environmental radiation exposure, or similar to the dose received during a cross-country airplane flight (concentrated into one short burst aimed at a small area).

My questions is, how should 0.150 mSv in the example be interpreted? I assume the dental x-ray radiation would result in a higher risk for the small area it focuses on. Similarly the same effective dose from the more diffuse background radiation would be a comparatively smaller risk to any area of the body taken individually (including the area focused on by the dental x-ray), but the sum of those smaller risks in all areas of the body would be equivalent to the more concentrated risk from the dental x-ray? In simpler terms, the dental x-ray might increase the risk of cancer in the area it targeted more than the equivalent background radiation would, but the background radiation would involve the same overall added risk of cancer (considering that it affects all areas of the body). Is my interpretation correct?

From this description, that seems to be the case?

An effective dose will carry the same effective risk to the whole body regardless of where it was applied, and it will carry the same effective risk as the same amount of equivalent dose applied uniformly to the whole body.

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    Thanks @CareyGregory - I've tried to clarify my question – Curtis Howard Oct 17 '20 at 14:30
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"Effective dose" is not a physical quantity, but a calculated summary value meant to evaluate the risk of some radiation exposure to other sources and to set occupational limits.

From Fisher & Fahey 2017:

Effective dose is a mathematical construct, concept, or surrogate of risk, used in radiation protection as the basis for calculating annual radiation limits to workers and members of the public from exposure to radiation and intakes of radionuclides.

The International Commission on Radiological Protection is the body that determines how to make these calculations. They weight the absorbed dose targeted to different organs by the risk of radiation exposure to those organs.

So yes, radiation exposure to the head has a higher risk to tissues in the head than elsewhere in the body, but the effective dose is a measure that allows for equivalent comparisons to be made when radiation is targeted elsewhere or received by the whole body.

The Fisher & Fahey paper goes into more depth on the appropriate and inappropriate uses for this measure and related measures, so I recommend it for further reading if you are interested.

Fisher, D. R., & Fahey, F. H. (2017). Appropriate use of effective dose in radiation protection and risk assessment. Health physics, 113(2), 102.

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