TOPIC 3: Energy Deposition in the Body

We’ve discussed the energy deposition in air and the energy deposition in matter.  Now let’s look at the energy deposition in the body.

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Not all types of radiation produce the same effect.  The dose equivalent .. Which we define with an “H” …takes into account the absorbed dose (D) and the type of radiation (Q).  The dose equivalent is applied to the whole body but does not take into account the sensitivity of each tissue or organ.

For example, for beta, X-ray, and gamma ray radiation, the dose equivalent is equal to the absorbed dose (expressed in rad), or a ratio of 1:1. However, when an alpha absorbed dose (D) is deposited within sensitive tissues of the body (NOT the dead layer of the skin), for the same absorbed dose, the dose equivalent (H) from the alphas is twenty times that of betas, gammas or X-rays because … as we learned earlier…it can do far more damage to sensitive tissue.

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The radiological term called dose equivalent is expressed in units of the rem in the traditional system and the sievert in the international system.

Known as the Roentgen Equivalent Man; the rem relates the amount of energy absorbed in human tissue (absorbed dose) due to the effective biological damage of the radiation.  Not all radiation has the same biological effect for the same amount of absorbed dose, as the quality factors showed us.  For instance, the alpha particle is harmless outside the body. Gamma-rays, however, go right through the skin and through the body.  We can see that the dose from alpha particles that are external to the skin are not a concern, but gamma radiation would be.  Only if the radionuclides that emit alphas are inhaled would they be of concern.  Once inhaled, the dose from alpha radiation in the lungs would be much more than for gamma-rays.  As we have seen, the dose equivalent would be twenty times that of the gammas. Dose equivalent is often expressed in terms of rem, or millirem.

Remember when we said that 1 mR = 1 mrad for practical use? Since the quality factor for gamma and X-radiation is one, we can extend that for practical purposes to 1 mR = 1 mrad = 1 mrem.

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The sievert (Sv) is the international unit for dose equivalent that has replaced the rem in most of the world.  One sievert is equivalent to 100 rem.  A sievert is an equivalent  energy per kilogram.  To determine the dose equivalent (Sv), multiply the absorbed dose (Gy) by the quality factor (Q) of the incident radiation.

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There is another radiological term known as the “Effective Dose”.  Different organs have different sensitivity to radiation.  The Effective Dose applies a weighting factor to each of the major body organs or tissues that are irradiated and multiplies them by their separate doses. Then it sums the products of the individual doses to each organ or tissue for a total body dose. That calculation is beyond the scope of this course.

We introduce the tissue weighting factors because they are used to calculate doses for partial body exposures.  For example, x-ray treatments to certain parts of a patient’s body, or a worker behind a lead shield but with hands touching the source.  We do not expect you to be able to do those calculations for this course.

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